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The moist-air entropy can be used to analyze and better understand the general circulation of the atmosphere or convective motions. Isentropic analyses are commonly based on studies of different equivalent potential temperatures, all of which are assumed to fully represent the entropy of moist air. It is, however, possible to rely either on statistical physics or the third law of thermodynamics when defining and computing the absolute entropy of moist air and to study the corresponding third-law potential temperature, which is different from the previous ones. The third law assumes that the entropy for the most stable crystalline state of all substances is zero when approaching absolute zero temperature. This paper shows that the way all these moist-air potential temperatures are defined has a large impact on (i) the plotting of the isentropes for a simulation of Hurricane Dumile, (ii) the changes in moist-air entropy computed for a steam cycle defined for this hurricane, (iii) the analyses of isentropic streamfunctions computed for this hurricane, and (iv) the computations of the heat input, the work function, and the efficiency defined for this steam cycle. The moist-air entropy is a state function and the isentropic analyses must be completely determined by the local moist-air conditions. The large differences observed between the different formulations of moist-air entropy are interpreted as proof that the isentropic analyses of moist-air atmospheric motions must be achieved by using the third-law potential temperature defined from general thermodynamics. [ABSTRACT FROM AUTHOR]

In Part I of this two-part paper, a formulation was developed to treat fragmentation in ice-ice collisions. In the present Part II, the formulation is implemented in two microphysically advanced cloud models simulating a convective line observed over the U.S. high plains. One model is 2D with a spectral bin microphysics scheme. The other has a hybrid bin-two-moment bulk microphysics scheme in 3D. The case consists of cumulonimbus cells with cold cloud bases (near 0°C) in a dry troposphere. Only with breakup included in the simulation are aircraft observations of particles with maximum dimensions >0.2 mm in the storm adequately predicted by both models. In fact, breakup in ice-ice collisions is by far the most prolific process of ice initiation in the simulated clouds (95%-98% of all nonhomogeneous ice), apart from homogeneous freezing of droplets. Inclusion of breakup in the cloud-resolving model (CRM) simulations increased, by between about one and two orders of magnitude, the average concentration of ice between about 0° and −30°C. Most of the breakup is due to collisions of snow with graupel/hail. It is broadly consistent with the theoretical result in Part I about an explosive tendency for ice multiplication. Breakup in collisions of snow (crystals >~1 mm and aggregates) with denser graupel/hail was the main pathway for collisional breakup and initiated about 60%-90% of all ice particles not from homogeneous freezing, in the simulations by both models. Breakup is predicted to reduce accumulated surface precipitation in the simulated storm by about 20%-40%. [ABSTRACT FROM AUTHOR]

Arctic climate change in winter is tightly linked to changes in the strength of surface temperature inversions, which occur frequently in the present climate as Arctic air masses form during polar night. Recent work proposed that, in a warmer climate, increasing low-cloud optical thickness of maritime air advected over high-latitude landmasses during polar night could suppress the formation of Arctic air masses, amplifying winter warming over continents and sea ice. But this mechanism was based on single-column simulations that could not assess the role of fractional cloud cover change. This paper presents two-dimensional cloud-resolving model simulations that support the single-column model results: low-cloud optical thickness and duration increase strongly with initial air temperature, slowing the surface cooling rate as the climate is warmed. The cloud-resolving model cools less at the surface than the single-column model, and the sensitivity of its cooling to warmer initial temperatures is also higher, because it produces cloudier atmospheres with stronger lower-tropospheric mixing and distributes cloud-top cooling over a deeper atmospheric layer with larger heat capacity. Resolving larger-scale cloud turbulence has the greatest impact on the microphysics schemes that best represent general observed features of mixed-phase clouds, increasing their sensitivity to climate warming. These findings support the hypothesis that increasing insulation of the high-latitude land surface by low clouds in a warmer world could act as a strong positive feedback in future climate change and suggest studying Arctic air formation in a three-dimensional climate model. [ABSTRACT FROM AUTHOR]

In the third part of the 'Cellular Statistical Models of Broken Cloud Fields' series the cloud statistics formalism developed in the first two parts is interpreted in terms of the theory of Markov processes. The master matrix introduced in this study is a unifying generalization of both the cloud fraction probability distribution function (PDF) and the Markovian transition probability matrix. To illustrate the new concept, the master matrix is used for computation of the moments of the cloud fraction PDF-in particular, the variance-which until now has not been analytically derived in the framework of the authors' previous work. This paper also serves as a bridge to the proposed future studies of the effects of sampling and averaging on satellite-based cloud masks. [ABSTRACT FROM AUTHOR]

Elevated heating of the atmosphere by large plateaus has been argued to influence regional climate in Asia and other regions, but the mechanisms that cause the troposphere to equilibrate at warmer temperatures over elevated terrain are not well understood. This paper quantitatively describes the physics that controls temperatures over elevated terrain in radiative-convective equilibrium (RCE). First, a cloud-system-resolving model (CSRM) is used to simulate RCE states over surfaces with various elevations. Then, a theory for the influence of surface elevation on temperatures in RCE is presented. Together with offline radiative transfer calculations, this theory is used to quantitatively attribute the magnitude of the elevated heating effect to top-of-atmosphere radiative flux changes caused by decreases in longwave absorption, shortwave scattering, and the moist lapse rate that occur as surface pressure drops. Sensitivity functions obtained through these offline calculations suggest that elevated heating is weaker in warmer climates, and additional CSRM simulations support this hypothesis. Under certain circumstances, even the sign of the elevated heating effect can change to produce cooler temperatures at a given pressure level as the surface is lifted in RCE. [ABSTRACT FROM AUTHOR]

Cloud-top generating cells (GCs) are a common feature atop stratiform clouds within the comma head of winter cyclones. The dynamics of cloud-top GCs are investigated using very high-resolution idealized WRF Model simulations to examine the role of shear in modulating the structure and intensity of GCs. Simulations were run for the same combinations of radiative forcing and instability as in Part II of this series, but with six different shear profiles ranging from 0 to 10 m s−1 km−1 within the layer encompassing the GCs. The primary role of shear was to modulate the organization of GCs, which organized as closed convective cells in simulations with radiative forcing and no shear. In simulations with shear and radiative forcing, GCs organized in linear streets parallel to the wind. No GCs developed in the initially stable simulations with no radiative forcing. In the initially unstable and neutral simulations with no radiative forcing or shear, GCs were exceptionally weak, with no clear organization. In moderate-shear (Δ u/Δ z = 2, 4 m s−1 km−1) simulations with no radiative forcing, linear organization of the weak cells was apparent, but this organization was less coherent in simulations with high shear (Δ u/Δ z = 6, 8, 10 m s−1 km−1). The intensity of the updrafts was primarily related to the mode of radiative forcing but was modulated by shear. The more intense GCs in nighttime simulations were either associated with no shear (closed convective cells) or strong shear (linear streets). Updrafts within GCs under conditions with radiative forcing were typically ~1-2 m s−1 with maximum values < 4 m s−1. [ABSTRACT FROM AUTHOR]

A conceptual bulk model for a dry, convective boundary layer with prescribed horizontally homogeneous and heterogeneous low-level radiative cooling rates is developed. For horizontally homogeneous radiative cooling, the response of the system to varying its prescribed parameters is explored and formulated in terms of nondimensional parameters. Large-eddy simulations with prescribed radiative cooling rates match the results of the bulk model well. It is found that, depending on the strength of the surface coupling, the height of the boundary layer (BL) either increases or decreases in response to increasing radiative BL cooling. Another property of the system is that, for increasing surface temperature, the BL temperature decreases if the prescribed radiative BL cooling rates are strong. This counterintuitive behavior is caused by the formulation of the entrainment rate at the inversion. Heterogeneous radiative BL cooling is found to cause a circulation induced by pressure deviations between the area of weak radiative BL cooling and the area of strong radiative BL cooling. Including the feedback of the induced circulation on the BL in a two-column model leads to a modified equilibrium state, in which a weakened horizontal BL flow of about 1 m s−1 is maintained for differences in radiative BL cooling rates larger than 1 K day−1. Such a circulation strength is comparable to a shallow circulation caused by surface temperature differences of a few kelvins. Spatial differences in radiative BL cooling should therefore be considered as a first-order effect for the formation of shallow circulations. [ABSTRACT FROM AUTHOR]

Based on long-term observations by the Atmospheric Radiation Measurement program at its Southern Great Plains site, a new composite case of continental shallow cumulus (ShCu) convection is constructed for large-eddy simulations (LES) and single-column models. The case represents a typical daytime nonprecipitating ShCu whose formation and dissipation are driven by the local atmospheric conditions and land surface forcing and are not influenced by synoptic weather events. The case includes early morning initial profiles of temperature and moisture with a residual layer; diurnally varying sensible and latent heat fluxes, which represent a domain average over different land surface types; simplified large-scale horizontal advective tendencies and subsidence; and horizontal winds with prevailing direction and average speed. Observed composite cloud statistics are provided for model evaluation. The observed diurnal cycle is well reproduced by LES; however, the cloud amount, liquid water path, and shortwave radiative effect are generally underestimated. LES are compared between simulations with an all-or-nothing bulk microphysics and a spectral bin microphysics. The latter shows improved agreement with observations in the total cloud cover and the amount of clouds with depths greater than 300 m. When compared with radar retrievals of in-cloud air motion, LES produce comparable downdraft vertical velocities, but a larger updraft area, velocity, and updraft mass flux. Both observations and LES show a significantly larger in-cloud downdraft fraction and downdraft mass flux than marine ShCu. [ABSTRACT FROM AUTHOR]

A stochastic multicloud model (SMCM) convective parameterization, which mimics the interactions at subgrid scales of multiple cloud types, is incorporated into the National Centers for Environmental Prediction (NCEP) Climate Forecast System, version 2 (CFSv2), model (CFSsmcm) in lieu of the preexisting simplified Arakawa-Schubert (SAS) cumulus scheme. A detailed analysis of the tropical intraseasonal variability (TISV) and convectively coupled equatorial waves (CCEW) in comparison with the original (control) model and with observations is presented here. The last 10 years of a 15-yr-long climate simulation are analyzed. Significant improvements are seen in the simulation of the Madden-Julian oscillation (MJO) and most of the CCEWs as well as the Indian summer monsoon (ISM) intraseasonal oscillation (MISO). These improvements appear in the form of improved morphology and physical features of these waves. This can be regarded as a validation of the central idea behind the SMCM according to which organized tropical convection is based on three cloud types, namely, the congestus, deep, and stratiform cloud decks, that interact with each other and form a building block for multiscale convective systems. An adequate accounting of the dynamical interactions of this cloud hierarchy thus constitutes an important requirement for cumulus parameterizations to succeed in representing atmospheric tropical variability. SAS fails to fulfill this requirement, which is evident in the unrealistic physical structures of the major intraseasonal modes simulated by CFSv2 as documented here. [ABSTRACT FROM AUTHOR]

The transient response of the tropical cyclone boundary layer is studied using linearized and nonlinear models, with particular focus on the frictionally forced vertical motion. The impulsively started, linearized tropical cyclone boundary layer is shown to adjust to its equilibrium solution via a series of decaying oscillations with the inertial period . In the nonlinear case, the oscillation period is slightly lengthened by inward advection of the slower-evolving flow from larger radii, but the oscillations decay more quickly. In an idealized cyclone with small sinusoidal oscillations superimposed on the gradient wind, the equilibrium nonlinear boundary layer acts as a low-pass filter with pass length scaling as , where is the 10-m frictional inflow. This filter is absent from the linearized boundary layer. The eyewall frictional updraft is similarly displaced inward of the radius of maximum winds (RMW) by a distance that scales with , owing to nonlinear overshoot of the inflowing air as it crosses the relatively sharp increase in I near the eyewall. This displacement is smaller (other things being equal) when the RMW is small, and greater when it is large, including in secondary eyewalls. The dependence of this distance on may explain, at least partially, why observed RMW are seldom less than 20 km, why storms with relatively peaked radial profiles of wind speed can intensify more rapidly, and why some secondary eyewalls initially contract rapidly with little intensification, then contract more slowly while intensifying. [ABSTRACT FROM AUTHOR]

Precipitating versions of the quasigeostrophic (QG) equations are derived systematically, starting from the equations of a cloud-resolving model. The presence of phase changes of water from vapor to liquid and vice versa leads to important differences from the dry QG case. The precipitating QG (PQG) equations, in their simplest form, have two variables to describe the full system: a potential vorticity (PV) variable and a variable M including moisture effects. A PV-and- M inversion allows the determination of all other variables, and it involves an elliptic partial differential equation (PDE) that is nonlinear because of phase changes between saturated and unsaturated regions. An example PV-and- M inversion is provided for an idealized cold-core cyclone with two vertical levels. A key point illustrated by this example is that the phase interface location is unknown a priori from PV and M, and it is discovered as part of the inversion process. Several choices of a moist PV variable are discussed, including subtleties that arise because of phase changes. Boussinesq and anelastic versions of the PQG equations are described, as well as moderate and asymptotically large rainfall speeds. An energy conservation principle suggests that the model has firm physical and mathematical underpinnings. Finally, an asymptotic analysis provides a systematic derivation of the PQG equations, which arise as the limiting dynamics of a moist atmosphere with phase changes, in the limit of rapid rotation and strong stratification in terms of both potential temperature and equivalent potential temperature. [ABSTRACT FROM AUTHOR]

A modified version of the Sawyer-Eliassen equation is applied to determine the impact of periodic diurnal heating on a balanced vortex. The TC diurnal cycle is a coherent signal that arises in the cirrus canopy. However, despite thorough documentation in the literature, the dynamical mechanism remains unknown. Recent work demonstrates that periodic radiative heating in the TC outflow layer is linked with an anomalous upper-level circulation; this heating is also associated with a cycle of latent heating in the lower troposphere that corresponds to a cycle in storm intensity. Using a method that is analogous to the Sawyer-Eliassen equation, but for solutions having the same time scale as time-periodic forcing, these distributions are analyzed to determine the effect of periodic diurnal heating on an axisymmetric vortex. Results for periodic heating in the lower troposphere show an overturning circulation that resembles the Sawyer-Eliassen solution. The model simulates positive perturbations in the azimuthal wind field of 2.5 m s−1 near the radius of maximum wind. Periodic heating near the top of the vortex produces a local overturning response in the region of heating and an inertia-buoyancy wave response in the storm environment. Comparison of the results from the modified Sawyer-Eliassen equation to those of an idealized axisymmetric solution for both heating distributions shows similarities in the structure of the perturbation wind fields, suggesting that the axisymmetric TC diurnal cycle is primarily a balanced response driven by periodic heating. [ABSTRACT FROM AUTHOR]

The two-dimensional (2D) size distribution of clouds in the horizontal plane plays a central role in the calculation of cloud cover, cloud radiative forcing, convective entrainment rates, and the likelihood of precipitation. Here, a simple method is proposed for calculating the area-weighted mean cloud size and for approximating the 2D size distribution from the 1D cloud-chord lengths measured by aircraft and vertically pointing lidar and radar. This simple method (which is exact for square clouds) compares favorably against the inverse Abel transform (which is exact for circular clouds) in the context of theoretical size distributions. Both methods also perform well when used to predict the size distribution of real clouds from a Landsat scene. When applied to a large number of Landsat scenes, the simple method is able to accurately estimate the mean cloud size. As a demonstration, the methods are applied to aircraft measurements of shallow cumuli during the Routine ARM Aerial Facility (AAF) Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) campaign, which then allow for an estimate of the true area-weighted mean cloud size. [ABSTRACT FROM AUTHOR]

Large-scale tracer transport in the upper troposphere and lower stratosphere (UTLS) is investigated using simulations of the Whole Atmosphere Community Climate Model (WACCM) over the period 1955-2099. The analyses are based on e90, an artificial passive tracer with constant emissions and atmospheric loss rates. The separate contributions of advection by the residual circulation, eddy mixing, and subgrid convection to total transport are explicitly evaluated. The results highlight distinct large-scale transport regimes in the tropics, characterized by efficient vertical tracer transport, and the extratropics, dominated by isentropic mixing. One novel result is the important role of vertical eddy mixing in the tropical upper troposphere. It is shown that interannual variability in e90 is largely driven by El Niño-Southern Oscillation and the quasi-biennial oscillation. The long-term trends emphasize a strong impact of a rising tropopause with climate change on UTLS dynamics and tracer transport. The analyses directly attribute the e90 trends to changes in the different transport components. Stronger residual circulation in the future leads to increased tracer concentrations in the tropical lower stratosphere. Enhanced eddy mixing increases e90 in the extratropical lowermost stratosphere, linked to an upward shift of wave dissipation tied to the tropopause rise. In the troposphere, reduced concentrations in the future are due to weaker convective transport out of the boundary layer and weaker extratropical isentropic eddy mixing. [ABSTRACT FROM AUTHOR]

The forcing of northerly low-level jets over the eastward-sloped terrain of the U.S. Great Plains was studied using a one-dimensional (1D) nonstationary analytical model based on the Boussinesq-approximated equations of motion and thermal energy. For northerly low-level jets, the forcing from diurnal changes in surface heating of the sloped terrain (Holton mechanism) is out of phase with the nocturnal inertial oscillation resulting from the cessation of turbulent mixing at sunset (Blackadar mechanism), which results in weaker northerly nocturnal low-level jets when compared to southerly nocturnal low-levels jets with the same-magnitude background pressure gradient forcing. Because of the Blackadar and Holton mechanisms acting out of phase, nocturnal northerly low-level jets cannot solely explain the northerly low-level jet maximum over the Great Plains found in climatological studies. It is shown that negative buoyancy values over the eastward-sloped terrain enhance the low-level northerly geostrophic wind, which can cause low-level jetlike wind profiles that do not necessarily depend on the diurnal cycle. However, nocturnal northerly low-level jets primarily caused by an inertial oscillation still occur when daytime mixing is strong and buoyancy is small at sunset. These conditions are possible when strong capping inversions are present in the daytime convective boundary layer. The occurrence of both types of northerly low-level jets, those caused by negative buoyancy values over the sloped terrain and those driven by an inertial oscillation, better explains the findings of previous low-level jet climatologies. [ABSTRACT FROM AUTHOR]

The relationship between energy transport and kinetic energy generation in a hurricane is analyzed. The hydrological cycle has a negative impact on the generation of kinetic energy. First, in a precipitating atmosphere, mechanical work must also be expended in order to lift water. Second, the injection of water vapor at low relative humidity and its removal through condensation and precipitation reduces the ability of a thermodynamic cycle to generate work. This reduction can be directly quantified in terms of the change in the Gibbs free energy between the water added and removed. A newly developed approach-namely, the mean airflow as Lagrangian dynamics approximation- is used to extract thermodynamic cycles from the standard output of a numerical simulation of a hurricane. While convection in the outer rainbands is inefficient at producing kinetic energy, the deepest overturning circulation associated with the rising air within the eyewall is an efficient heat engine that produces about 70% as much kinetic energy as a comparable Carnot cycle. This confirms that thermodynamic processes play a central role in hurricane formation and intensification and that the thermodynamic cycles in a hurricane are characterized by high generation of kinetic energy that differ significantly from those found in atmospheric convection. [ABSTRACT FROM AUTHOR]

In this study, the authors numerically investigate the response of an axisymmetric tropical cyclone (TC) vortex to the vertical fluxes of momentum, heat, and moisture induced by roll vortices (rolls) in the boundary layer. To represent the vertical fluxes induced by rolls, a two-dimensional high-resolution Single-Grid Roll-Resolving Model (SRM) is embedded at multiple horizontal grid points in the mesoscale COAMPS for Tropical Cyclones (COAMPS-TC) model domain. Idealized experiments are conducted with the SRM embedded within 3 times the radius of maximum wind of an axisymmetric TC. The results indicate that the rolls induce changes in the boundary layer wind distribution and cause a moderate (approximately 15%) increase in the TC intensification rate by increasing the boundary layer convergence in the eyewall region and induce more active eyewall convection. The numerical experiments also suggest that the roll-induced tangential momentum flux is most important in contributing to the TC intensification process, and the rolls generated at different radii (within the range considered in this study) all have positive contributions. The results are not qualitatively impacted by the initial TC vortex or the setup of the vertical diffusivity in COAMPS-TC. [ABSTRACT FROM AUTHOR]

The Whole Atmosphere Community Climate Model, version 4 (WACCM4), is used to investigate the influence of stratospheric conditions on the development of sudden stratospheric warmings (SSWs). To this end, targeted experiments are performed on selected modeled SSW events. Specifically, the model is reinitialized three weeks before a given SSW, relaxing the surface fluxes, winds, and temperature below 10 km to the corresponding fields from the free-running simulation. Hence, the tropospheric wave evolution is unaltered across the targeted experiments, but the stratosphere itself can evolve freely. The stratospheric zonal-mean state is then altered 21 days prior to the selected SSWs and rerun with an ensemble of different initial conditions. It is found that a given tropospheric evolution concomitant with the development of an SSW does not uniquely determine the occurrence of an event and that the stratospheric conditions are relevant to the subsequent evolution of the stratospheric flow toward an SSW, even for a fixed tropospheric evolution. It is also shown that interpreting the meridional heat flux at 100 hPa as a proxy of the tropospheric injection of wave activity into the stratosphere should be regarded with caution and that stratospheric dynamics critically influence the heat flux at that altitude. [ABSTRACT FROM AUTHOR]

Convection-permitting numerical experiments using the Weather Research and Forecasting (WRF) Model are performed to explore the influence of monsoonal onshore wind speed and moisture content on the intensity and diurnal variations of coastal rainfall over south China during the mei-yu seasons. The focus of the analyses is on a pair of 10-day WRF simulations with diurnally cyclic-in-time lateral boundary conditions averaged over the high versus low onshore wind speed days of the 2007-09 mei-yu seasons. Despite differences in the rainfall intensity, the spatial distributions and diurnal variations of rainfall in both simulations verified qualitatively well against the mean estimates derived from ground-based radar observations, averaged respectively over either the high-wind or low-wind days. Sensitivity experiments show that the pattern of coastal rainfall spatial distribution is mostly controlled by the ambient onshore wind speed. During the high-wind days, strong coastal rainfall is concentrated along the coastline and reaches its maximum in the early morning. The coastal lifting induced by the differential surface friction and small hills is the primary cause for the strong coastal rainfall, while land breeze enhances coastal lifting and precipitation from evening to early morning. In the low-wind days, on the other hand, coastal rainfall is mainly induced by the land-sea-breeze fronts, which has apparent diurnal propagation perpendicular to the coastline. With stronger land-sea temperature contrast, the land-sea breeze is stronger during the low-wind days. Both in the high-wind and low-wind days, the coastal rainfall intensity is sensitive to the incoming moisture in the upstream oceanic airflow, especially to the moisture content in the boundary layer. [ABSTRACT FROM AUTHOR]

In situ data collected by three research aircraft in four geographical locations are analyzed to determine the relationship between cloud-base temperature, drop size distribution, and the development of supercooled water drops and ice in strong updraft cores of convective clouds. Data were collected in towering cumulus and feeder cells in the Caribbean, over the Gulf of Mexico, over land near the Gulf Coast, over land in the southeastern United States, and the high plains in Colorado and Wyoming. Convective clouds in the Caribbean, over the Gulf of Mexico and its coast, and over the southeastern United States all develop millimeter-diameter supercooled drops in updraft cores. Clouds over the high plains do not generate supercooled large drops, and rarely are drops >70 μm observed in updraft cores. Commensurate with the production of supercooled large drops, ice is generated and rapidly glaciates updraft cores through a hypothesized secondary ice process that is based on laboratory observations of large drops freezing and emitting tiny ice particles. Clouds over the high plains do not experience the secondary ice process and significant concentrations of supercooled liquid in the form of small drops are carried much higher (up to −35.5°C) in the updraft cores. An empirical relationship that estimates the maximum level to which supercooled liquid water will be transported, based on cloud-base drop size distribution and temperature, is developed. Implications have applications for modeling the transport of water vapor and particles into the upper troposphere and hygroscopic seeding of cumulus clouds. [ABSTRACT FROM AUTHOR]

To investigate the effectiveness of emission reductions on the concentrations of gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM) at a rural site in Maryland (MD08), long-term (2005-14) measurements of speciated atmospheric mercury were analyzed using concentration-weighted trajectory (CWT) analysis. CWT results suggested that the number of major source regions contributing to GEM, GOM, and reactive mercury (RM = GOM + PBM) over the eastern United States and southeastern Canada declined over time. Across much of these regions, source contributions in 2011-14 decreased by up to 20% for GEM, by greater than 60% for GOM, and by 20%-60% for PBM compared to 2006-08, largely because of the decreases in power-plant mercury emissions since 2009. Changes in the spatial distribution of the source regions were also observed over time. Increases in source contributions of GEM after 2011 over the northeastern United States and southeastern Canada were predominantly from emission increases in metal and steel production and forest fires. Source contribution increases in PBM were more widespread, which can be attributed potentially to mercury transformation processes in the air or wood combustion rather than industrial sources. [ABSTRACT FROM AUTHOR]

With the Weather Research and Forecasting (WRF) Model specifically configured to simulate the intensification and evolution of an extratropical baroclinic wave, this study first investigates why cold fronts are characteristically longer, narrower, and more intense than warm fronts in the extratropical atmosphere. It is found that the differential thermal advection by the geostrophic and ageostrophic wind components in the two frontal regions results in a greater thermal contrast across the cold front. The length of the cold front is essentially the length scale of the intensifying baroclinic wave (i.e., on the order of radius of deformation). The frontal system as a whole moves eastward under the influence of a steering flow. In addition, the cold front outpaces the warm front eastward, making the western portion of the warm front progressively occluded and the eastern portion of the warm front shorter. The dynamical processes tend to move the cold front eastward, whereas the diabatic heating processes tend to move it westward, contributing to the narrowness of the cold front. This study also investigates whether, how, and why an upper-level front (ULF) would synergistically interact with a surface front (SF). It is found that a favorable circumstance for such interaction to occur in an observed extratropical cyclone and in the WRF Model simulation is when the ULF and SF are roughly parallel to one another with the ULF aloft located a few hundred kilometers to the west of the SF. The relative importance of 'forcing' for the ageostrophic circulation associated with the geostrophic circulation, diabatic heating, and friction are diagnosed in such interaction. [ABSTRACT FROM AUTHOR]

The regional response of the global ocean to low-frequency changes in atmospheric pressure loading, ηib, is analyzed as it occurs in the Max Planck Institute for Meteorology (MPI-M) coupled ocean–atmosphere climate model in response to increased atmospheric CO2 concentrations. Results suggest that long-term changes in ηib can lead to increases in high-latitude sea level by up to 5 and 10 cm, respectively, after doubling and quadrupling the atmospheric CO2 content. At low latitudes, sea level will decrease simultaneously between 2 and 4 cm through the combined effects of changes in the atmospheric circulation and through the increase of its moisture content. In subpolar regions, associated rates of sea level increase are of the order of 0.4–0.6 mm yr-1 for quadrupled atmospheric CO2 concentrations, while in mid- and low latitudes, sea level will decrease at a rate of 0.2 mm yr-1. Differences between doubling and quadrupling CO2 concentrations indicate regionally dependent nonlinearities in the changing climate system. The analysis suggests that in some regions (including the coasts of northern Europe) low-frequency ηib changes could be as large as 10%–20% of a global sea level increase anticipated over the next 100 yr. While not being a dominant effect, amplitudes of long-period ηib changes are large enough to be included in future estimates of climate-related regional sea level change. Increasing the vertically integrated atmospheric CO2 content by 4 and 12 kg m-2 (on global average), in response to doubling and quadrupling atmospheric CO2 concentrations, suggests associated reductions of global sea level by 0.6 and 1.7 cm, respectively. The differences between two different model solutions are significant, especially in the Southern Ocean, where they show significantly different atmospheric mass and pressure distributions, and at low latitudes, where differences resemble the contribution of increased moisture content added to the inverted-barometer (IB) effect in the MPI-M solution. [ABSTRACT FROM AUTHOR]

A high-resolution numerical model with idealized topography is used to investigate the degree to which a coastal upwelling jet separates from the shelf as it flows around a submarine bank depending on the wind strength and the horizontal scale of the bank. Experiments were run using several wind forcing magnitudes and submarine banks with different geometries, so as to explore a wide range of the flow strength as measured by the Rossby number (Ro) and the ratio of the squares of the internal Rossby radius of deformation and curvature of the topography as denoted by the Burger number (Bu). The intensity of the jet separation is strongly dependent on both parameters, with maximum separation with increasing Ro and Bu close to 1, when large amounts of upwelled water are exported toward deeper waters. For small Bu, separation is minimal and independent of Ro. For high Ro, the degree of separation decreases at large Bu since the bank acts only as a small perturbation to the flow. Term balances in the along-shelf momentum equation reveal that the primary balance over the bank is between the nonlinear and the ageostrophic terms. In an asymmetric bank, the radius of curvature in the upstream half of the bank dominates in terms of determining the offshore deflection of a water particle at the surface. The asymmetry increases the cross-isobath transport but not the offshore deflection of the jet. [ABSTRACT FROM AUTHOR]

Recent studies of regime behavior in the extratropical variability have been based on a nonlinear extension to principal component analysis. Multimodality has been identified in the nonlinear principal component, and the multimodality has been interpreted as evidence for the existence of multiple circulation regimes. Here, multimodality is shown to be abundant in nonlinear principal component analysis when applied to sufficiently isotropic data even if these data are inherently unimodal. It is recommended that the nonlinear principal component analysis should not be used for detection of multimodality and regime behavior. [ABSTRACT FROM AUTHOR]

A formula is derived for the rate of change of circulation around an updraft perimeter at a constant elevation. This quantity depends on the continuous propagation of points on the edge, so an expression for local propagation of the edge is obtained from Petterssen's formula for the motion of an isopleth and the vertical equation of motion. On the edge of an updraft in inviscid anelastic flow, the local propagation velocity along the outward normal is equal to the local nonhydrostatic vertical pressure-gradient force (NHVPGF) divided by the magnitude of the local vertical-velocity gradient. Circulation around an updraft perimeter increases at a rate equal to the line integral around the edge of vertical vorticity times the outward propagation velocity. Formulas are also found for the propagation of an updraft's centroid at a given height and for the acceleration of an updraft's vertical helicity. All of the formulas are tested on exact Beltrami-flow solutions of the governing equations. The relevance of two paradigms of supercell dynamics to local edge propagation and circulation growth of updrafts is evaluated by decomposing the NHVPGF into linearly and nonlinearly forced parts and examining results of supercell simulations in different types of shear. Propagation across the shear and rate of increase of circulation depend mostly on the nonlinear part of the NHVPGF (as in the vertical-wind-shear paradigm) for updrafts in nearly unidirectional shear and on the linear part (as in the helicity paradigm) for updrafts in shear that turns markedly with height. [ABSTRACT FROM AUTHOR]

ENSO teleconnections were originally regarded as a single train of stationary Rossby waves generated by a compact region of enhanced (reduced for La Niña) equatorial convective heating. While more recent studies have greatly enhanced this dynamical picture, the dominant conceptual model of the teleconnections still identifies this monopolar convective heat source as the ultimate driver of the teleconnections. This note presents evidence that the surrounding regions of diabatic cooling are just as important as equatorial heating in producing the ENSO teleconnections. In simulations with a linear diagnostic model, heating and cooling anomalies derived from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis make comparable contributions to the upper-level eddy height anomalies. In particular, remote cooling is just as important as local heating in determining the central longitude of the subtropical El Niño anticyclones. The same diagnosis is applied to the ENSO response of an atmospheric general circulation model (AGCM) forced by observed sea surface temperatures in an integration performed by the NASA Seasonal-to-Interannual Prediction Project (NSIPP). Despite differences in the climatological basic state and diabatic heating, positive and negative heating anomalies play the same complimentary roles for the simulated ENSO response as they do for the observed ENSO pattern. [ABSTRACT FROM AUTHOR]

The climate during the Maunder Minimum is compared with current conditions in GCM simulations that include a full stratosphere and parameterized ozone response to solar spectral irradiance variability and trace gas changes. The Goddard Institute for Space Studies (GISS) Global Climate/Middle Atmosphere Model (GCMAM) coupled to a q-flux/mixed-layer model is used for the simulations, which begin in 1500 and extend to the present. Experiments were made to investigate the effect of total versus spectrally varying solar irradiance changes; spectrally varying solar irradiance changes on the stratospheric ozone/climate response with both preindustrial and present trace gases; and the impact on climate and stratospheric ozone of the preindustrial trace gases and aerosols by themselves. The results showed that 1) the Maunder Minimum cooling relative to today was primarily associated with reduced anthropogenic radiative forcing, although the solar reduction added 40% to the overall cooling. There is no obvious distinguishing surface climate pattern between the two forcings. 2) The global and tropical response was greater than 1°C, in a model with a sensitivity of 1.2°C (W m[sup -2] )[sup -1] . To reproduce recent low-end estimates would require a sensitivity one-fourth as large. 3) The global surface temperature change was similar when using the total and spectral irradiance prescriptions, although the tropical response was somewhat greater with the former, and the stratospheric response greater with the latter. 4) Most experiments produce a relative negative phase of the North Atlantic Oscillation/Arctic Oscillation (NAO/AO) during the Maunder Minimum, with both solar and anthropogenic forcing equally capable, associated with the tropical cooling and relative poleward Eliassen–Palm (EP) flux refraction. 5) A full stratosphere appeared to be necessary for the negative AO/NAO phase, as was the case with this model for global warming experiments, unless the cooling was very large, while the ozone response played a minor role and did not influence surface temperature significantly. 6) Stratospheric ozone was most affected by the difference between present-day and preindustrial atmospheric composition and chemistry, with increases in the upper and lower stratosphere during the Maunder Minimum. While the estimated UV reduction led to ozone decreases, this was generally less important than the anthropogenic effect except in the upper midstratosphere, as judged by two different ozone photochemistry schemes. 7) The effect of the reduced solar irradiance on stratospheric ozone and on climate was similar in Maunder Minimum and current atmospheric conditions. [ABSTRACT FROM AUTHOR]

The wintertime atmospheric circulation responses to observed patterns of North Atlantic sea surface temperature and sea ice cover trends in recent decades are studied by means of experiments with an atmospheric general circulation model. Here the relationship between the forced responses and the dominant pattern of internally generated atmospheric variability is focused on. The total response is partioned into a portion that projects onto the leading mode of internal variability (the indirect response) and a portion that is the residual from that projection (the direct response). This empirical decomposition yields physically meaningful patterns whose distinctive horizontal and vertical structures imply different governing mechanisms. The indirect response, which dominates the total geopotential height response, is hemispheric in scale with resemblance to the North Atlantic Oscillation or Northern Hemisphere annular mode, and equivalent barotropic in the vertical from the surface to the tropopause. In contrast, the direct response is localized to the vicinity of the surface thermal anomaly (SST or sea ice) and exhibits a baroclinic structure in the vertical, with a surface trough and upper-level ridge in the case of a positive heating anomaly, consistent with theoretical models of the linear baroclinic response to extratropical thermal forcing. Both components of the response scale linearly with respect to the amplitude of the forcing but nonlinearly with respect to the polarity of the forcing. The deeper vertical penetration of anomalous heating compared to cooling is suggested to play a role in the nonlinearity of the response to SST forcing. [ABSTRACT FROM AUTHOR]

This article presents a reply to comments on an article about sudden stratospheric warmings (SSW). The critics focus on the tropospheric precursor signals in the composite map analysis of SSW. Second, they call attention to previous work on the potential role of Eurasian snow cover in forcing or helping to sustain annular mode variations in the troposphere and stratosphere. The authors of the article show 1000-, 250-, and 50-hPa composite geopotential height anomaly charts for the Northern Hemisphere during the onset, growth, mature, decline, and decay phases of SSW.