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This is the second part of a two-part paper whose main objective is to improve the characterization of atmospheric motion vectors (AMVs) and their errors to guide developments in the use of AMVs in numerical weather prediction (NWP). AMVs tend to exhibit considerable systematic and random errors. These errors can arise in the AMV derivation or the interpretation of AMVs as single-level point estimates of wind. An important difficulty in the study of AMV errors is the scarcity of collocated observations of clouds and wind. The study uses instead a simulation framework: geostationary imagery for Meteorological Satellite-8 ( Meteosat-8) is generated from a high-resolution simulation with the Weather Research and Forecasting regional model, and AMVs are derived from sequences of these simulated images. The NWP model provides the 'truth' with a sophisticated description of the atmosphere. This second part focuses on alternative interpretations of AMVs. The key results are 1) that interpreting the AMVs as vertical and horizontal averages of wind can give some benefits over the traditional single-level interpretation (improvements in RMSVD of 5% for high-level AMVs and 20% for low-level AMVs) and 2) that there is evidence that AMVs are more representative of either a wind average over the model cloud layer or wind at a representative level within the cloud layer than of wind at the model cloud top or cloud base. [ABSTRACT FROM AUTHOR]

This paper describes a dynamic framework for an atmospheric general circulation spectral model in which a reference stratified atmospheric temperature and a reference surface pressure are introduced into the governing equations so as to improve the calculation of the pressure gradient force and gradients of surface pressure and temperature. The vertical profile of the reference atmospheric temperature approximately corresponds to that of the U.S. midlatitude standard atmosphere within the troposphere and stratosphere, and the reference surface pressure is a function of surface terrain geopotential and is close to the observed mean surface pressure. Prognostic variables for the temperature and surface pressure are replaced by their perturbations from the prescribed references. The numerical algorithms of the explicit time difference scheme for vorticity and the semi-implicit time difference scheme for divergence, perturbation temperature, and perturbation surface pressure equation are given in detail. The modified numerical framework is implemented in the Community Atmosphere Model version 3 (CAM3) developed at the National Center for Atmospheric Research (NCAR) to test its validation and impact on simulated climate. Both the original and the modified models are run with the same spectral resolution (T42), the same physical parameterizations, and the same boundary conditions corresponding to the observed monthly mean sea surface temperature and sea ice concentration from 1971 to 2000. This permits one to evaluate the performance of the new dynamic framework compared to the commonly used one. Results show that there is a general improvement for the simulated climate at regional and global scales, especially for temperature and wind. [ABSTRACT FROM AUTHOR]

Katabatic winds on ice sheets and glaciers are buoyancy-driven flows, much like turbidity currents in the ocean. These winds are driven by radiative cooling of the ice surface and are not resolved by the typical horizontal and vertical discretization of general circulation models; therefore, a parameterization of their magnitude is desirable. In this paper, it is shown that the simplest such parameterization, based on the classical Prandtl model of slope winds, is physically inadmissible, and an improved model, which removes this irregularity, is presented. It is also shown that the improved model allows favorable comparison with both observations and regional numerical models. [ABSTRACT FROM AUTHOR]

Improved understanding of transport issues and source–receptor relationships on the interregional scale is dependent on reducing the uncertainties in the ability to define complex three-dimensional wind fields evolving in time. The numerical models used for this purpose have been upgraded substantially in recent years by introducing finer grid resolution, better representation of subgrid-scale physics, and practical four-dimensional data assimilation (FDDA) techniques that reduce the accumulation of errors over time. The impact of these improvements for interregional transport is investigated in this paper using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) and the Second-Order Closure Integrated Puff (SCIPUFF) dispersion model to simulate the 1983 Cross-Appalachian Tracer Experiment (CAPTEX-83) episode 1 of 18–19 September 1983. Combining MM5 and SCIPUFF makes it possible to verify predicted tracer concentrations against observed surface concentrations collected during the CAPTEX-83 study. Conclusions from this study are as follows. 1) Not surprisingly, a baseline model configuration reflecting typical capabilities of the late 1980s (70-km horizontal grid, 15 vertical layers, older subgrid physics, and no FDDA) produced large meteorological errors that severely degraded the accuracy of the surface tracer concentrations predicted by SCIPUFF. 2) Improving the horizontal and vertical resolution of the MM5 to 12 km (typical for current operational model) and 32 layers led to some improvements in the statistical skill, but the further addition of more advanced physics produced much greater reductions of simulation errors. 3) The use of FDDA, along with 12-km resolution and improved physics, produced the overall best performance. 4) Further reduction of the horizontal grid size to 4 km had a detrimental effect on meteorological and plume-dispersion solutions in this case because of misrepresentation of convection associated with a cold front by the MM5's explicit moist physics. [ABSTRACT FROM AUTHOR]

In this paper, a new method of dual-Doppler radar wind analysis based on a three-dimensional variational data assimilation (3DVAR) approach is proposed. In it, a cost function, including background term and radial observation term, is minimized through a limited memory, quasi-Newton conjugate-gradient algorithm with the mass continuity equation imposed as a weak constraint. In the method, the background error covariance matrix, though simple in this case, is modeled by a recursive filter. Furthermore, the square root of this matrix is used to precondition the minimization problem. The current method is applied to Doppler radar observation of a supercell storm, and the analysis results are compared to a conceptual model and previous research. It is shown that the horizontal circulations, both within and around the storms, as well as the strong updraft and the associated downdraft, are well analyzed. Because no explicit integration of the anelastic mass continuity equation is involved, error accumulation associated with such integration is avoided. As a result, the method is less sensitive to the vertical boundary uncertainties. [ABSTRACT FROM AUTHOR]

The relaxed eddy accumulation (REA) method has become a standard tool for scalar flux measurements near the surface during field experiments. With this technique, turbulent fluxes of relatively slowly reacting species are measured using the difference in the mean concentration between air samples accumulated during updrafts and downdrafts. A prototype of a relaxed eddy accumulator adapted to air-sea flux measurements aboard a research vessel was developed at the Centre National de Recherches Meteorologiques (CNRM) and was implemented on board the research vessel L'Atalante during the Programme Océan Multidisciplinaire Méso-Echelle (POMME) experiment (March-April 2001). When this technique is used on a mobile platform, a major difficulty is the real-time calculation of the true vertical wind corrected for platform motions. The different stages of this computation are described in this paper. In addition, a ground-based version of the system was tested over a maize field during the Expérience sur Sites pour Contraindre les Modèles de Pollution Atmosphérique et de Transport d'Emission (ESCOMPTE) experiment. With this simplified prototype in which the motion corrections were not needed, the effect of filtering the vertical velocity signal was evaluated and the accuracy of the gas analysis system was estimated. Water vapor, carbon dioxide (CO2), and nitrogen oxides flux over the maize field was measured by this technique. The validation of the system was carried out through comparisons between fluxes obtained by other methods (eddy covariance for CO2 and latent heat flux during ESCOMPTE, and bulk method for latent heat flux during POMME). Good correlations were observed between latent heat and CO2 fluxes measured by eddy covariance and REA techniques for the ESCOMPTE dataset. During the POMME experiment, the correlation between REA and the bulk method is reasonably good (r = 0.83) for latent heat flux. However, since the bulk method is not a direct method, this correlation is slightly lower than the comparisons during the ESCOMPTE experiment. Eventually, the scatter is of the same order of magnitude as that observed between various air-sea flux techniques (for instance, between eddy-covariance and bulk air-sea fluxes). This result leads the authors to consider that their system can be used aboard ships in the future to measure fluxes of trace species at the ocean-atmosphere interface, provided that the analyzer used is accurate enough to detect and measure the concentration difference between the two reservoirs. [ABSTRACT FROM AUTHOR]

Using the NCEP–NCAR reanalysis, the 40-yr ECMWF Re-Analysis (ERA-40), and precipitation data from the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) and the Australian Bureau of Meteorology, the variability and circulation features influencing southwest Western Australia (SWWA) winter rainfall are investigated. It is found that the climate of southwest Australia bears a strong seasonality in the annual cycle and exhibits a monsoon-like atmospheric circulation, which is called the southwest Australian circulation (SWAC) because of its several distinct features characterizing a monsoonal circulation: the seasonal reversal of winds, alternate wet and dry seasons, and an evident land–sea thermal contrast. The seasonal march of the SWAC in extended winter (May–October) is demonstrated by pentad data. An index based on the dynamics’ normalized seasonality was introduced to describe the behavior and variation of the winter SWAC. It is found that the winter rainfall over SWWA has a significant positive correlation with the SWAC index in both early (May–July) and late (August–October) winter. In weaker winter SWAC years, there is an anticyclonic anomaly over the southern Indian Ocean resulting in weaker westerlies and northerlies, which are not favorable for more rainfall over SWWA, and the opposite combination is true in the stronger winter SWAC years. The SWAC explains not only a large portion of the interannual variability of SWWA rainfall in both early and late winter but also the long-term drying trend over SWWA in early winter. The well-coupled SWAC–SWWA rainfall relationship seems to be largely independent of the well-known effects of large-scale atmospheric circulations such as the southern annular mode (SAM), El Niño–Southern Oscillation (ENSO), Indian Ocean dipole (IOD), and ENSO Modoki (EM). The result offers qualified support for the argument that the monsoon-like circulation may contribute to the rainfall decline in early winter over SWWA. The external forcing of the SWAC is also explored in this study. [ABSTRACT FROM AUTHOR]

Hourly observational records and 6-hourly reanalysis data were used to investigate the influences of large-scale forcings on the diurnal variation of summer rainfall along the Yangtze River (YR). The results show that long-duration (more than six hours) rainfall events dominate the summer rainfall along the YR. These events tend to start during the night and to peak after several hours of development. The eastward-delayed initiation of the nocturnal long-duration rainfall events is thought to be due to the diurnal clockwise rotation of the low-tropospheric circulation, especially the accelerated nocturnal southwesterlies. In the early evening, the anomalous easterly flow toward the Tibetan Plateau (TP) causes low-level convergence over the Plateau’s eastern slope that induces the formation of rainfall in the upper YR valley. The anomalous wind sequentially rotates clockwise to a southerly flow at midnight and accelerates the meridional wind in the middle valley, resulting in the initiation of rainfall between 2300 and 0300 LST. In the early morning, the accelerated southwesterlies in southern China, when combined with decelerated winds in the north of the YR, causes a strong convergence along the YR and contributes to the early morning rainfall in the lower valley. Furthermore, the development of the convection systems is suppressed in the afternoon by the mid- and low-level warm advection downstream from the TP. This helps explain why long-duration events do not typically start in the afternoon in the upper YR valley. [ABSTRACT FROM AUTHOR]

The authors present results from laboratory experiments and numerical simulations of the barotropic circulation in a basin with sloping boundaries forced by a surface stress. Focus is placed on flows with large-scale Rossby numbers that are significantly smaller than unity. The results of the laboratory experiments and simulations show that cyclonic circulation follows the isobaths, the flow pattern being independent of the strength of the forcing. For anticyclonic circulation, the flow pattern changes with forcing strength. It is similar to the cyclonic topographically steered pattern for weak forcing, and it develops strong cross-slope flows for strong forcing. Linear dynamics are symmetric between cyclonic and anticyclonic circulations and give a good description of the cyclonic and weakly forced anticyclonic circulations. The analysis of the nonlinear dynamics shows that topographically steered cyclonic flows are all stable and steady energy-minimum solutions to the inviscid nonlinear equations. This implies that the nonlinear terms (advection of relative vorticity) are always small for the topographically steered cyclonic flow. For anticyclonic flow, the situation is very different. It is possible that no anticyclonic topographically steered flow is ever a solution to the steady inviscid equations. And if such a steady anticyclonic flow does exist, it is likely to be unstable, since it must correspond to a saddle point in energy rather than to a minimum or a maximum. The nonlinear terms are important when the Rossby number is larger than the Ekman number, which is the case for the anticyclonic experiments with strongest forcing. For these experiments, the advection of relative vorticity prevents the flow from following topography, creating locations with strong relative vorticity and cross-slope flow. The development of cross-slope flow can be understood from the conservation of potential vorticity in basins with irregular topography. The separation of anticyclonic flow from steep topography shown in the laboratory experiments and the theoretical analysis herein are in agreement with features like the Gulf Stream separation from the continental slope at Cape Hatteras, North Carolina. [ABSTRACT FROM AUTHOR]

The potential to predict cotton yields up to one month before planting in the southeastern United States is assessed in this research. To do this, regional atmospheric variables that are related to historic summer rainfall and cotton yields were identified. The use of simulations of those variables from a global circulation model (GCM) for estimating cotton yields was evaluated. The authors analyzed detrended cotton yields (1970–2004) from 48 counties in Alabama and Georgia, monthly rainfall from 53 weather stations, monthly reanalysis data of 850- and 200-hPa winds and surface temperatures over the southeast U.S. region, and monthly predictions of the same variables from the ECHAM 4.5 GCM. Using the reanalysis climate data, it was found that meridional wind fields and surface temperatures around the Southeast were significantly correlated with county cotton yields (explaining up to 52% of the interannual variability of observed yields), and with rainfall over most of the region, especially during April and July. The tendency for cotton yields to be lower during years with atmospheric circulation patterns that favor higher humidity and rainfall is consistent with increased incidence of disease in cotton during flowering and harvest periods under wet conditions. Cross-validated yield estimations based on ECHAM retrospective simulations of wind and temperature fields forced by observed SSTs showed significant predictability skill (up to 55% and 60% hit skill scores based on terciles and averages, respectively). It is concluded that there is potential to predict cotton yields in the Southeast by using variables that are forecast by numerical climate models. [ABSTRACT FROM AUTHOR]

A cluster analysis of wind measurements from two meteorological stations in the Lower Fraser Valley, British Columbia, Canada, has been performed to identify mesoscale circulation regimes that are common to days on which ozone mixing ratios at one or more measuring stations in the region’s fixed monitoring network exceed the National Ambient Air Quality Objective of 82 ppb. The analysis, using 20 yr of data (1984–2003), identifies the following four regimes: two with morning winds at the coastal Vancouver International Airport (YVR) meteorological station from the northwest direction and two with southerly YVR morning winds. Not all exceedance days are associated with sea-breeze circulations, but days with southerly morning winds have a higher proportion of well-developed sea-breeze circulations. Composite synoptic patterns associated with each regime all show high pressure over the eastern Pacific Ocean with a thermal trough over Washington State and southwestern British Columbia. Composite ozone patterns, corresponding to each mesoscale circulation regime and taken at the hour of maximum ozone concentration, show similar general features, including strong ozone titration in and around the urban source regions and higher values downwind. This suggests that precursor buildup, prior to the exceedance day, plays an important role in the spatial ozone pattern on exceedance days. A simple multiple linear regression of the plume centroid with the number of days elapsed from the start of the analysis period suggests the centroid of the ozone plume has shifted eastward since 1990. There also appears to be a north–south shift in the ozone plume. It is impossible to tell if these shifts are due to changes in emission levels or to changes in spatial emission patterns, because both changes have occurred over the study period. [ABSTRACT FROM AUTHOR]

The idealized 2D model developed in Part I of this study is used to study the West African monsoon sensitivity to large-scale forcing. Using ECWMF reanalyses, a large-scale forcing is introduced in the 2D model in terms of temperature and humidity advection. A coherent structure of cooling–moistening near the surface and drying–warming in the 2–4-km layer is found in the Saharan heat low region. The effect of the advective forcing is to block the monsoon propagation by strengthening the northerly flux and by an increase of convective inhibition. The heat low thus appears to play a key role in the monsoon northward penetration through its temperature and humidity budget. Ultimately, warmer low levels and/or more moist midlevels in the heat low favor a more northerly position of the ITCZ. A detailed view of the continental diurnal cycle is also presented. Potential temperature and humidity budgets are performed in the deep convective and heat low area. The moistening process to sustain deep convection is made through nocturnal advection at low levels and daytime turbulence that redistributes humidity vertically. The same mechanism occurs in the heat low except that the vertical transfers by turbulence help maintain the dryness of the low levels. A possible mechanism of interaction between the deep convective zone and the Saharan heat low is also proposed that involves gravity waves in the upper troposphere. [ABSTRACT FROM AUTHOR]

The article discusses a study which utilized an existing conventional primitive-equation atmospheric general circulation models (AGCMs) and an ocean general circulation models (OGCMs), wherein subgrid-scale convection and mixed-layer turbulence are parameterized. The particular AGCM used for the simulation is called the AGCM for the Earth Simulator. The original code was adopted from an earlier version of a Japanese community AGCM that had been jointly developed by the Center for Climate System Research, the University of Tokyo and the Japanese National Institute for Environmental Studies.

The effects of convective momentum transport (CMT) on global climate simulations are examined in this study. Comparison between two sets of 20-yr (1979–98) integration using the NCAR Community Climate Model version 3 (CCM3) illustrates that the inclusion of CMT in the convection scheme systematically modifies the climate mean state over the equatorial region. The convective momentum tendencies slow down the equatorward flow at higher latitudes near the surface and weaken the equatorial convergence and convection. This reduces the convective heating and drying around the equator and produces an improved meridional distribution within the upward branch of the Hadley circulation. The major heating peak during the boreal winter is moved to south of the equator at about 10°S, which is closer to the heat budget residuals of the ECMWF reanalysis data. The responses of meridional wind to the reduced heating result in the secondary meridional circulation within the intertropical convergence zone. [ABSTRACT FROM AUTHOR]

Previous studies suggest that the enhanced meridional extent of some South American low-level jet events (known as Chaco jets) is a consequence of a positive feedback between the low-level wind and strong convection that is usually observed at their exit region. To assess how this interaction takes place, a Chaco low-level jet event observed between 18 and 19 December 2002 (i.e., during the South America Low-Level Jet Experiment) and the associated mesoscale convective system that evolved at its exit region have been selected to perform numerical experiments where diabatic heating effects associated with phase changes can be quantified. This case study has also been used to analyze the diurnal oscillations related to planetary boundary layer (PBL) mechanisms in order to describe whether the observed evolution of the low-level wind can be explained either by PBL-related forcing or by the interaction with convection. The sensitivity experiments confirm that there is a positive feedback at low levels between convection and the northerly wind flow that becomes accelerated and also aids in the identification of a strong coupling between organized convection and the upper-level circulation, resulting in an increase of the upper-level jet strength downstream of the simulated precipitation area. A conceptual model of how these systems (i.e., convection, low- and upper-level jets) mutually interact is proposed, which differs from coupling mechanisms documented for the Great Plains low-level jet. [ABSTRACT FROM AUTHOR]

The Pearl River Delta (PRD) region, located in the southern part of Guangdong Province in China, is one of the most rapidly developing regions in the world. The evolution of local and regional sea-breeze circulation (SBC) is believed to be responsible for forming meteorological conditions for high air-pollution episodes in the PRD. To understand better the impacts of urbanization and its associated urban heat island (UHI) on the local- and regional-scale atmospheric circulations over PRD, a number of high-resolution numerical experiments, with different approaches to treat the land surface and urban processes, have been conducted using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). The results show that an accurate urban land-use dataset and a proper urban land-use parameterization are critical for the mesoscale model to capture the major features of the observed UHI effect and land–sea-breeze circulations in the PRD. Stronger UHI in the PRD increases the differential temperature gradient between urbanized areas and nearby ocean surface and hence enhances the mesoscale SBC. The SBC front consequently penetrates farther inland to overcome the prevailing easterly flow in the western part of inland Hong Kong. Additional sensitivity studies indicate that further industrial development and urbanization will strengthen the daytime SBC as well as increase the air temperature in the lowest 2 km of the atmosphere. [ABSTRACT FROM AUTHOR]

The Community Climate System Model version 3 (CCSM3) has a dipolelike pattern with a cold bias in the northern Tropics and a warm bias in the southeastern Tropics, which is reminiscent of the observed pattern of climate variability in boreal spring. Along the equator, in contrast, in boreal spring CCSM3 exhibits striking westerly winds with easterly winds in the upper troposphere, in turn reminiscent of the observed pattern of climate variability in boreal summer. The westerly winds cause a deepening of the eastern thermocline that keeps the east warm despite enhanced coastal upwelling. Thus, the bias in the seasonal cycle of the coupled model appears to project at least partially onto the spatial patterns of natural climate variability in this sector. Information about the origin of the bias in CCSM3 is deduced from a comparison of CCSM3 with a simulation using specified historical SST to force the Community Atmospheric Model version 3 (CAM3). The patterns of bias in CAM3 resemble those apparent in CCSM3, including the appearance of substantially intensified subtropical bands of sea level pressure (SLP), indicating that the problem may be traced to difficulties in the atmospheric component model. Positive SLP bias also appears in the western tropical region, which may be related to deficient Amazonian precipitation. The positive SLP bias seems to be the cause of the anomalous westerly trade winds in boreal spring, and those in turn appear to be responsible for the anomalous deepening of the thermocline in the southeastern Tropics. [ABSTRACT FROM AUTHOR]

Dynamical mechanisms of atmospheric regime behavior are investigated in the context of a quasigeostrophic three-level T21 model of the wintertime atmospheric circulation over the Northern Hemisphere. The model, driven by realistic orography and using a thermal forcing determined by a newly developed tuning procedure, is shown to possess a reasonable climatology and to simulate the Arctic Oscillation quite realistically. It exhibits pronounced internally generated interannual and decadal variability and, in particular, circulation regimes that agree fairly well with observed ones. Two known hypotheses about the origin of regime behavior, as it occurs in the model herein are addressed: (i) multiple equilibria and (ii) chaotic itinerancy between attractor ruins. The first hypothesis is falsified at very high probability, while the second is likely to be true. [ABSTRACT FROM AUTHOR]

Spatial structure of annular modes shows a remarkable resemblance to that of the recent trend in the observed circulation (Thompson et al.). This study performs a series of multilevel primitive equation model simulations to examine the extent to which the annular mode is capable of predicting changes in the zonal-mean flow response to external heat perturbations. Each of these simulations represents a statistically steady state and differs from each other in the values of the imposed tropical heating (&Hscript;) and high-latitude cooling (&Cscript;). Defining the annular mode as the first empirical orthogonal function (EOF1) of zonal-mean tropospheric zonal wind, it is found that the “climate predictability” is generally high in the small &Cscript;–large &Hscript; region of the parameter space, but is markedly low in the large &Cscript;–small &Hscript; region. In the former region, EOF1 represents meridional meandering of the midlatitude jet, while in the latter region, EOF1 and EOF2 combine to represent coherent poleward propagation of zonal-mean flow anomalies. It is also found that the climate predictability tends to be higher with respect to changes in &Cscript; than to changes in &Hscript;. The implications of these findings for the Southern Hemisphere climate predictability are also presented. [ABSTRACT FROM AUTHOR]

Five independent multicentury reconstructions of the July Canadian Drought Code and one reconstruction of the mean July–August temperature were developed using a network of 120 well-replicated tree-ring chronologies covering the area of the eastern Boreal Plains to the eastern Boreal Shield of Canada. The reconstructions were performed using 54 time-varying reconstruction submodels that explained up to 50% of the regional drought variance during the period of 1919–84. Spatial correlation fields on the six reconstructions revealed that the meridional component of the climate system from central to eastern Canada increased since the mid–nineteenth century. The most obvious change was observed in the decadal scale of variability. Using 500-hPa geopotential height and wind composites, this zonal to meridional transition was interpreted as a response to an amplification of long waves flowing over the eastern North Pacific into boreal Canada, from approximately 1851 to 1940. Composites with NOAA Extended Reconstructed SSTs indicated a coupling between the meridional component and tropical and North Pacific SST for a period covering at least the past 150 yr, supporting previous findings of a summertime global ocean–atmosphere–land surface coupling. This change in the global atmospheric circulation could be a key element toward understanding the observed temporal changes in the Canadian boreal forest fire regimes over the past 150 yr. [ABSTRACT FROM AUTHOR]

Detailed observations of boundary layer structure were acquired on 14 September 2001, prior to and during the landfall of Tropical Storm Gabrielle. The Mobile Integrated Profiling System (MIPS) and the Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) were collocated at the western Florida coastline near Venice, very close to the wind center at landfall. Prior to landfall, the boundary layer was rendered weakly stable by a long period of evaporational cooling and mesoscale downdrafts within extensive stratiform precipitation that started 18 h before landfall. The cool air mass was expansive, with an area within the 23°C surface isotherm of about 50 000 km2. East-northeasterly surface flow transported this cool air off the west coast of Florida, toward the convergent warm core of the Gabrielle, and promoted the development of shallow warm and cold fronts that were prominent during the landfall phase. Airflow properties of the boundary layer around the coastal zone are examined using the MIPS and SMART-R data. Wind profiles exhibited considerable temporal variability throughout the period of observations. The stable offshore flow within stratiform precipitation exhibited a modest jet that descended from about 600 to 300 m within the 20-km zone centered on the coastline. In contrast, the onshore flow on the western side of the wind center produced a more turbulent boundary layer that exhibited a well-defined top varying between 400 and 1000 m MSL. The horizontal variability of each boundary layer is examined using high-resolution Doppler radar scans at locations up to 15 km on either side of the coastline, along the mean flow direction of the boundary layer. These analyses reveal that transitions in boundary layer structure for both the stable and unstable regimes were most substantial within 5 km of the coastline. [ABSTRACT FROM AUTHOR]

A high wind event (14–15 February 2001) over the Hawaiian Islands associated with a cold front is simulated using the National Centers for Environmental Prediction (NCEP) Mesoscale Spectral Model (MSM) coupled with an advanced land surface model (LSM). During this period, a strong high pressure cell moved to the northeast of the Hawaiian Islands following the passage of the cold front. The cell then merged with the semipermanent subtropical high and resulted in windy conditions across the state of Hawaii. Analyses of soundings from Lihue on Kauai and Hilo on the Big Island reveal a mean-state critical level below 400 hPa, a strong cross-barrier flow (∼13 m s-1), and the presence of a trade wind inversion. The MSM–LSM predicts downslope windstorms on the lee sides of mountains or ridges with tops beneath the trade wind inversion and within ocean channels between islands. In the case of high mountains with a peak height above the trade wind inversion, weak winds are simulated on the lee side. Around the corners of the islands and in gaps between mountains, gap winds and downslope windstorms are both important for the development of localized leeside windstorms. The localized windstorms over the Hawaiian Islands develop as a result of interactions between large-scale airflow and the complex local topography. Since the terrain is not adequately resolved by the 10-km RSM–LSM, it is no surprise that these windstorms are better simulated by the high-resolution nonhydrostatic MSM–LSM than the 10-km RSM–LSM. [ABSTRACT FROM AUTHOR]

Comments on the article "A Theory for Strong Long-Lived Squall Lines," by R. Rotunno, J.B. Klemp and M.L. Weisman, published previously in the "Journal of Atmospheric Sciences." Proposal of an optimal state of squall lines made by Weisman and colleagues, in which the relative strength of the circulation associated with the storm-generated cold pool and the circulation associated with the environmental shear are balanced; Opinion that ambient shear is an important factor in the dynamics of squall lines; Assessment of the two-dimensional vorticity-streamfunction model that shows that the maximum vertical displacement of low-level air parcels occurs when only low-level shear is present in the environmental wind profile.

The response of the zonal-mean zonal winds in the tropical upper troposphere to thermal forcing in the Tropics is studied using an idealized general circulation model with 18 vertical levels and simplified atmospheric physics. The model produces a conventional general circulation, with deep easterly flow over the equator, when integrated using zonally invariant and hemispherically symmetric boundary conditions, but persistent equatorial superrotation (westerly zonal-mean flow over the equator) is obtained when steady longitudinal variations in diabatic heating are imposed at low latitudes. The superrotation is driven by horizontal eddy momentum fluxes associated with the stationary planetary wave response to the applied tropical heating. The strength of the equatorial westerlies is ultimately limited by vertical steady eddy momentum fluxes, which are downward in the tropical upper troposphere, and by the zonally averaged circulation in the meridional plane, which erodes the mean westerly shear via vertical advection. The transition to superrotation can be prevented by specifying offsetting zonally invariant heating and cooling anomalies on either side of the equator to create a “solstitial” basic state with a single dominant Hadley cell straddling the equator. Superrotation is restricted in the solstitial climate because the strength of the mean meridional overturning is enhanced, which increases the efficiency of vertical advection, and because the cross-equatorial flow in the upper troposphere provides an easterly zonal acceleration that offsets some of the momentum flux convergence associated with tropical eddy heating. The cross-equatorial flow aloft also reduces the stationary planetary wave response in the summer hemisphere. These results suggest that hemispheric asymmetry in the mean meridional circulation is responsible for maintaining the observed mean easterly flow in the tropical upper troposphere against the westerly torques associated with tropical wave sources. [ABSTRACT FROM AUTHOR]

This study identifies the common large-scale environments associated with the development of derecho- producing convective systems (DCSs) from a large number of events. Patterns are identified using statistical clustering of the 500-mb geopotential heights as guidance. The majority of the events (72%) fall into three main patterns that include a well-defined upstream trough (40%), a ridge (20%), and a zonal, low-amplitude flow (12%), which is identified as an additional warm-season pattern. Consequently, the environmental large-scale patterns idealized in past studies only depict a portion of the full spectrum of the possibilities associated with the development of DCSs. In addition, statistics of derecho proximity-sounding parameters are presented relative to the derecho life cycle as well as relative to the forcing for upward motion. It is found that the environments ahead of maturing derechos tend to moisten at low levels while remaining relatively dry aloft. In addition, derechos tend to decay as they move into environments with less instability and smaller deep-layer shear. Low-level shear (instability) is found to be significantly higher (lower) for the more strongly forced events, while the low-level storm-relative inflow tends to be much deeper for the more weakly forced events. Furthermore, discrepancies are found in both low- level and deep-tropospheric shear parameters between observations and the shear profiles considered favorable for strong, long-lived convective systems in idealized simulations. This study highlights the need to examine DCS simulations within more realistic environments to help reconcile these disparities in observations and idealized models and to provide improved information to forecasters. [ABSTRACT FROM AUTHOR]

Basic concepts of buoyancy are reviewed and considered first in light of simple parcel theory and then in a more complete form. It is shown that parcel theory is generally developed in terms of the density (temperature) difference between an ascending parcel and an “environment” surrounding that parcel. That is, buoyancy is often understood as a relative quantity that apparently depends on the choice of a base-state environmental profile. However, parcel theory is most appropriately understood as a probe of the static stability of a sounding to finite vertical displacements of hypothetical parcels within the sounding rather than as a useful model of deep convection. The thermal buoyancy force, as measured by the temperature difference between a parcel and the base state, and vertical perturbation pressure gradient force together must remain independent of the base state. The vertical perturbation pressure gradient force can be decomposed to include a term due to thermal buoyancy and another due to the properties of motion in the flow. Some thought experiments are presented to illustrate the ambiguous relevance of the base state. It is concluded that buoyancy is not a relative quantity in that it cannot be dependent on the choice of an essentially arbitrary reference state. Buoyancy is the static part of an unbalanced vertical pressure gradient force and, as such, is determined locally, not relative to some arbitrary base state outside of a parcel. This has direct application to the diagnosis of buoyancy from numerical simulations—done properly, such a diagnosis must include not only the thermal buoyancy term but also the perturbation pressure gradient force due to buoyancy. [ABSTRACT FROM AUTHOR]

Wind-forced across-shelf flow on a continental shelf is often described as a two-dimensional balance between flow within a surface boundary layer generated by alongshelf winds and a compensating subsurface flow. Typically, the effects of across-shelf winds are ignored because transport in the surface Ekman layer parallel to winds is assumed zero. Recent studies, however, have shown that the traditional view of across-shelf transport breaks down in shallow waters where friction is important. In this study, a series of two-dimensional simulations is used to show that across-shelf winds can result in across-shelf velocities in the upper few meters of the water column that are similar in magnitude to those generated by alongshelf winds. This circulation has important consequences for the transport of surface-dwelling material, such as blue crab larvae off of Delaware and Chesapeake Bays. The effects of both alongshelf and across-shelf winds on simulated surface dwelling tracer patches are examined. These studies show that across-shelf winds can account for a large amount of surface on- and offshore transport within the stratified outer shelf and, in fact, can be the primary mechanism for across-shelf transport within the friction-dominated inner shelf. Simple estimates of the parameters that describe this transport agree very well with output from the simulations. [ABSTRACT FROM AUTHOR]

The article presents a discussion of the shortcomings of nonlinear principal component analysis (NLPCA) in detecting atmospheric circulation regimes. A study on NLPCA model in year 2005 suggested that only a limited class of approximation shapes are possible and the parameterization of the associated time series is arbitrary up to a homeomorphism. More information on the subject matter has been discussed.