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There exist different theories representing the effects of surface gravity waves on oceanic flow fields. In the past, the author has conjectured that the vertically integrated, two-dimensional fluid equations of motion put forward by Longuet-Higgins and Stewart are correct and that theories that differ from their theory cannot be entirely correct; this paper explores these differences. Longuet-Higgins and Stewart deduced vertically integrated, two-dimensional equations featuring a wave radiation stress term in the fluid dynamic, momentum equation. More recently, the author has proposed vertically dependent, three-dimensional equations that have required correction but when vertically integrated, agreed with the earlier, two-dimensional equations. This paper derives both vertically independent and vertically dependent equations from the same base and, importantly, using the same expression for pressure in the belief that the paper will contribute to the understanding and clarification of this seemingly difficult topic in ocean dynamics. An error in the classical papers by Longuet-Higgins and Stewart has been detected. Although the final phase-averaged result was correct, the error has had consequences in the development of vertically dependent equations. The prognostic equations in this paper are for the Eulerian current plus Stokes drift; toward the end of the paper these equations are contrasted with prognostic equations for the Eulerian current alone. [ABSTRACT FROM AUTHOR]

In regards to the recent paper by Pedlosky and Spall ( Journal of Physical Oceanography, November 2015), this comment maintains that the steady-state solutions of Rossby waves in a uniform eastward current past an island have waves on the upstream side that are not caused by the island because of inappropriate boundary conditions and the assumed form of the solution. The solutions are interesting but are not the solutions to the problem as posed in their paper. Similar upstream waves in a time-dependent numerical model are also inconsistent with linear Rossby wave theory, though the reasons for their presence are uncertain. [ABSTRACT FROM AUTHOR]

A number of studies have suggested that heat waves will increase in frequency, intensity, and duration under anthropogenic global warming. However, changes to heat extremes and temperature distributions in general are less understood in regions of complex terrain and substantial land-water contrasts. Such surface variations are important in the northwest United States, where synoptic- and mesoscale circulations interact with local topography and land-water interfaces to produce heat waves that have substantial impacts on fire weather, air quality, wind energy, and the population in general. Thus it is crucial to identify how the synoptic- and mesoscale circulations that drive such local extremes will change in a warming world. This paper analyzes changes to the conditions associated with heat extremes over the northwest United States by utilizing global and regional climate models. A maximum in 700-hPa warming is projected over the northwest United States and southwest Canada, with low-level warming attenuated near the coast. Soil moisture declines are projected over the region, which further enhances future extremes. It is found that low-level zonal wind distributions over the northwest United States become narrower, leading to fewer days with strong offshore flow. This is important since extreme warming events are associated with the strongest offshore/downslope flow. Historical and future regional temperature distributions are described and it is shown that although CMIP5 models predict increases in heat extremes for west Oregon and Washington, these increases are less than for inland areas. [ABSTRACT FROM AUTHOR]

Recent field observations suggest that cloud-top precipitation generating cells (GCs) are ubiquitous in the warm-frontal and comma-head regions of midlatitude winter cyclones. The presence of fallstreaks emanating from the GCs and their persistence either to the surface or until merging into precipitation bands suggests that GCs are a critical component of the precipitation process in these cyclones. This paper is the second part of a three-part series that investigates the dynamics of GCs through very-high-resolution idealized Weather Research and Forecasting (WRF) Model simulations. This paper assesses the role of cloud-top instability paired with nighttime, daytime, or no radiative forcing on the development and maintenance (or lack) of GCs. Under initially unstable conditions at cloud top, GCs develop regardless of radiative forcing but only persist clearly with radiative forcing. Cloud-top destabilization due to longwave cooling leads to development of GCs even under initially neutral and stable conditions, providing a physical explanation for the observed ubiquity of GCs atop winter cyclones. GCs do not develop in initially stable simulations with no radiation. Decreased range in vertical velocity spectra under daytime radiative forcing is consistent with offset of the destabilizing influence of longwave cooling by shortwave heating. [ABSTRACT FROM AUTHOR]

The Ekman boundary layer (EBL) is a central problem in geophysical fluid dynamics that emerges when the pressure gradient force, the Coriolis force, and the frictional force interact in a flow. The unsteady version of the problem, which occurs when these forces are not in equilibrium, is solvable analytically only for a limited set of forcing variability regimes, and the resulting solutions are intricate and not always easy to interpret. In this paper, large-eddy simulations (LESs) of neutral atmospheric EBLs are conducted under various unsteady forcings to reveal the range of physical characteristics of the flow. Subsequently, it is demonstrated that the dynamics of the unsteady EBL can be reduced to a second-order ordinary differential equation that is very similar to the dynamical equation of a damped oscillator, such as a mass-spring-damper system. The validation of the proposed reduced model is performed by comparing its analytical solutions to LES results, revealing very good agreement. The reduced model can be solved for a wide range of variable forcing conditions, and this feature is exploited in the paper to elucidate the physical origin of the inertia (mass), energy storage (spring), and energy dissipation (damper) attributes of Ekman flows. [ABSTRACT FROM AUTHOR]

Natural small-scale microphysical and dynamical mechanisms are identified in a winter orographic snowstorm over the Sierra Madre Range of Wyoming during an intensive observational period (IOP) from the AgI Seeding Cloud Impact Investigation (ASCII; January-March 2012). A suite of high-resolution radars, including a ground-based scanning X-band dual-polarization Doppler on Wheels radar, vertically pointing Ka-band Micro Rain Radar (MRR), and airborne W-band Wyoming Cloud Radar (WCR), and additional in situ and remote sensing instruments are used in the analysis. The analysis focuses on a deep postfrontal period on 16 January 2012 (IOP2) when clouds extended throughout the troposphere and cloud liquid water was absent following the passage of a baroclinic front. A turbulent shear layer was observed in this postfrontal environment that was created by a midlevel cross-barrier jet riding over a decoupled Arctic air mass that extended above mountaintop. MRR and WCR observations indicate additional regions of turbulence aloft that favor rapid particle growth at upper levels of the cloud. Plunging flow in the lee of the Sierra Madre was also observed during this case, which caused sublimation of snow up to 20 km downwind. The multi-instrument analysis in this paper suggests that 1) shear-induced turbulent overturning cells do exist over cold continental mountain ranges like the Sierra Madre, 2) the presence of cross-barrier jets favors these turbulent shear zones, 3) this turbulence is a key mechanism in enhancing snow growth, and 4) snow growth enhanced by turbulence primarily occurs through deposition and aggregation in these cold (<−15°C) postfrontal continental environments. [ABSTRACT FROM AUTHOR]

For wind-generated waves, the wind-wave triplets (reference wind speed, significant wave height, and spectral peak wave period) are intimately connected through the fetch- or duration-limited wave growth functions. The full set of the triplets can be obtained knowing only one of the three, together with the input of fetch (duration) information using the pair of fetch-limited (duration limited) wave growth functions. The air-sea energy and momentum exchanges are functions of the wind-wave triplets, and they can be quantified with the wind-wave growth functions. Previous studies have shown that the wave development inside hurricanes follows essentially the same growth functions established for steady wind forcing conditions. This paper presents the analysis of wind-wave triplets collected inside Hurricane Bonnie 1998 at category 2 stage along 10 transects radiating from the hurricane center. A fetch model is formulated for any location inside the hurricane. Applying the fetch model to the 2D hurricane wind field, the detailed spatial distribution of the wave field and the associated energy and momentum exchanges inside the hurricane are investigated. For the case studied, the energy and momentum exchanges display two local maxima resulting from different weightings of wave age and wind speed. Referenced to the hurricane heading, the exchanges on the right half plane of the hurricane are much stronger than those on the left half plane. Integrated over the hurricane coverage area, the right-to-left ratio is about 3:1 for both energy and momentum exchanges. Computed exchange rates with and without considering wave properties differ significantly. [ABSTRACT FROM AUTHOR]

In this paper, an ideal model of the role of mesoscale eddies in the Kuroshio intruding into the South China Sea (SCS) is developed, which represents the northwestern Pacific and the SCS as two rectangular basins connected by a gap. In the case of considering only intrinsic ocean variability, a time-dependent western boundary current (WBC) driven by steady wind is modeled under both eddy-resolving and noneddy-resolving resolutions. Almost all simulated WBCs intrude into the adjacent sea in the form of the Loop Current with multiple-state transitions and eddy-shedding processes, which have aperiodic variations on intraseasonal or interannual scales, determined by the eddy-induced WBC variation. For the parameters considered in this paper, the WBC intrusion exhibits a 30-90-day cycle in the presence of the subgrid-scale eddy forcing (SSEF) but a 300-500-day cycle in the absence of SSEF. Moreover, the roles of the grid-scale and subgrid-scale eddies in the WBC intrusion are studied by using the dynamically consistent decomposition developed by Berloff. Based on the large-sample composite analysis of the intrusion events, it is found that the Loop Current intensity is mainly determined by baroclinic processes through grid-scale, eddy-eddy interaction and subgrid-scale, eddy-flow interaction. The intrusion position and period are mainly regulated by the SSEF to the west of gap, where the balance between relative vorticity and isopycnal thickness SSEFs determines eddy detachment. Generally, the relative vorticity SSEF therein tends to induce WBC intrusion. However, the isopycnal thickness SSEF tends to induce eddy shedding, and WBC retreat thus determines the intrusion cycle through counteracting relative vorticity SSEF. [ABSTRACT FROM AUTHOR]

The lack of understanding on the soil moisture-precipitation feedback mechanisms remains a large source of uncertainty for land-atmosphere coupled models. Previous observation-based studies on the soil moisture-precipitation feedback in Illinois have shown contradictory results. This paper extends earlier research by providing a more holistic analysis considering different scales based on an 11-yr (2003-13) hourly soil moisture dataset, which makes it possible to revisit the disputed hypothesis on the correlation between warm-season soil moisture and subsequent precipitation. This study finds a strong positive correlation between late spring/early summer state-average soil moisture at the root-zone depths and subsequent state-average summer precipitation. On the daily to weekly time scale, however, no relation is found. Moreover, regional analysis suggests that precipitation variability over central Illinois can be best explained by the soil moisture variability in northwest Illinois. Using a back-trajectory method [Water Accounting Model-2 layers (WAM-2layers)] from May to July, the evaporative sources of precipitation in Illinois are identified. The pattern of the source regions shows little interannual variability, while the strength of the sources changes significantly and the Gulf of Mexico contributes more during wet years. However, strong influences (teleconnections) of sea surface temperatures on the subsequent precipitation variability in Illinois are not found on a seasonal scale. The long time scale of the soil moisture-precipitation correlation and the weak influences of SSTs and climate indices may suggest that precipitation variability in spring/summer in Illinois is mostly related to continental-scale soil moisture-precipitation feedback. [ABSTRACT FROM AUTHOR]

The processes responsible for double-peak latitudinal structures in the time-averaged tropical lower-stratospheric upwelling, centered near 70 hPa and 20°N/S, previously noted in ERA-Interim and other reanalysis and model datasets, are considered. It is demonstrated that the structure of the wave force resolved by ERA-Interim consistently balances the angular momentum transport associated with the double peak. Analysis of the corresponding structures in diabatic heating rates from ERA-Interim indicates that the peaks arise predominantly from the meridional structure in ozone concentrations and the associated absorption of both shortwave and longwave radiation. Additional smaller contributions arise from local absorption of longwave radiation emitted from the relatively warm layers above and below, as well as from cloud-related radiative effects and nonradiative diabatic heating. The temperature at 70 hPa is slightly higher near 20°N/S than at the equator, opposite of what would be expected if the latitudinal structure in radiative heating were associated with local relaxation. It is proposed on the basis of this analysis that the primary cause of the peaks in upwelling is the externally imposed (i.e., nonrelaxational) part of the radiative heating field. The dynamical plausibility of this hypothesis is investigated in a companion paper. [ABSTRACT FROM AUTHOR]

The air-sea flux of greenhouse gases [e.g., carbon dioxide (CO2)] is a critical part of the climate system and a major factor in the biogeochemical development of the oceans. More accurate and higher-resolution calculations of these gas fluxes are required if researchers are to fully understand and predict future climate. Satellite Earth observation is able to provide large spatial-scale datasets that can be used to study gas fluxes. However, the large storage requirements needed to host such data can restrict its use by the scientific community. Fortunately, the development of cloud computing can provide a solution. This paper describes an open-source air-sea CO2 flux processing toolbox called the 'FluxEngine,' designed for use on a cloud-computing infrastructure. The toolbox allows users to easily generate global and regional air-sea CO2 flux data from model, in situ, and Earth observation data, and its air-sea gas flux calculation is user configurable. Its current installation on the Nephalae Cloud allows users to easily exploit more than 8 TB of climate-quality Earth observation data for the derivation of gas fluxes. The resultant netCDF data output files contain >20 data layers containing the various stages of the flux calculation along with process indicator layers to aid interpretation of the data. This paper describes the toolbox design, which verifies the air-sea CO2 flux calculations; demonstrates the use of the tools for studying global and shelf sea air-sea fluxes; and describes future developments. [ABSTRACT FROM AUTHOR]

Submesoscale dynamics are hypothesized to play a leading-order role in setting the stratification of the mixed layer via the interaction of submesoscale eddies and surface forcing. Previous studies of such interactions have generally focused on the time-evolving characteristics of submesoscale turbulence, such as the spindown of a baroclinically unstable front. This paper focuses instead on the equilibrium dynamics of the oceanic mixed layer, where forcing and dissipation are in balance, through a combination of scaling analysis and numerical simulations. The steady dynamics are well described by a turbulent thermal wind balance, with external forcing parameterized by a strong vertical diffusivity κ. Scaling laws are developed for the characteristic vertical length scale L υ, ageostrophic velocity scales U and V, buoyancy frequency N2, and eddy buoyancy flux , which are appropriate for a turbulent mixed layer whose stratification is equilibrated against strong vertical mixing. A suite of numerical simulations is developed to test these scalings for different values of κ and lateral buoyancy gradient. The scaling relations are shown to be very robust across all simulations, and this allows the new scaling for to be directly compared against an extant parameterization in the forcing scenarios explored here. [ABSTRACT FROM AUTHOR]

The equatorial eastern Indian Ocean (EIO) upwelling occurs in the Indian Ocean warm pool, differing from the equatorial Pacific and Atlantic upwelling that occurs in the cold tongue. By analyzing observations and performing ocean model experiments, this paper quantifies the remote versus local forcing in causing interannual variability of the equatorial EIO upwelling from 2001 to 2011 and elucidates the associated processes. For all seasons, interannual variability of thermocline depth in the EIO, as an indicator of upwelling, is dominated by remote forcing from equatorial Indian Ocean winds, which drive Kelvin waves that propagate along the equator and subsequently along the Sumatra-Java coasts. Upwelling has prominent signatures in sea surface temperature (SST) and chlorophyll- a concentration but only in boreal summer-fall (May-October). Local forcing plays a larger role than remote forcing in producing interannual SST anomaly (SSTA). During boreal summer-fall, when the mean thermocline is relatively shallow, SSTA is primarily driven by the upwelling process, with comparable contributions from remote and local forcing effects. In contrast, during boreal winter-spring (November-April), when the mean thermocline is relatively deep, SSTA is controlled by surface heat flux and decoupled from thermocline variability. Advection affects interannual SSTA in all cases. The remote and local winds that drive the interannual variability of the equatorial EIO upwelling are closely associated with Indian Ocean dipole events and to a lesser degree with El Niño-Southern Oscillation. [ABSTRACT FROM AUTHOR]

There are now several observations of internal solitary waves passing through a critical point where the coefficient of the quadratic nonlinear term in the variable coefficient Korteweg-de Vries equation changes sign, typically from negative to positive as the wave propagates shoreward. This causes a solitary wave of depression to transform into a train of solitary waves of elevation riding on a negative pedestal. However, recently a polarity change of a different kind was observed in Laoshan Bay, China, where a periodic wave train of elevation waves converted to a periodic wave train of depression waves as the thermocline rose on a rising tide. This paper describes the application of a newly developed theory for this phenomenon. The theory is based on the variable coefficient Korteweg-de Vries equation for the case when the coefficient of the quadratic nonlinear term undergoes a change of sign and predicts that a periodic wave train will pass through this critical point as a linear wave, where a phase change occurs that induces a change in the polarity of the wave, as observed. A two-layer model of the density stratification and background current shear is developed to make the theoretical predictions specific and quantitative. Some numerical simulations of the variable coefficient Korteweg-de Vries equation, and also the extended variable coefficient Korteweg-de Vries equation, are reported that confirm the theoretical predictions and are in good agreement with the observations. [ABSTRACT FROM AUTHOR]

This paper uses the energetics framework developed by Scotti and White to provide a critical assessment of the widely used Thorpe-scale method, which is used to estimate dissipation and mixing rates in stratified turbulent flows from density measurements along vertical profiles. This study shows that the relevant displacement scale in general is not the rms value of the Thorpe displacement. Rather, the displacement field must be Reynolds decomposed to separate the mean from the turbulent component, and it is the turbulent component that ought to be used to diagnose mixing and dissipation. In general, the energetics of mixing in an overall stably stratified flow involves potentially complex exchanges among the available potential energy and kinetic energy associated with the mean and turbulent components of the flow. The author considers two limiting cases: shear-driven mixing, where mixing comes at the expense of the mean kinetic energy of the flow, and convective-driven mixing, which taps the available potential energy of the mean flow to drive mixing. In shear-driven flows, the rms of the Thorpe displacement, known as the Thorpe scale is shown to be equivalent to the turbulent component of the displacement. In this case, the Thorpe scale approximates the Ozmidov scale, or, which is the same, the Thorpe scale is the appropriate scale to diagnose mixing and dissipation. However, when mixing is driven by the available potential energy of the mean flow (convective-driven mixing), this study shows that the Thorpe scale is (much) larger than the Ozmidov scale. Using the rms of the Thorpe displacement overestimates dissipation and mixing, since the amount of turbulent available potential energy (measured by the turbulent displacement) is only a fraction of the total available potential energy (measured by the Thorpe scale). Corrective measures are discussed that can be used to diagnose mixing from knowledge of the Thorpe displacement. In a companion paper, Mater et al. analyze field data and show that the Thorpe scale can indeed be much larger than the Ozmidov scale. [ABSTRACT FROM AUTHOR]

This paper concerns the collation, quality control, and analysis of single-point field measurements from fixed sensors mounted on offshore platforms. In total, the quality-controlled database contains 122 million individual waves, of which 3649 are rogue waves. Geographically, the majority of the field measurements were recorded in the North Sea, with supplementary data from the Gulf of Mexico, the South China Sea, and the North West shelf of Australia. The significant wave height ranged from 0.12 to 15.4 m, the peak period ranged from 1 to 24.7 s, the maximum crest height was 18.5 m, and the maximum recorded wave height was 25.5 m. This paper will describe the offshore installations, instrumentation, and the strict quality control procedure employed to ensure a reliable dataset. An examination of sea state parameters, environmental conditions, and local characteristics is performed to gain an insight into the behavior of rogue waves. Evidence is provided to demonstrate that rogue waves are not governed by sea state parameters. Rather, the results are consistent with rogue waves being merely extraordinary and rare events of the normal population caused by dispersive focusing. [ABSTRACT FROM AUTHOR]

Motivated by the potential of oceanic mesoscale eddies to drive intrinsic low-frequency variability, this paper examines geostrophic turbulence in the frequency-wavenumber domain. Frequency-wavenumber spectra, spectral fluxes, and spectral transfers are computed from an idealized two-layer quasigeostrophic (QG) turbulence model, a realistic high-resolution global ocean general circulation model, and gridded satellite altimeter products. In the idealized QG model, energy in low wavenumbers, arising from nonlinear interactions via the well-known inverse cascade, is associated with energy in low frequencies and vice versa, although not in a simple way. The range of frequencies that are highly energized and engaged in nonlinear transfer is much greater than the range of highly energized and engaged wavenumbers. Low-frequency, low-wavenumber energy is maintained primarily by nonlinearities in the QG model, with forcing and friction playing important but secondary roles. In the high-resolution ocean model, nonlinearities also generally drive kinetic energy to low frequencies as well as to low wavenumbers. Implications for the maintenance of low-frequency oceanic variability are discussed. The cascade of surface kinetic energy to low frequencies that predominates in idealized and realistic models is seen in some regions of the gridded altimeter product, but not in others. Exercises conducted with the general circulation model suggest that the spatial and temporal filtering inherent in the construction of gridded satellite altimeter maps may contribute to the discrepancies between the direction of the frequency cascade in models versus gridded altimeter maps seen in some regions. Of course, another potential reason for the discrepancy is missing physics in the models utilized here. [ABSTRACT FROM AUTHOR]

This paper describes how topographic variations can transform a small-amplitude, linear, coastal-trapped wave (CTW) into a nonlinear wave or an eddy train. The dispersion relation for CTWs depends on the slope of the shelf. Provided the cross-shelf slope varies sufficiently slowly along the shelf, the local structure of the CTW adapts to the local geometry and the wave transformation can be analyzed by the Wentzel-Kramers-Brillouin-Jeffreys (WKBJ) method. Two regions of parameter space are straightforward: adiabatic transmission (where, at the incident wave frequency, a long wave exists everywhere along the shelf) and short-wave reflection (where somewhere on the shelf no long wave exists at the incident frequency, but the stratification is sufficiently weak that a short reflected wave can coexist with the incident wave). This paper gives the solutions for these two cases but concentrates on a third parameter regime, which includes all sufficiently strongly stratified flows, where neither of these behaviors is possible and the WKBJ method fails irrespective of how slowly the topography changes. Fully nonlinear integrations of the equation for the advection of the bottom boundary potential vorticity show that the incident wave in this third parameter regime transforms into a nonlinear wave when topographic variations are gradual or into an eddy train when the changes are abrupt. [ABSTRACT FROM AUTHOR]

Barrier layers are generated when the surface mixed layer is shallower than the layer where temperature is well mixed, in geographical regions where salinity plays a key role in setting up upper-ocean density stratification. In the tropical oceans, thick barrier layers are also found in a latitude range where spiraling trajectories from surface in situ drifters suggest the presence of predominantly cyclonic submesoscale-like vortices. The authors explore these dynamical processes and their interplay in the present paper, focusing on the tropical South Atlantic Ocean and using a high-resolution modeling approach. The objective is threefold: to investigate the mean dynamics contributing to barrier-layer formation in this region, to study the distribution and seasonality of submesoscale features, and to verify whether and how the submesoscale impacts barrier-layer thickness. The model used is the Regional Ocean Modeling System (ROMS) in its Adaptive Grid Refinement in Fortran (AGRIF) online-nested configuration with a horizontal resolution ranging between 9 and 1 km. The simulated circulation is first described in terms of mean and submesoscale dynamics, and the associated seasonal cycle. Mechanisms for barrier-layer formation are then investigated. The results confirm previous hypotheses by Mignot et al. on the relevance of enhanced winter mixing deepening the isothermal layer, whereas the salinity stratification is sustained by advection of surface fresh waters and subsurface salinity maxima. Finally, submesoscale effects on barrier-layer thickness are studied, quantifying their contribution to vertical fluxes of temperature and salinity. Submesoscale vortices associated with salinity fronts are found to have a significant effect, producing thicker barrier layers (by ~20%-35%) and a shallower mixed layer because of their restratifying effect on salinity. [ABSTRACT FROM AUTHOR]

The ITCZ lies at the ascending branch of the tropical meridional overturning circulation, where near-surface meridional mass fluxes vanish. Near the ITCZ, column-integrated energy fluxes vanish, forming an atmospheric energy flux equator (EFE). This paper extends existing approximations relating the ITCZ position and EFE to the atmospheric energy budget by allowing for zonal variations. The resulting relations are tested using reanalysis data for 1979-2014. The zonally varying EFE is found as the latitude where the meridional component of the divergent atmospheric energy transport (AET) vanishes. A Taylor expansion of the AET around the equator relates the ITCZ position to derivatives of the AET. To a first order, the ITCZ position is proportional to the divergent AET across the equator; it is inversely proportional to the local atmospheric net energy input (NEI) that consists of the net energy fluxes at the surface, at the top of the atmosphere, and zonally across longitudes. The first-order approximation captures the seasonal migrations of the ITCZ in the African, Asian, and Atlantic sectors. In the eastern Pacific, a third-order approximation captures the bifurcation from single- to double-ITCZ states that occurs during boreal spring. In contrast to linear EFE theory, during boreal winter in the eastern Pacific, northward cross-equatorial AET goes along with an ITCZ north of the equator. EFE and ITCZ variations driven by ENSO are characterized by an equatorward (poleward) shift in the Pacific during El Niño (La Niña) episodes, which are associated with variations in equatorial ocean energy uptake. [ABSTRACT FROM AUTHOR]

This paper reports a new finding and related mechanism: the forcing effect of the tropical Atlantic (TA) sea surface temperature (SST) on the atmosphere-ocean coupling in the western North Pacific (WNP) and northern Indian Ocean (NIO). Since the early 1980s, the TA SST has increased and, notably, exhibited an enhanced interannual statistical relationship with the WNP subtropical high and NIO SST in boreal summer. Empirical diagnostics reveal the following spatial pattern linking the TA SST and the atmosphere-ocean coupling in the Pacific and Indian Ocean: 1) a cyclonic (anticyclonic) circulation pair straddling the equator over the eastern Pacific, 2) an anticyclonic (cyclonic) circulation pair straddling the equator in the WNP and Indian Ocean, 3) overturning circulation with ascending (descending) and descending (ascending) anomalies over the TA and tropical western Pacific, respectively, and 4) positive (negative) SST anomaly in the TA and NIO. The characteristics of this pattern are consistent with those of a WNP-NIO coupling pattern identified in a previous study. Empirical diagnostics and numerical simulations indicate that the TA SST serves as a forcing to induce low-level divergence and streamfunction anomalies in the Indian Ocean and the western Pacific. The latter in turn induces anomalous heat storage in the NIO and enhances the WNP-NIO coupling system, which is an intrinsic pattern engendered by the atmosphere-ocean interaction in the region. Without the remote influence of the TA SST forcing, the WNP-NIO coupling pattern and its impacts on the summer monsoon and TC variability in South Asia, East Asia, and the WNP would be considerably less significant than observed. [ABSTRACT FROM AUTHOR]

The paper demonstrates that sudden stratospheric warmings (SSWs) can be simulated in an ensemble of dry dynamical cores that miss the typical SSW forcing mechanisms like moist processes, land-sea contrasts, or topography. These idealized general circulation model (GCM) simulations are driven by a simple Held-Suarez-Williamson (HSW) temperature relaxation and low-level Rayleigh friction. In particular, the four dynamical cores of NCAR's Community Atmosphere Model, version 5 (CAM5), are used, which are the semi-Lagrangian (SLD) and Eulerian (EUL) spectral-transform models and the finite-volume (FV) and the spectral element (SE) models. Three research themes are discussed. First, it is shown that SSW events in such idealized simulations have very realistic flow characteristics that are analyzed via the SLD model. A single vortex-split event is highlighted that is driven by wavenumber-1 and -2 wave-mean flow interactions. Second, the SLD simulations are compared to the EUL, FV, and SE dynamical cores, which sheds light on the impact of the numerical schemes on the circulation. Only SLD produces major SSWs, while others only exhibit minor stratospheric warmings. These differences are caused by SLD's more vigorous wave-mean flow interactions in addition to a warm pole bias, which leads to relatively weak polar jets in SLD. Third, it is shown that tropical quasi-biennial oscillation (QBO)-like oscillations and SSWs can coexist in such idealized HSW simulations. They are present in the SLD dynamical core that is used to analyze the QBO-SSW interactions via a transformed Eulerian-mean (TEM) analysis. The TEM results provide support for the Holton-Tan effect. [ABSTRACT FROM AUTHOR]

This paper addresses the validity of the gradient wind balance approximation during the intensification phase of a tropical cyclone in Ooyama's three-layer model. For this purpose, the sensitivity to various model modifications is examined, given by the inclusion of (i) unbalanced dynamics in the free atmosphere, (ii) unbalanced dynamics in the slab boundary layer, (iii) a height-parameterized boundary layer model, and (iv) a rigid lid. The most rapid intensification occurs when the model employs the unbalanced slab boundary layer, while the simulation with the balanced boundary layer reveals the slowest intensification. The simulation with the realistic height-parameterized boundary layer model exhibits an intensification rate that lies in between. Intensification is induced by a convective ring in all experiments, but a distinct contraction of the radius of maximum gradient wind only takes place with unbalanced boundary layer dynamics. In all experiments the rigid lid and the balance approximation for the free atmosphere have no crucial impact on intensification, and a linear stability analysis cannot explain the found sensitivity to intensification. Most likely the nonlinear momentum advection term plays an important role in the boundary layer. It is found on the basis of a diagnostic radial mass flux equation that the source term for latent heat provides the largest contribution to intensification and contraction. Furthermore, it turns out that the position of the convective ring inside or outside of the radius of maximum gradient wind (RMGW) is of vital importance for intensification and most likely explains the large impact of boundary layer imbalance. [ABSTRACT FROM AUTHOR]

A linear response function (LRF) relates the mean response of a nonlinear system to weak external forcings and vice versa. Even for simple models of the general circulation, such as the dry dynamical core, the LRF cannot be calculated from first principles owing to the lack of a complete theory for eddy-mean flow feedbacks. According to the fluctuation-dissipation theorem (FDT), the LRF can be calculated using only the covariance and lag-covariance matrices of the unforced system. However, efforts in calculating the LRFs for GCMs using FDT have produced mixed results, and the reason(s) behind the poor performance of the FDT remain(s) unclear. In Part I of this study, the LRF of an idealized GCM, the dry dynamical core with Held-Suarez physics, is accurately calculated using Green's functions. In this paper (Part II), the LRF of the same model is computed using FDT, which is found to perform poorly for some of the test cases. The accurate LRF of Part I is used with a linear stochastic equation to show that dimension reduction by projecting the data onto the leading EOFs, which is commonly used for FDT, can alone be a significant source of error. Simplified equations and examples of 2 × 2 matrices are then used to demonstrate that this error arises because of the nonnormality of the operator. These results suggest that errors caused by dimension reduction are a major, if not the main, contributor to the poor performance of the LRF calculated using FDT and that further investigations of dimension-reduction strategies with a focus on nonnormality are needed. [ABSTRACT FROM AUTHOR]

The deployment and recovery of autonomous or remotely piloted platforms from research vessels have become a way of significantly extending the capabilities and reach of the research fleet. This paper describes the use of ship-launched and ship-recovered Boeing-Insitu ScanEagle unmanned aerial vehicles (UAVs). The UAVs were instrumented to characterize the marine atmospheric boundary layer (MABL) structure and dynamics, and to measure ocean surface processes during the October 2012 Equatorial Mixing (EquatorMix) experiment in the central Pacific and during the July 2013 Trident Warrior experiment off the Virginia coast. The UAV measurements, including atmospheric momentum and radiative, sensible, and latent heat fluxes, are complemented by measurements from ship-based instrumentation, including a foremast MABL eddy-covariance system, lidar altimeters, and a digitized X-band radar system. During EquatorMix, UAV measurements reveal longitudinal atmospheric roll structures not sampled by ship measurements, which contribute significantly to vertical fluxes of heat and momentum. With the nadir-looking UAV lidar, surface signatures of internal waves are observed, consistent and coherent with measurements from ship-based X-band radar, a Hydrographic Doppler Sonar System, and a theoretical model. In the Trident Warrior experiment, the instrumented UAVs were used to demonstrate real-time data assimilation of meteorological data from UAVs into regional coupled ocean-atmosphere models. The instrumented UAVs have provided unprecedented spatiotemporal resolution in atmospheric and oceanographic measurements in remote ocean locations, demonstrating the capabilities of these platforms to extend the range and capabilities of the research fleet for oceanographic and atmospheric studies. [ABSTRACT FROM AUTHOR]

Because of the drifting nature of the ground track of Jason-1 during its geodetic mission (GM), there are 1200 overlap events where the Jason-1 GM and Jason-2 tracks align perfectly (less than their altimeter footprint radius) over thousands of kilometers. These overlap events sample homogeneously all longitudes and all time differences ( dt) ranging from a few minutes to 10 days or more. When dt is almost zero, the difference is characterized by altimeter noise and its modulation by waves. As dt increases, the rapid ocean variability is revealed. The first statistical analysis of the 1200 events yields variance maps, spectra, autocorrelation, and space-time scales that are consistent with past observations (e.g., the 3-day phase of ERS-1). This paper highlights the value of this Jason-1 GM overlap dataset for more sophisticated studies of the rapid ocean variability. There are two major limitations: 1) the noise level of Jason-class altimeters prevents analyzing scales smaller than 80 km and 2) short time differences also absorb a fraction of the derivative of slower signals. These findings are important if a geodetic mission is considered for Jason-2 in the coming years (e.g., when the satellite starts to exhibit aging problems): a well-chosen geodetic orbit for Jason-2 has the potential to collect a better distribution of overlap events with Jason-3. To that extent, thousands of orbits were screened to find the options that would provide good geodetic and mesoscale sampling and also maximize the overlap sampling of a tentative Jason-2 GM phase. [ABSTRACT FROM AUTHOR]

This paper discusses the benefits of using a hybrid ensemble Kalman filter and four-dimensional variational (4D-Var) data assimilation (DA) system rather than a 4D-Var system employing the National Meteorological Center (NMC, now known as NCEP) method (4D-Var-Bnmc) to predict severe weather events. An adjoint-based 4D-Var system was employed with a background error covariance matrix constructed from the NMC method and perturbations in a local ensemble transform Kalman filter system. The DA systems are based on the Japan Meteorological Agency's nonhydrostatic model. To reduce the sampling noise, three types of implementation (the spatial localization, spectral localization, and neighboring ensemble approaches) were tested. The assimilation of a pseudosingle observation of sea level pressure located at a tropical cyclone (TC) center yielded analysis increments physically consistent with what is expected of a mature TC in the hybrid systems at the beginning of the assimilation window, whereas analogous experiments performed using the 4D-Var-Bnmc system did not. At the end, the structures of the 4D-Var-based increments became similar to one another, while the analysis increment by the 4D-Var-Bnmc system was broad in the horizontal direction. Realistic DA experiments showed that all of the hybrid systems provided initial conditions that yielded more accurate TC track and intensity forecasts than those achievable by the 4D-Var-Bnmc system. The hybrid systems also yielded some statistically significant improvements in forecasting local heavy rainfall events in terms of fraction skill scores when a 160 km × 160 km window size was used. The overall skills of the hybrid systems were relatively independent of the choice of implementation. [ABSTRACT FROM AUTHOR]

Large-scale synoptic circulations have a profound effect on western U.S. summer weather and climate. Heat waves, water availability, the distribution of monsoonal moisture, fire-weather conditions, and other phenomena are impacted by the position and amplitude of large-scale synoptic circulations. Furthermore, regional weather is modulated by the interactions of the large-scale flow with terrain and land-water contrasts. It is therefore crucial to understand projected changes in large-scale circulations and their variability under anthropogenic global warming. Although recent research has examined changes in the jet stream, storm tracks, and synoptic disturbances over the Northern Hemisphere under global warming, most papers have focused on the cold season. In contrast, this work analyzes the projected trends in the spatial distribution and amplitude of large-scale synoptic disturbances over the western United States and eastern Pacific during July and August. It is shown that CMIP5 models project weaker mean midtropospheric gradients in geopotential height as well as attenuated temporal variability in geopotential height, temperature, vorticity, vertical motion, and sea level pressure over this region. Most models suggest reduced frequency of troughs and increased frequency of ridges over the western United States. These changes in the variability of synoptic disturbances have substantial implications for future regional weather and climate. [ABSTRACT FROM AUTHOR]

This paper addresses the effect of interannual variability in jet stream orientation on weather systems over the North Atlantic basin (NAB). The observational analysis relies on 65 yr of NCEP-NCAR reanalysis (1948-2012). The total daily kinetic energy of the geostrophic wind (GTKE) is taken as a measure of storm activity over the North Atlantic. The NAB is partitioned into four rectangular regions, and the winter average of GTKE is calculated for each quadrant. The spatial GTKE average over all four quadrants shows striking year-to-year variability and is strongly correlated with the North Atlantic Oscillation (NAO). The GTKE strength in the northeast quadrant is closely related to the diffluence angle of the jet stream in the northwest quadrant. To gain insight into the relationship between the diffluence angle and its downstream impact, a quasigeostrophic baroclinic model is used. The results show that an initially zonal jet persists at its initial latitude over 30 days or longer, while a tilted jet propagates meridionally according to the Rossby wave group velocity, unless kept stationary by external forcing. A Gulf Stream-like narrow sea surface temperature (SST) front provides the requisite forcing for an analytical steady-state solution to this problem. This SST front influences the atmospheric jet in the northwest quadrant: it both strengthens the jet and tilts it northward at higher levels, while its effect is opposite at lower levels. Reanalysis data confirm these effects, which are consistent with thermal wind balance. The results suggest that the interannual variability found in the GTKE may be caused by intrinsic variability of the thermal Gulf Stream front. [ABSTRACT FROM AUTHOR]

This paper discusses whether and how a baroclinic atmosphere can equilibrate with very small bottom friction in a dry primitive equation general circulation model. The model is forced by a Newtonian relaxation of temperature to a prescribed temperature profile, and it is damped by a linear friction near the lower boundary. When friction is decreased by four orders of magnitude, kinetic energy dissipation by friction gradually becomes negligible, while 'energy recycling' becomes dominant. In this limit kinetic energy is converted back into potential energy at the largest scales, thus closing the energy cycle without significant frictional dissipation. The momentum fluxes are of opposite sign in the upper and lower atmosphere: in the upper atmosphere, eddies converge momentum into the westerly jets; however, in the lower atmosphere, the eddies diverge momentum out of the westerly jets. The secondary circulation driven by the meridional eddy momentum fluxes thus acts to increase the baroclinicity of the westerly jet. This regime may be relevant for the Jovian atmosphere, where the frictional time scale may be much larger than the radiative damping time scale. [ABSTRACT FROM AUTHOR]

This paper reports a diagnosis of the structure and dynamics of upper-level fronts (ULFs) simulated with a high-resolution Weather Research and Forecasting Model with diabatic heating versus one without diabatic heating. The ULFs of both simulations develop in about 6 days as integral parts of intensifying baroclinic waves. Each has a curvilinear structure along the southern edge of a relatively narrow long tongue of high potential vorticity in which stratospheric air is subducted to different tropospheric levels by synoptic-scale subsidence. It resembles a veil in the sky of varying thickness across the midsection upstream of the trough of the baroclinic wave. The 3D frontogenetical function is shown to be a necessary and sufficient metric for quantifying the rate of development of ULFs. Its value is mostly associated with the contribution of the 3D ageostrophic velocity component. Upper-level frontogenesis is attributable to the joint direct influence of the vortex-stretching process and the deformation property of the 3D ageostrophic flow component. The model also generates a spectrum of vertically propagating mesoscale gravity waves. The ULFs simulated with and without diabatic heating processes are qualitatively similar. The ULF is considerably more intense when there is heating. The heating, however, does not make a significant direct contribution to but indirectly does so through its impacts on the subsidence field of the baroclinic wave. [ABSTRACT FROM AUTHOR]

Vertical shear commonly organizes atmospheric convection into coherent multiscale structures. The associated countergradient vertical transport of horizontal momentum by organized convection can enhance the wind shear and transport kinetic energy upscale. However, organized convection and its upscale effects are not represented by traditional mass-flux-based parameterizations. The present paper sets the archetypal dynamical models, originally formulated by the second author, into a parameterization context by utilizing a nonhydrostatic anelastic model with segmentally constant approximation (NAM-SCA). Using a two-dimensional framework as a starting point, NAM-SCA spontaneously generates propagating tropical squall lines in a sheared environment. High numerical efficiency is achieved through a novel compression methodology. The numerically generated archetypes produce vertical profiles of convective momentum transport that are consistent with the analytic archetype. [ABSTRACT FROM AUTHOR]

The intertropical convergence zone (ITCZ) has been the focus of considerable research in recent years, with much of this work concerned with how the latitude of maximum tropical precipitation responds to natural climate variability and to radiative forcing. The width of the ITCZ, however, has received little attention despite its importance for regional climate and for understanding the general circulation of the atmosphere. This paper investigates the ITCZ width in simulations with an idealized general circulation model over a wide range of climates. The ITCZ, defined as the tropical region where there is time-mean ascent, displays rich behavior as the climate varies, widening with warming in cool climates, narrowing in temperate climates, and maintaining a relatively constant width in hot climates. The mass and energy budgets of the Hadley circulation are used to derive expressions for the area of the ITCZ relative to the area of the neighboring descent region, and for the sensitivity of the ITCZ area to changes in climate. The ITCZ width depends primarily on four quantities: the net energy input to the tropical atmosphere, the advection of moist static energy by the Hadley circulation, the transport of moist static energy by transient eddies, and the gross moist stability. Different processes are important for the ITCZ width in different climates, with changes in gross moist stability generally having a weak influence relative to the other processes. The results are likely to be useful for analyzing the ITCZ width in complex climate models and for understanding past and future climate change in the tropics. [ABSTRACT FROM AUTHOR]

Large-scale climatic indices have been used as predictors of precipitation totals and extremes in many studies and are used operationally in weather forecasts to circumvent the difficulty in obtaining robust dynamical simulations of precipitation. The authors show that the sea level pressure North Pacific high (NPH) wintertime anomaly, a component of the Northern Oscillation index (NOI), provides a superior covariate of interannual precipitation variability in Northern California, including seasonal precipitation totals, drought, and extreme precipitation intensity, compared to traditional ENSO indices such as the Southern Oscillation index (SOI), the multivariate ENSO index (MEI), Niño-3.4, and others. Furthermore, the authors show that the NPH anomaly more closely reflects the influence of Pacific basin conditions over California in general, over groups of stations used to characterize statewide precipitation in the Sierra Nevada range, and over the southern San Francisco Bay region (NASA Ames Research Center). This paper uses the term prediction to refer to the estimation of precipitation (the predictand) from a climate covariate (the predictor), such as a climate index, or atmospheric moisture. In this sense, predictor and predictand are simultaneous in time. Statistical models employed show the effectiveness of the NPH winter anomaly as a predictor of total winter precipitation and daily precipitation extremes at the Moffett Field station. NPH projected by global climate models is also used in conjunction with atmospheric humidity [atmospheric specific humidity (HUS) at the 850-hPa level] to obtain projections of mean and extreme precipitation. The authors show that future development of accurate forecasts of NPH anomalies issued several months in advance is important for forecasting total winter precipitation and is expected to directly benefit water resource management in California. Therefore, the authors suggest that investigating the lead-time predictability of NPH anomalies is an important direction for future research. [ABSTRACT FROM AUTHOR]

This paper aims to test the validity, utility, and limitations of the lateral eddy diffusivity concept in a coastal environment through analyzing data from coupled drifter and dye releases within the footprint of a high-resolution (800 m) high-frequency radar south of Martha's Vineyard, Massachusetts. Specifically, this study investigates how well a combination of radar-based velocities and drifter-derived diffusivities can reproduce observed dye spreading over an 8-h time interval. A drifter-based estimate of an anisotropic diffusivity tensor is used to parameterize small-scale motions that are unresolved and underresolved by the radar system. This leads to a significant improvement in the ability of the radar to reproduce the observed dye spreading. [ABSTRACT FROM AUTHOR]

A spectral framework for quantifying the geometric/kinematic and dynamic/energetic properties of breaking ocean waves was proposed by Phillips in 1985. Phillips assumed a constant breaking strength coefficient to link the kinematic/geometric breaking crest properties to the associated excess energy and momentum fluxes from the waves to the upper ocean. However, a scale-dependent (spectral) breaking strength coefficient is needed, but is unavailable from measurements. In this paper, the feasibility of a parametric mean effective breaking strength coefficient valid for a wide range of sea states is investigated. All available ocean breaking wave datasets were analyzed and complemented with wave model behavior. Robust evidence is found supporting a single linear parameter relationship between the effective breaking strength and wave age or significant wave steepness. Envisaged applications for the effective breaking strength are described. [ABSTRACT FROM AUTHOR]

An established analytical technique for modeling internal tide generation by barotropic flow over bottom topography in the ocean is the Green function-based approach. To date, however, for realistic ocean studies this method has relied on the WKB approximation. In this paper, the complete Green function method, without the WKB approximation, is developed and tested, and in the process, the accuracy of the WKB approximation for realistic ridge geometries and ocean stratifications is considered. For isolated Gaussian topography, the complete Green function approach is shown to be accurate via close agreement with the results of numerical simulations for a wide range of height ratios and criticality; in contrast, the WKB approach is found to be inaccurate for small height ratios in the subcritical regime and all tall topography that impinges on the pycnocline. Two ocean systems are studied, the Kaena and Wyville Thomson Ridges, for which there is again excellent agreement between the complete Green function approach and numerical simulations, and the WKB approximate solutions have substantial errors. This study concludes that the complete Green function approach, which is typically only modestly more computationally expensive than the WKB approach, should be the go-to analytical method to model internal tide generation for realistic ocean ridge scenarios. [ABSTRACT FROM AUTHOR]

This paper examines the trajectories followed by intense intrusions of moist air into the Arctic polar region during autumn and winter and their impact on local temperature and sea ice concentration. It is found that the vertical structure of the warming associated with moist intrusions is bottom amplified, corresponding to a transition of local conditions from a 'cold clear' state with a strong inversion to a 'warm opaque' state with a weaker inversion. In the marginal sea ice zone of the Barents Sea, the passage of an intrusion also causes a retreat of the ice margin, which persists for many days after the intrusion has passed. The authors find that there is a positive trend in the number of intrusion events crossing 70°N during December and January that can explain roughly 45% of the surface air temperature and 30% of the sea ice concentration trends observed in the Barents Sea during the past two decades. [ABSTRACT FROM AUTHOR]

This paper studies operators inspired by the fluctuation-dissipation theorem that consider the seasonality (nonstationarity) of the climate system under conditions of limited sample size relevant to application of the method to observational records. The approach is used to predict the steady-state response of an atmospheric general circulation model to localized temperature perturbations. A seasonal operator nominally requires a much larger data sample than a stationary operator; the authors study some strategies to overcome this. First, two methods for approximating the seasonality of the system are examined. Second, an alternative 'transpose approach' to the standard dimension reduction is considered that is more efficient and accurate for small sample sizes and additionally enables the use of a kernel, which provides a convenient way to incorporate prior physical understanding into the operator. All operators show considerable skill in predicting seasonal responses for a variety of variables (temperature, winds, rainfall, and cloud cover) and better skill in predicting the annual-mean ones. A comparison of these predictions to ones done on the same system with temporally fixed boundary conditions shows unexpectedly that skill is, if anything, improved by the presence of a seasonal cycle. The authors suggest that the extra complexity due to a seasonal system is outweighed by the added information due to the seasonal forcing and the effect of seasonality in smoothing out prediction errors. [ABSTRACT FROM AUTHOR]

A global climate model is used to study the effect of flattening the orography of the Antarctic Ice Sheet on climate. A general result is that the Antarctic continent and the atmosphere aloft warm, while there is modest cooling globally. The large local warming over Antarctica leads to increased outgoing longwave radiation, which drives anomalous southward energy transport toward the continent and cooling elsewhere. Atmosphere and ocean both anomalously transport energy southward in the Southern Hemisphere. Near Antarctica, poleward energy and momentum transport by baroclinic eddies strengthens. Anomalous southward cross-equatorial energy transport is associated with a northward shift in the intertropical convergence zone. In the ocean, anomalous southward energy transport arises from a slowdown of the upper cell of the oceanic meridional overturning circulation and a weakening of the horizontal ocean gyres, causing sea ice in the Northern Hemisphere to expand and the Arctic to cool. Comparison with a slab-ocean simulation confirms the importance of ocean dynamics in determining the climate system response to Antarctic orography. This paper concludes by briefly presenting a discussion of the relevance of these results to climates of the past and to future climate scenarios. [ABSTRACT FROM AUTHOR]

High-frequency radar (HFR) surface current data are an increasingly utilized tool for capturing complex dynamics of coastal ocean systems worldwide. The radar is uniquely capable of sampling relevant temporal and spatial scales of nearshore processes that impact event response activities and basic coastal ocean research. HFR is a shore-based remote sensing system and is therefore subject to data gaps, which are predominately due to environmental effects, like increased external noise or low signal due to ocean surface conditions. Many applications of these surface current data require that these gaps be filled, such as Lagrangian numerical models, to estimate material transport and dispersion. This study introduces a new penalized least squares regression method based on a three-dimensional discrete cosine transform method to reconstruct hourly HFR surface current data with a horizontal resolution of 6 km. The method explicitly uses both time and space variability to predict the missing value. Furthermore, the method is fast, robust, and requires relatively low computer memory storage. This paper evaluates the method against two scenarios of common data gaps found in HFR networks currently deployed around the world. The validation is based on observed surface current maps along the mid-Atlantic coast of the United States with specific vectors removed to replicate these common gap scenarios. The evaluation shows that the new method is robust and particularly well suited to fill a more common scenario with complete data coverage surrounding an isolated data gap. It is shown that the real-time application of the method is suitable for filling data gaps in large oceanography datasets with high accuracy. [ABSTRACT FROM AUTHOR]

Clouds are both produced by and interact with the mesoscale and synoptic-scale structure of extratropical cyclones (ETCs) in ways that are still not well understood. Cloud-scale radiative and latent heating modifies the thermal environment, leading to a response in the dynamics that can in turn feed back on cloud distribution and microphysical properties. Key to the structure of ETCs is the warm conveyor belt (WCB); the poleward-ascending airstream that produces the bulk of the clouds and precipitation. This paper examines a long-lived WCB that persisted over the western North Atlantic Ocean in nearly the same location for several days. During this time, the storm was sampled multiple times by NASA's A-Train satellite constellation, and a clear transition from stratiform to convective clouds was observed. Examination of coincident temperature and water vapor data reveals destabilization of the thermodynamic profile after the cyclone reached maturity. CloudSat radar reflectivity from two sequential overpasses of the warm front depicts a change from stratiform to convective cloud structure, and high-frequency microwave data reveal an increase in the amount of ice hydrometeors. The presence of convection may serve to strengthen the warm frontal trough while slowing the movement of the primary low pressure center. The stratiform-convective transition cannot be detected from passive measurements of cloud-top pressure. The results demonstrate the effectiveness of multivariate satellite observations for examining the outcome of dynamic processes in ETCs, and highlight the need for more rapid temporal profiling in future remote sensing observing systems. [ABSTRACT FROM AUTHOR]

The response of the atmosphere to zonally symmetric applied heating and mechanical forcing is considered, allowing for the fact that the response may include a change in the wave force (or 'wave drag'). A scaling argument shows that an applied zonally symmetric heating is effective in driving a steady meridional circulation provided that the wave force (required to satisfy angular momentum constraints) is sufficiently sensitive to changes in the mean flow in the sense that the ratio is large, where K is a measure of the sensitivity of the wave force; α, N, and f are the radiative damping rate, buoyancy frequency, and Coriolis parameter, respectively; and and are the horizontal and vertical length scales of the heating, respectively. Furthermore, in the 'narrow heating' regime where this ratio is large, the structure of the meridional circulation response is only weakly dependent on the details of the wave force. The scaling arguments are verified by experiments in a dry dynamical circulation model. Consistent with the scaling prediction, the regime does not apply when the width of the imposed heating is increased. The narrow-heating regime is demonstrated to be relevant to the double peak in tropical lower-stratospheric upwelling considered in a companion paper, supporting the hypothesis that this feature is radiatively driven. Similar arguments are applied to show that a narrow zonally symmetric applied mechanical forcing is primarily balanced by a change in wave force. This provides an explanation for the recently identified compensation between resolved and parameterized waves in driving modeled trends in the Brewer-Dobson circulation. [ABSTRACT FROM AUTHOR]

Stationary and low-frequency Rossby waves are the primary drivers of extratropical weather variations on monthly and longer time scales. They take the form of persistent highs and lows, which, for example, shape subtropical dry zones and guide extratropical storms. More generally, stationary-eddy circulations, including zonally anomalous tropical overturning circulations, set up large zonal variations in net precipitation (precipitation minus evaporation, P − E). This paper investigates the response of stationary eddies and the zonally asymmetric hydrological cycle to global warming in an idealized GCM, simulating a wide range of climates by varying longwave absorption. The stationary eddies are forced by two idealized zonal asymmetries: a midlatitude Gaussian mountain and an equatorial ocean heat source. Associated with changes in stationary eddies are changes in the zonal variation of the hydrological cycle. Particularly in the subtropics, these simulations show a nearly constant or decreasing amplitude of the zonally anomalous hydrological cycle in climates warmer than modern despite the wet gets wetter, dry gets drier effect associated with increasing atmospheric moisture content. An approximation for zonally anomalous P − E, based on zonal-mean surface specific humidity and stationary-eddy vertical motion, disentangles the roles of thermodynamic and dynamic changes. The approximation shows that changes in the zonally asymmetric hydrological cycle are predominantly controlled by changes in lower-tropospheric vertical motion in stationary eddies. [ABSTRACT FROM AUTHOR]

Direct covariance flux (DCF) measurements taken from floating platforms are contaminated by wave-induced platform motions that need to be removed before computation of the turbulent fluxes. Several correction algorithms have been developed and successfully applied in earlier studies from research vessels and, most recently, by the use of moored buoys. The validation of those correction algorithms has so far been limited to short-duration comparisons against other floating platforms. Although these comparisons show in general a good agreement, there is still a lack of a rigorous validation of the method, required to understand the strengths and weaknesses of the existing motion-correction algorithms. This paper attempts to provide such a validation by a comparison of flux estimates from two DCF systems, one mounted on a moored buoy and one on the Air-Sea Interaction Tower (ASIT) at the Martha's Vineyard Coastal Observatory, Massachusetts. The ASIT was specifically designed to minimize flow distortion over a wide range of wind directions from the open ocean for flux measurements. The flow measurements from the buoy system are corrected for wave-induced platform motions before computation of the turbulent heat and momentum fluxes. Flux estimates and cospectra of the corrected buoy data are found to be in very good agreement with those obtained from the ASIT. The comparison is also used to optimize the filter constants used in the motion-correction algorithm. The quantitative agreement between the buoy data and the ASIT demonstrates that the DCF method is applicable for turbulence measurements from small moving platforms, such as buoys. [ABSTRACT FROM AUTHOR]

Using daily rain gauge records for Madagascar and nearby islands, this paper investigates rainfall intraseasonal variability at local and regional scales during the austral summer season (November-February), as well as the respective influences of recurrent convective regimes over the southwest Indian Ocean (SWIO) and the Madden-Julian oscillation (MJO). The results show a general consistency between local-scale rainfall variability in Madagascar and regional-scale features of climate variability. The influence of tropical temperate troughs in their mature phase and/or their easternmost locations is first underlined. The development of such systems over southern Africa and the Mozambique Channel can be considered as precursors for Malagasy wet spells, especially over the southern part of the island. Regional and local effects of the MJO are weaker on average, and only concern the northwest of the island and the north of the Mozambique Channel. MJO and convective regimes are finally shown to explain distinct fractions of regional rainfall variability. [ABSTRACT FROM AUTHOR]

Understanding salt dynamics is important to adequately model salt intrusion, baroclinic forcing, and sediment transport. In this paper, the importance of the residual salt transport due to tidal advection in well-mixed tidal estuaries is studied. The water motion is resolved in a consistent way with a width-averaged analytical model, coupled to an advection-diffusion equation describing the salt dynamics. The residual salt balance obtained from the coupled model shows that the seaward salt transport driven by river discharge is balanced by the landward salt transport due to tidal advection and horizontal diffusion. It is found that the tidal advection behaves as a diffusion process, and this contribution is named tidal advective diffusion. The horizontal diffusion parameterizes processes not explicitly resolved in the model and is called the prescribed diffusion. The tidal advective diffusion results from the correlation between the tidal velocity and salinity and can be explicitly calculated with the dominant semidiurnal water motion. The sensitivity analysis shows that tidal advective diffusivity increases with increasing bed roughness and decreasing vertical eddy viscosity. Furthermore, tidal advective diffusivity reaches its maximum for moderate water depth and moderate convergence length. The relative importance of tidal advective diffusion is investigated using the residual salt balance, with the prescribed diffusion coefficient obtained from the measured salinity field. The tidal advective diffusion dominates the residual salt transport in the Scheldt estuary, and other processes significantly contribute to the residual salt transport in the Delaware estuary and the Columbia estuary. [ABSTRACT FROM AUTHOR]

This paper compares simple theoretical expressions relating vertical velocity, perturbation pressure, updraft size, and dimensionality for cumulus convection, derived in Part I, with numerical solutions of the anelastic buoyant perturbation pressure Poisson equation and vertical velocity w. A range of thermal buoyancy profiles representing shallow to deep moist convection are tested for both two-dimensional (2D) and three-dimensional (3D) updrafts. The theoretical expressions give similar results for w and perturbation pressure difference from updraft top to base Δ p compared to the numerical solutions over a wide range of updraft radius R. The theoretical expressions are also consistent with 2D and 3D fully dynamical updraft simulations initiated by warm bubbles of varying width. Implications for nonhydrostatic modeling in the 'gray zone,' with a horizontal grid spacing Δ x of O(1-10) km where convection is generally underresolved, are discussed. The theoretical and numerical solutions give a scaling of updraft velocity with R (~Δ x) consistent with fully dynamical 2D and 3D simulations in the gray zone, with a rapid decrease of maximum w at relatively small R and a slower decrease at large R. These results suggest that an incorrect representation of perturbation pressure may be an important contributor to biases in convective strength at these resolutions. The theoretical solutions also provide a concise physical interpretation of the 'virtual mass' coefficient in convection parameterizations and can be easily incorporated into these schemes to provide a consistent scaling of perturbation pressure effects with R, updraft height, and the buoyancy profile. [ABSTRACT FROM AUTHOR]

This paper presents high-resolution numerical solutions of a nonlinear zonally symmetric slab model of the intertropical convergence zone (ITCZ) boundary layer. The boundary layer zonal and meridional flows are forced by a specified pressure field, which can also be interpreted as a specified geostrophically balanced zonal wind field u g( y). One narrow on-equatorial peak in boundary layer pumping is produced when the forcing is easterly geostrophic flow along the equator and two narrow peaks in boundary layer pumping are produced on opposite sides of the equator (a double ITCZ) when the forcing is westerly geostrophic flow along the equator. In the case when easterlies are surrounding a westerly wind burst, once again a double ITCZ is produced, but the ITCZs have significantly more intense boundary layer pumping than the case of only westerly geostrophic flow. A comparison of the numerical solutions to those of classical Ekman theory suggests that the meridional advection term υ(∂ υ/∂ y) plays a vital role in strengthening and narrowing boundary layer pumping regions while weakening and broadening boundary layer suction regions. [ABSTRACT FROM AUTHOR]

This paper demonstrates the impact of the enhancement in detectability by the dual-frequency precipitation radar (DPR) on board the Global Precipitation Measurement (GPM) core observatory. By setting two minimum detectable reflectivities-12 and 18 dB Z-artificially to 6 months of GPM DPR measurements, the precipitation occurrence and volume increase by ~21.1% and ~1.9%, respectively, between 40°S and 40°N. GPM DPR is found to be able to detect light precipitation, which mainly consists of two distinct types. One type is shallow precipitation, which is most significant for convective precipitation over eastern parts of subtropical oceans, where deep convection is typically suppressed. The other type is probably associated with lower parts of anvil clouds associated with organized precipitation systems. While these echoes have lower reflectivities than the official value of the minimum detectable reflectivity, they are found to mostly consist of true precipitation signals, suggesting that the official value may be too conservative for some sort of meteorological analyses. These results are expected to further the understanding of both global energy and water budgets and the diabatic heating distribution. [ABSTRACT FROM AUTHOR]