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ABSTRACT The eastern boundary region off Angola encompasses a highly productive ecosystem important for the food security of the coastal population. The fish-stock distribution, however, undergoes large variability on intraseasonal, [...]

3) EASTERN NORTH PACIFIC AND CENTRAL NORTH PACIFIC BASINS--M. C. Kruk, C. J. Schreck, and T. Evans (i) Seasonal activity The eastern North Pacific (ENP) basin is officially split into [...]

e. Atmospheric circulation I) MEAN SEA LEVEL PRESSURE AND RELATED MODES OF VARIABILITY--R. Allan and C. K. Folland Throughout 2014, Southern Oscillation index (SOI) thresholds for El Nino were often [...]

Author(s): Daniel E. Horton (corresponding author) [1, 2]; Nathaniel C. Johnson [3, 4, 5]; Deepti Singh [1]; Daniel L. Swain [1]; Bala Rajaratnam [1, 2, 6]; Noah S. Diffenbaugh [1, [...]

Rapid Arctic warming and sea-ice reduction in the Arctic Ocean are widely attributed to anthropogenic climate change (1-3). The Arctic warming exceeds the global average warming because of feedbacks that include sea-ice reduction (4,5) and other dynamical and radiative feedbacks (6-13). We find that the most prominent annual mean surface and tropospheric warming in the Arctic since 1979 has occurred in northeastern Canada and Greenland. In this region, much of the year-to-year temperature variability is associated with the leading mode of large-scale circulation variability in the North Atlantic, namely, the North Atlantic Oscillation (14,15). Here we show that the recent warming in this region is strongly associated with a negative trend in the North Atlantic Oscillation, which is a response to anomalous Rossby wave-train activity originating in the tropical Pacific. Atmospheric model experiments forced by prescribed tropical sea surface temperatures simulate the observed circulation changes and associated tropospheric and surface warming over northeastern Canada and Greenland. Experiments from the Coupled Model Intercomparison Project Phase 5 (ref. 16) models with prescribed anthropogenic forcing show no similar circulation changes related to the North Atlantic Oscillation or associated tropospheric warming. This suggests that a substantial portion of recent warming in the northeastern Canada and Greenland sector of the Arctic arises from unforced natural variability., The recent Fifth Assessment Report of the Intergovernmental Panel on Climate Change (1) anticipates the continuing retreat of sea ice and warming accompanying future anthropogenic emissions of greenhouse gases and [...]

This scientific assessment examines changes in three climate extremes--extratropical storms, winds, and waves--with an emphasis on U.S. coastal regions during the cold season. There is moderate evidence of an increase in both extratropical storm frequency and intensity during the cold season in the Northern Hemisphere since 1950, with suggestive evidence of geographic shifts resulting in slight upward trends in offshore/coastal regions. There is also suggestive evidence of an increase in extreme winds ' (at least annually) over parts of the ocean since the early to mid-1980s, but the evidence over the U.S. land surface is inconclusive. Finally, there is moderate evidence of an increase in extreme waves in winter along the Pacific coast since the 1950s, but along other U.S. shorelines any tendencies are of modest magnitude compared with historical variability. The data for extratropical cyclones are considered to be of relatively high quality for trend detection, whereas the data for extreme winds and waves are judged to be of intermediate quality. In terms of physical causes leading to multidecadal changes, the level of understanding for both extratropical storms and extreme winds is considered to be relatively low, while that for extreme waves is judged to be intermediate. Since the ability to measure these changes with some confidence is relatively recent, understanding is expected to improve in the future for a variety of reasons, including increased periods of record and the development of 'climate reanalysis' projects., This scientific assessment examines changes in climate extremes associated with extratropical storms, winds, and waves, with an emphasis on U.S. coastal regions during the cold season. Weather and climate extremes [...]

(Adopted by the AVIS Council on 19 September 2013) The American Meteorological Society recommends that appropriate institutions expand drought planning, preparedness, research, coordination, early warning, and communication efforts at the [...]

The Madden-Julian oscillation exerts broad influences on global weather and climate as its center of convection moves from the tropical Indian Ocean into the Pacific. Weather events under the influence of the MJO include precipitation, surface temperature, tropical cyclones, tornadoes, flood, wildfire, and lightning, among others. Several climate phenomena are also affected by the MJO. They are the monsoons, E1 Nino-Southern Oscillation, the North Atlantic Oscillation, the Pacific and North American pattern, the Arctic and Antarctic Oscillations or northern and southern annual modes, the Indian Ocean dipole, the Wyrtki jets, and the Indonesian Through-flow. This article provides a brief summary of the connections between the MJO and these weather and climate phenomena. These connections demonstrate the critical role of the MJO in the weather-climate continuum and its prediction., The conceptual separation of weather and climate is deeply rooted in our daily experience, as Herbertson (1901) put it: 'Climate is what on an average we may expect, weather is [...]

Temperature data over the past five decades show faster warming of the global land surface during the night than during the day (1). This asymmetric warming is expected to affect [...]

The recent decades-long decline in East African rainfall (1) suggests that multidecadal variability is an important component of the climate of this vulnerable region. Prior work based on analysing the [...]

In this paper we study a mathematical model for atmospheric circulation. If the wind directions of the atmosphere were averaged over large sections of the Earth's surface, a circulation pattern evolves--developed by the heating of the sun and also due to the Earth's rotation. To describe this complex weather system more than 105 variables would be involved. We propose a low order model based on a system proposed by Lorenz [4], which models a modified Hadley circulation. (This model will be referred to as the Lorenz-84 Model.) The model involves three dependent variables : x representing the poleward temperature gradient, y and z representing the cosine and sine phases of a sequence of eddies which are large-scale and superposed and t, the independent variable representing time. Keywords: circulation, Lorenz, attractors, Introduction The Earth's atmosphere is in constant circulation due to the Earth's atmosphere being heated by the Sun and the Earth's rotation. Mega scale circulatory events develop in the atmosphere [...]

Estimation of evaporation from three models namely: Penman, Priestley-Taylor, and Linacre for calculating evaporation rate were compared with the observed values of data collected from pan evaporimeter. Based on two years data, the most promising and reliable model for Algardabiya reservoir in Libya has been established. Various statistical measures such as RMSE, MBE, [R.sup.2] and t test have been chosen for performance evaluation of these models. The study revealed that the models resulted in reasonable accuracy. The errors in the model prediction were 2.95, 1.68, and 1.15 (mm/d) for Penman, Priestley-Taylor, and Linacre models, respectively. Linacre model, which requires only temperature data, was found to be the most reliable. Key words: semi-arid region, Algardabiya reservoir, evaporation, modeling, class A pan. L'estimation de l'evaporation a partir de trois modeles (Penman, Priestley-Taylor et Linacre) de calcul du taux d'evaporation a ete comparee aux (valeurs observees des) donnees colligees d'un bassin d'evaporation. En se basant sur deux annees de donnees, le modele le plus prometteur et le plus fiable pour le reservoir d'Algardabiya en Libye a ete determine. Diverses mesures statistiques, telles que l'ecart-type, l'erreur de justesse moyenne (MBE), [R.sup.2] et le test t, ont ete choisies pour evaluer le rendement de ces modeles. L'etude a montre que les modeles offraient une precision raisonnable. Les erreurs dans la prevision du modele etaient de 2,95, 1,68 et 1,15 (mm/j) respectivement pour les modeles Penman, Priestley-Taylor et Linacre. Le modele de Linacre, qui ne demande que des donnees de temperature, s' est avere le plus fiable. Mots-cles: region semi-aride, reservoir Algardabiya, evaporation, modelisation, bassin de classe A. [Traduit par la Redaction], 1. Introduction Evaporation is one of the major components of the hydrologic cycle and it describes the loss of water from water bodies to the air over a long period [...]

An important consideration when interpreting eddy-covariance (EC) flux-tower measurements is the spatial distribution of forest land surface cover and soil type within the EC flux-tower footprint. At many EC flux-tower sites, there is a range of geospatial data available with the ability to estimate the spatial distribution of forest land cover and soils. Developing methods that utilize multiple geospatial data sets will result in more thorough estimates of ecosystem C stock distributions. The objective of this study was to develop, apply, and validate methods to obtain comprehensive estimates of the spatial distribution of ecosystem C stock components from live-tree, detritus, and soil pools within an EC flux-tower foot-print. First, a set of geospatial data sets was collected and assessed for its predictive ability for the measured aboveground C stocks. Next, large tree and snag aboveground C stocks were estimated using two methods: (i) a geospatial regression model, and (ii) most similar neighbor (k-MSN) spatial prediction methodology, and the results were compared with those of a multiple linear regression model using light detection and ranging (LiDAR) data alone. Finally, we applied the spatial prediction methodology to estimate the spatial distribution of other C stock components (including soil C and woody debris). Resume: Il est important de tenir compte du type de sol et de couvert forestiers dans l'empreinte de la tour lorsqu'on inter-prete des mesures de correlation turbulente prises au sommet d'une tour a flux. Dans plusieurs stations o'est installee une tour a flux pour mesurer la correlation turbulente, une serie de donnees geospatiales qui permettent d'estimer la distribution spatiale du couvert et des sols forestiers sont disponibles. Le developpement de methodes qui ont recours a de multiples ensembles de donnees geospatiales devrait se traduire par des estimations plus completes de la distribution du carbone (C) dans l'ecosysteme. L'objectif de cette etude consistait a developper, appliquer et valider des methodes pour obtenir des esti-mations plus detaillees de la distribution spatiale des composantes du stock de C dans l'ecosysteme, soit la biomasse vivante, les debris ligneux et le sol, a l'interieur de l'empreinte d'une tour a flux utilisee pour mesurer la correlation turbulente. Dans un premier temps, des ensembles de donnees geospatiales ont ete rassembles et leur valeur predictive a ete evaluee dans le cas des stocks mesures de C aerien. Ensuite, les stocks de C aerien dans les chicots et les gros arbres ont ete estimes en utilisant deux methodes: (i) un modele de regression spatiale et (ii) une methode de prediction spatiale, les k voisins les plus similaires (k-MSN). Les resultats ont ete compares a un modele de regression lineaire multiple utilisant seulement des donnees obtenues par la methode de detection et telemetrie par la lumiere (LiDAR). Finalement, nous avons applique la methode de prediction spatiale pour estimer la distribution spatiale des autres composantes du stock de C (incluant le C dans le sol et les debris ligneux). [Traduit par la Redaction], Introduction Global carbon (C) cycling is strongly influenced by forest C dynamics as forests cover 4.1 billion hectares of the Earth's surface and are estimated to contain 1146 Pg C [...]

Water is transferred from the atmosphere to the land surface mainly via various forms of atmospheric precipitation, but the contribution of surface condensation and direct deposition (e.g., dew, hoar frost, occult precipitation, etc.) should not be neglected. Wet deposition (including surface condensation and direct deposition) scavenges pollutants from the atmosphere and contributes significantly to their transport. The composition of the runoff formed on contact with various surfaces might be further modified by the components of dry deposition collected on these surfaces or by substances leaching from the surfaces themselves. Samples of wet precipitation or direct deposition might be good indicators of the state of the atmospheric environment and can be used to trace the migration of chemicals emitted from various sources, including anthropogenic ones. This paper discusses the unique challenges present at every stage of the analytical procedures used for the determination of the pollutants in atmospheric precipitation and deposition. A brief review of the results of pollutant and toxicity determination in samples of various forms of atmospheric precipitation and (or) deposition is also included, together with a discussion of the application of chemometric techniques for data interpretation. The aim of this paper is to help interested readers discover the field of pollutant determination in wet deposition and to broaden the scope of their existing research. Key words: atmospheric pollution, wet deposition, precipitation, determination methods. Le transfert de l'eau de l'atmosphere a la surface terrestre s'effectue surtout via diverses formes de precipitations atmospheriques, mais on ne doit pas negliger la contribution de la condensation ou deposition en surface (rosee, frima, precipitations occultes, etc.). La deposition humide (incluant la condensation de surface et la deposition directe) ramasse les polluants de l'atmosphere et contribue significativement a leur transport. La composition de l'ecoulement forme au contact avec diverses surfaces peut etre encore modifiee par des composantes de depots secs accumulees sur ces surfaces ou par des substances lessivees a partir des surfaces elles-memes. Les echantillons de depots directs ou precipitations humides peuvent constituer de bons indicateurs de l'etat de l'environnement atmospherique et peuvent etre utilises suivre la migration des substances chimiques emises a partir de diverses sources, incluant celles d'origine anthropique. On discute les defis uniques rencontres aux differentes etapes des procedes analytiques utilises pour determiner les polluants des depots et des precipitations atmospheriques. On inclut egalement une breve revue des resultats de la determination des polluants et de leur toxicite dans des echantillons de diverses formes de precipitation ou deposition, ainsi qu'une discussion de l'application des techniques chemometrique pour l'interpretation des donnees. Le but de cette revue vise a aider les lecteurs interesses a prendre connaissance du champ de la determination des polluants dans les depositions humides et d'elargir l'envergure de la recherche existante. Mots-cles : pollution atmospherique, deposition humide, precipitation, methodes de determination., Introduction Atmospheric pollution adversely affects the natural environment, human health, and the products of human activity. In spite of various preventative measures, levels of atmospheric pollution still climb in some [...]

A regional network in experimental mode over the last two years monitors environmental variables and develops scientifically based forecasts with easy-to-use oceanographic, meteorological, and hydrological products for decision makers in the six collaborating countries.

The sensitivity of the midlatitude storm track and eddy-driven wind to the sea surface temperature (SST) boundary forcing is studied over a wide range of perturbations using both simple and comprehensive general circulation models over aquaplanet lower boundary conditions. Under the single-jet circulation regime similar to the conditions of the present climate in the Northern Hemisphere winter or the Southern Hemisphere summer, the eddy-driven jet shifts monotonically poleward with both the global mean and the equator-to-pole gradient of the SST. The eddy-driven jet can have a reverse relationship to the gradient if it is well separated from the subtropical jet and Hadley cell boundary in a double-jet circulation regime. A simple scaling is put forward to interpret the simulated sensitivity of the storm-track/eddy-driven westerly wind position within the single-jet regime in both models. The rationale for the scaling is based on the notion that the wave activity flux can propagate horizontally away from the source region, resulting in a broader distribution of eddy potential vorticity (PV) flux in the upper troposphere than that of the flux in the opposite direction in the lower troposphere. As a consequence, the position of the maximum of the eddy-driven westerlies tends to be controlled by the profile of the relatively sharp-peaked low-level PV flux, which is dominated by the eddy heat flux component of the Eliassen-Palm (EP) flux. Thus, the position of the eddy-driven surface westerlies may be inferred from the vertical EP flux coming out of the lower troposphere. The vertical EP flux can be parameterized by a measure of baroclinicity, whose latitudinal variations show a linear relationship with the meridional displacement of the eddy-driven westerlies and the storm track. This relationship still holds well within the single-jet regime, even when only the variation of static stability is taken into consideration in estimating the baroclinicity (the temperature gradient component of which is fixed). To the extent that the static stability is deterministically constrained by and hence can be predicted from the given SST conditions through a moist scaling for the midlatitude stratification, one may, given SST perturbations, predict which way the storm track and eddy-driven wind should shift with respect to a chosen reference climate state. The resultant anomaly-wise scaling turns out to be valid for both the idealized and comprehensive models, regardless of the details in the model physics. By corollary, it can be argued that the poleward shift of storm track found in the global warming simulations by fully coupled climate models may be attributed, at least partially, to the increase in the subtropical and midlatitude static stability with global warming. DOI: 10.1175/2010JAS3477.1

A diagnostic relationship between finite-amplitude wave activity and the associated adiabatic adjustments to the zonal-mean zonal wind and temperature is developed in the quasigeostrophic (QG) framework and is applied to a 23-yr segment (1979-2001) of the 40-yr ECMWF Re-Analysis (ERA-40) data. Wave activity is defined in terms of an instantaneous areal displacement of QG potential vorticity (PV) from zonal symmetry. Unlike previous forms, the tendency of wave activity equals exactly the negative of the eddy PV flux (Eliassen--Palm flux divergence) in the conservative limit, even at finite amplitude. This allows one to integrate the transformed Eulerian mean (TEM) theory in time and quantify the departure (adiabatic adjustment) of the zonal-mean state from an eddy-free reference state in terms of the observed wave activity. The structure of wave activity identifies synoptic eddies in the extratropics and planetary waves in the high latitudes of winter-to-spring stratosphere. In addition, a thin layer of high wave activity is found at the top of the lowermost stratosphere (~17 km) in the summer extratropics. The reference state is constructed by 'zonalizing' the PV contours conservatively (preserving area) on the isobaric surface and by inverting the resultant PV gradient for the mean flow. The adjustment associated with wave activity depends on the assumed surface boundary condition for the reference state. With a no-slip condition, the observed zonal-mean temperature is on average ~33 (90) K higher than the reference state in the troposphere (stratosphere) of the Arctic winter, while the zonal-mean zonal wind is ~30 m [s.sup.-1] slower in the upper stratosphere. Since the reference state filters out the advective eddy-mean flow interaction, it fluctuates less than the zonal-mean state, potentially improving the signal-to-noise ratio for climate diagnosis. DOI: 10.1175/2010JAS3503.1

This paper discusses a meteorological phenomenon that occurred in the Basin of Mexico in August 1521. The only known description appears in Book XII of the Florentine Codex, which is an account of the Spanish conquest of Mexico written in Nahuatl in the mid-sixteenth century. According to it, just before the fall of Mexico-Tenochtitlan a heavy storm accompanied by a whirlwind struck the basin. The whirlwind hovered for a while above Tlatelolco, Tenochtitlan's twin city, before moving to the lake where it disappeared. Analyzing the account in the context of Nahua culture and contrasting it with contemporary European descriptions of tornadoes and waterspouts shows that the described phenomenon was a tornado. This conclusion receives further support from eighteenth- and nineteenth-century pictorial and written evidence showing that tornadoes do occur in the territory now occupied by Mexico City. Since the tornado of Tlatelolco predates the Cambridge, Massachusetts, tornado of 1680, it represents the earliest documented tornado in the Americas. DOI: 10.1175/2010BAMS2874.1

A large-domain large-eddy simulation of a tropical deep convection system is used as a benchmark to derive and test a mixed subgrid-scale (SGS) scheme for scalar and momentum fluxes in cloud-resolving models (CRMs). The benchmark simulation resolves a broad range of scales ranging from mesoscale organizations, through gravity waves and individual clouds, down to energy-containing turbulent eddies. A spectral analysis shows that the vertical-velocity kinetic energy peaks at scales from hundreds of meters in the lower cloud layer to several kilometers higher up: these scales are typical grid sizes of today's CRMs. The analysis also shows that a significant portion of the scalar and momentum fluxes in the benchmark simulation are carried by motions smaller than several kilometers (i.e., smaller than a typical grid resolution of CRMs). The broad range of scales of the benchmark simulation is split into two components: filter scale (mimicking CRM resolvable scale) and subfilter scale (mimicking CRM SGS), using filter widths characteristic of a typical CRM grid spacing. The local relationship of the subfilter-scale fluxes to the filter-scale variables is examined. This leads to a mixed SGS scheme to represent the SGS fluxes of scalars and momentum in CRMs. A priori tests show that the mixed SGS scheme yields spatial distributions of subfilter-scale fluxes that correlate much better with those retrieved from the benchmark when compared with an eddy viscosity/diffusivity scheme that is commonly used in today's CRMs. DOI: 10.1175/2010JAS3565.1

This paper explores the hypothesis that a tropical cyclone (TC) may produce baroclinic waves through the divergent circulation that arises from its low-level inflow and upper-level outflow. The model setting is a quasigeostrophic (QG) two-layer fluid in which the effect of the tropical cyclone is parameterized through a source term on the QG potential vorticity equation. Equations predicting the spectral subset of baroclinic waves that are excited through linear resonance are derived. The near-TC pattern of the baroclinic waves in the streamfunction field typically takes the form of a ridge-trough couplet whose phase with respect to the TC varies with the speed and direction of the TCs motion vector. The predictions from the linearized theory are verified in two ways: 1) fully nonlinear simulations are shown and 2) comparison is made to the observed upper-level ridge-trough couplets produced by recurring TCs in the Navy's Operational Global Prediction System (NOGAPS). The implications of this work for the predictability of downstream impacts from recurving TCs are briefly described. DOI: 10.1175/2010JAS3459.1

This study examines the importance of horizontal photon transport effects, which are not considered in the 1D calculations of solar radiative heating used by most atmospheric dynamical models. In particular, the paper analyzes the difference between 2D and 1D radiative calculations for 2D vertical cross sections of clouds that were observed at three sites over 2-3-yr periods. The results show that 2D effects increase multiyear 24-h average total solar absorption by about 4.1, 1.2, and 0.3 W [m.sup.-2] at tropical, midlatitude, and arctic sites, respectively. However, 2D effects are often much larger than these average values, especially for high sun and for convective clouds. The results also reveal a somewhat unexpected behavior, namely, that horizontal photon transport often enhances solar heating even for oblique sun. These findings underscore the need for fast radiation calculation methods that can allow atmospheric dynamical simulations to consider the inherently multidimensional nature of shortwave radiative processes. DOI: 10.1175/2010JAS3506.1

In recent years, aircraft data from mountain waves have been primarily analyzed using velocity and temperature power spectrum and momentum flux estimation. Herein it is argued that energy flux wavelets (i.e., pressure-velocity wavelet cross-spectra) provide a more powerful tool for locating and classifying different types of mountain waves. In the first part of the paper, pressure-velocity cross-spectra using various linear mountain-wave solutions are shown to be capable of disentangling collocated waves with different propagation directions and wavelength. A field of group velocity vectors can also be determined. In the second part, the energy flux wavelet technique is applied to five cases of mountain waves entering the stratosphere from the Terrain-Induced Rotor Experiment (T-REX) in 2006. Perturbation pressure along the flight track is determined using aircraft static pressure corrected hydrostatically with GPS altitude. In four of the cases, collocated long up-propagating and short down-propagating waves are seen in the stratosphere. These waves have strong, but opposite, p'w' cospectra. In one of these cases, a patch of turbulence is collocated with the up and down waves. In two other cases, trapped waves riding on the tropopause inversion layer (TIL) are seen. These trapped waves have p'w' quadrature spectra that reverse sign across the tropopause. These newly discovered wave types may arise from secondary wave generation (i.e., a nonlinear transfer of energy from the long vertically propagating waves to shorter modes). DOI: 10.1175/2009JAS3285.1

Stratospheric variability is examined in a vertically extended version of the Met Office global climate model. Equatorial variability includes the simulation of an internally generated quasi-biennial oscillation (QBO) and semiannual oscillation (SAO). Polar variability includes an examination of the frequency of sudden stratospheric warmings (SSW) and annular mode variability. Results from two different horizontal resolutions are also compared. Changes in gravity wave filtering at the higher resolution result in a slightly longer QBO that extends deeper into the lower stratosphere. At the higher resolution there is also a reduction in the occurrence rate of sudden stratospheric warmings, in better agreement with observations. This is linked with reduced levels of resolved waves entering the high-latitude stratosphere. Covariability of the tropical and extratropical stratosphere is seen, linking the phase of the QBO with disturbed NH winters, although this linkage is sporadic, in agreement with observations. Finally, tropospheric persistence time scales and seasonal variability for the northern and southern annular modes are significantly improved at the higher resolution, consistent with findings from other studies. DOI: 10.1175/2010JAS3338.1

Despite the fact that asymmetries in hurricanes, such as spiral rainbands, polygonal eyewalls, and mesovortices, have long been observed in radar and satellite imagery, many aspects of their origin, space--time structure, and dynamics still remain unsolved, particularly their role on the vortex intensification. The underlying inner-core dynamics need to be better understood to improve the science of hurricane intensity forecasting. To fill this gap, a simple 2D barotropic 'dry' model is used to perform two experiments starting respectively with a monopole and a ring of enhanced vorticity to mimic hurricane-like vortices during incipient and mature stages of development. The empirical normal mode (ENM) technique, together with the Eliassen--Palm (EP) flux calculations, are used to isolate wave modes from the model datasets to investigate their space--time structure, kinematics, and the impact on the changes in the structure and intensity of the simulated hurricane-like vortices. From the ENM diagnostics, it is shown in the first experiment how an incipient storm described by a vortex monopole intensifies by 'inviscid damping' of a 'discrete-like' vortex Rossby wave (VRW) or quasi mode. The critical radius, the structure, and the propagating properties of the quasi mode are found to be consistent with predictions of the linear eigenmode analysis of small perturbations. In the second experiment, the fastest growing wavenumber-4 unstable VRW modes of a vortex ring reminiscent of a mature hurricane are extracted, and their relation with the polygonal eyewalls, mesovortices, and the asymmetric eyewall contraction are established in consistency with results previously obtained from other authors. DOI: 10.1175/2010JAS3499.1

Large-eddy simulations of warm, trade wind cumulus clouds are conducted for a range of aerosol conditions with a focus on precipitating clouds. Individual clouds are tracked over the course of their lifetimes. Precipitation rate decreases progressively as aerosol increases. For larger, precipitating clouds, the polluted clouds have longer lifetimes because of precipitation suppression. For clean aerosol conditions, there is good agreement between the average model precipitation rate and that calculated based on observed radar reflectivity Z and precipitation rate R relationships. Precipitation rate can be expressed as a power-law function of liquid water path (LWP) and [N.sub.d], to reasonable accuracy. The respective powers for LWP and [N.sub.d] are of similar magnitude compared to those based on observational studies of stratocumulus clouds. The time-integrated precipitation rate represented by a power-law function of LWP, [N.sub.d], and cloud lifetime is much more reliably predicted than is R expressed in terms of LWP and [N.sub.d] alone. The precipitation susceptibility ([S.sub.o] = -dlnR/dln[N.sub.d]) that quantifies the sensitivity of precipitation to changes in [N.sub.d] depends strongly on LWP and exhibits nonmonotonic behavior with a maximum at intermediate LWP values. The relationship between [S.sub.o] and precipitation efficiency is explored and the importance of including dependence on [N.sub.d] in the latter is highlighted. The results provide trade cumulus cloud population statistics, as well as relationships between microphysical/macrophysical properties and precipitation, that are amenable for use in larger-scale models. DOI: 10.1175/2010JAS3484.1

There are different techniques for sensing wavefront phase distortions due to atmospheric turbulence. Curvature sensors are practical because their sensitivity is adjustable to the prevailing atmospheric conditions. Even at the best sites, the turbulence intensity has been found to vary at times over only a few minutes and regularly over longer periods. Two methods to automatically adjust the sensitivity of a curvature sensor are proposed: First, the defocus distance can be adjusted prior to the adaptive-optics (AO) loop through the acquisition of a long-exposure image and can then be kept constant. Second, the defocus distance can be changed during the AO loop, based on the voltage values sent to the deformable mirror. We demonstrate that the performance increase--assessed in terms of the image Strehl ratio--can be significant. OCIS codes: 110.0115, 110.1080.

An initial-value problem is employed with a GCM to investigate the role of the convectively driven Rossby and Kelvin waves for tropopause transition layer (TTL) upwelling in the tropics. The convective heating is mimicked with a prescribed heating field, and the Lagrangian upwelling is identified by examining the evolution of passive tracer fields whose initial distribution is identical to the initial heating field. This study shows that an overturning circulation, induced by the tropical Rossby waves, is capable of generating the TTL upwelling. Even when the heating is placed in the eastern Pacific, the TTL upwelling occurs only over the western tropical Pacific, indicating that the background flow plays a crucial role. The results from a Rossby wave source analysis suggest that a key feature of the background flow is the strong absolute vorticity gradient associated with the Asian subtropical jet. In addition, static stability is relatively weak over the western Pacific, suggesting that this may also contribute to the TTL upwelling in that region. The background flow also modulates the internal Kelvin waves in such a manner that the coldest region in the TTL (resembling the observed 'cold trap') occurs over the western tropical Pacific. As a consequence, the upwelling air, induced by the meridional momentum flux of the Rossby wave, passes through the cold trap generated by the Kelvin wave. Since in reality the background flow is shaped by the convective heating, the climatological western tropical Pacific heating is ultimately responsible for both the TTL upwelling and the cold trap; however, both processes are realized indirectly through its impact on the background flow and the generation of the tropical waves. DOI: 10.1175/2010JAS3434.1

The year 2009 was a major drought year for the Indian summer monsoon with a seasonal deficit of rainfall by 21.6%. Standard oceanic predictors such as ENSO and the Indian Ocean dipole are not consistent for these dry spells. There are a host of other parameters such as the Himalayan ice cover, the Eurasian snow cover, the passage of intraseasonal waves, and even accumulated effects of Asian pollution that have been considered for analysis of the dry spells of the monsoon. This paper presents another factor, the western Asian desert air incursions toward central India, and emphasizes the formation of a blocking high over western Asia as an important feature for these dry spells. The blocking high advects descending very dry air toward central India, portrayed using swaths of three-dimensional trajectories. This is a robust indicator for dry spells of the monsoon during the last several decades. This dry air above the 3-km level over central India strongly inhibits the vertical growth of deep convection. Some of the interesting antecedents of the formation of the blocking high include an eastward and somewhat northward extension of the ITCZ over North Africa, a stronger than normal local Hadley cell over North Africa, a strong subtropical jet stream over the southern Mediterranean, and strong conversions of anticyclonic shear vorticity to anticyclonic curvature vorticity. The dynamical antecedents of the aforementioned scenario in this study are related to many aspects of North African weather features. They are portrayed using both reanalysis datasets and ensemble modeling using a suite of coupled atmosphere--ocean models. DOI: 10.1175/2010JAS3440.1

Cumulus clouds involve processes on a vast range of scales--including cloud droplets, turbulent mixing, and updrafts and downdrafts--and it is often difficult to determine how processes on different scales interact with each other. In this article, several multiscale asymptotic models are derived for cumulus cloud dynamics in order to (i) provide a systematic scale analysis on each scale and (ii) clarify the nature of interactions between different scales. In terms of scale analysis, it is shown that shallow cumulus updrafts can be described by balanced dynamics with a balance between source terms and ascent/descent; this is a cloud-scale version of so-called weak-temperature-gradient models. In terms of multiscale interactions, a model is derived that connects these balanced updrafts to the fluctuations within the balanced updraft envelope. These fluctuations describe parcels and updraft pulses, and this model encompasses some of the multiscale aspects of entrainment. In addition to this shallow cumulus model, to provide a broad picture of general cumulus dynamics, multiscale models are also derived for other scales; these include models for parcels and subparcel turbulent mixing and models for deep cumulus. Broadly speaking, the differences between the shallow and deep cases convey the notion that shallow cumulus dynamics are parcel dominated, whereas deep cumulus dynamics are updraft dominated; this is largely due to the difference in the apparent magnitude of the background temperature stratification. In addition to their use in guiding theory, the multiscale models also provide a framework for multiscale numerical simulations. DOI: 10.1175/2010JAS3380.1

Despite the fact that asymmetries in hurricanes (e.g., spiral rainbands, polygonal eyewalls, and mesovortices) have long been observed in radar and satellite imagery, many aspects of their dynamics remain unsolved, particularly in the formation of the secondary eyewall. The underlying associated dynamical processes need to be better understood to advance the science of hurricane intensity forecasting. To fill this gap, a simple 2D barotropic 'dry' model is used to simulate a hurricane-like concentric rings vortex. The empirical normal mode (ENM) technique, together with Eliassen-Palm (EP) flux calculations, are used to isolate wave modes from the model datasets to investigate their impact on the changes in the structure and intensity of the simulated hurricane-like vortex. From the ENM diagnostics, it is shown that asymmetric disturbances outside a strong vortex ring with a large vorticity skirt may relax to form concentric rings of enhanced vorticity that contain a secondary wind maximum. The fact that the critical radius for some of the leading modes is close to the location where the secondary ring of enhanced vorticity develops suggests that a wave-mean flow interaction mechanism based on vortex Rossby wave (VRW) dynamics may explain important dynamical aspects of concentric eyewall genesis (CEG). DOI: 10.1175/2010JAS3500.1

This study tested a numerical and theoretical prediction that the stratosphere and troposphere are coupled through the effect of stratospheric vertical wind shear on baroclinic waves. Wavelengths, phase speeds, and background quasigeostrophic potential vorticity gradients were analyzed over the Pacific and Atlantic during strong and weak stratospheric polar vortex events and interpreted in terms of the counterpropagating Rossby wave perspective on baroclinic instability. Effects of zonal variations in the background flow were included in the analysis of phase speeds. Observed changes in wave packet average wavelength and phase speed support the vertical shear hypothesis for stratosphere--troposphere coupling; however, changes in the intrinsic phase speed contradict the hypothesis. This inconsistency was resolved by considering the change in zonal wind speed in the lower stratosphere, which accounts for most of the change in phase speeds during strong and weak vortex events. Changes in the average wavelengths and meridional wave activity flux are also consistent with this modified hypothesis involving the stratospheric zonal wind. The results demonstrate that a simple mechanism for stratosphere--troposphere coupling can be found in the observational record. DOI: 10.1175/2010JAS3279.1