Advection Of Dry Air Research Articles

Biomass burning impact on black carbon aerosol mass concentration at a coastal site: Case studies

During 25 March – 5 April 2010 intense wildfires in the Kaliningrad region (Russia) occurred. The resultant smoke plume blanketing the Lithuanian western part was seen in satellite images. Concurrently, an extremely high black carbon (BC) aerosol mass concentration was observed at the background Preila site (55°55' N, 21°00' E, 5 m a.s.l., Lithuania). The surface measurements and calculation of Angstrom exponent of the absorption coefficient carried out separately for shorter and longer wavelengths (i. e., λ =370–520 nm and λ =590–950 nm) showed that high levels of BC aerosol were related to the transport of air masses rich in biomass burning products from the Kaliningrad region caused by active grass burning. During this event the BC aerosol mass concentration of 1-hour average reached 13000 ngm−3, while normally annual mean concentration values are about 750 ngm−3. The transport of the burning products from fire areas is associated with southeastern flow and strong advection of warm and dry air from South Europe in the lower troposphere. During the event the highest mean values of Angstrom exponent of the absorption coefficient α370−520 and α590−950 were observed (2.0±0.4 and 1.6±0.3, respectively). The mean values of Angstrom exponent of the absorption coefficient during the study period obviously indicate that a major part of carbon mass in aerosol particles transferred by the regional air masses comes from the wildfire location. : black carbon aerosol, aethalometer, biomass burning, Angstrom exponent of the absorption coefficient

Diabatic heating, divergent circulation and moisture transport in the African monsoon system

AbstractThe dynamics of the West African monsoon system is studied through the diagnosis of the roles of diabatic heating in the divergent circulation and moisture transport. The divergent circulation is partitioned into latent‐heating and non‐latent‐heating (the sum of surface sensible heat flux and radiative heating) driven components based on its field properties and its relationship with diabatic heating profiles. Roles of latent and non‐latent diabatic heating in the moisture transport of the monsoon system are thus distinguished.The gradient in surface sensible heat flux between the Saharan heat‐low and the Gulf of Guinea drives a shallow meridional circulation, which transports moisture far into the continent on the northern side of the monsoon rain band and thereby promotes the seasonal northward migration of monsoon precipitation. In contrast, the circulation directly associated with latent heating is deep and the corresponding moisture convergence maximum is within the region of precipitation and thus enhances local monsoon precipitation. Meanwhile, latent heating also induces dry air advection from the north. The seasonal northward migration of precipitation is encouraged by neither of the two effects. On the other hand, the divergent circulation forced by remote latent heating influences local moisture distribution through advection. Specifically by bringing Saharan air from the north, and driving moisture to the adjacent oceans, global latent heating has an overall drying effect over the Sahel. Copyright © 2009 Royal Meteorological Society

Synoptic-Scale Controls of Persistent Low Temperature and Icy Weather over Southern China in January 2008

Abstract In January 2008, central and southern China experienced persistent low temperatures, freezing rain, and snow. The large-scale conditions associated with the occurrence and development of these snowstorms are examined in order to identify the key synoptic controls leading to this event. Three main factors are identified: 1) the persistent blocking high over Siberia, which remained quasi-stationary around 65°E for 3 weeks, led to advection of dry and cold Siberian air down to central and southern China; 2) a strong persistent southwesterly flow associated with the western Pacific subtropical high led to enhanced moisture advection from the Bay of Bengal into central and southern China; and 3) the deep inversion layer in the lower troposphere associated with the extended snow cover over most of central and southern China. The combination of these three factors is likely responsible for the unusual severity of the event, and hence a long return period.

Climatology of Surface Meteorology, Surface Fluxes, Cloud Fraction, and Radiative Forcing over the Southeast Pacific from Buoy Observations

Abstract A 5-yr climatology of the meteorology, including boundary layer cloudiness, for the southeast Pacific region is presented using observations from a buoy located at 20°S, 85°W. The sea surface temperature and surface air temperature exhibit a sinusoidal seasonal cycle that is negatively correlated with surface pressure. The relative humidity, wind speed, and wind direction show little seasonal variability. But the advection of cold and dry air from the southeast varies seasonally and is highly correlated with the latent heat flux variations. A simple model was used to estimate the monthly cloud fraction using the observed surface downwelling longwave radiative flux and surface meteorological parameters. The annual cycle of cloud fraction is highly correlated to that of S. A. Klein: lower-tropospheric stability parameter (0.87), latent heat flux (−0.59), and temperature and moisture advection (0.60). The derived cloud fraction compares poorly with the International Satellite Cloud Climatology Project (ISCCP)-derived low-cloud cover but compares well (0.86 correlation) with ISCCP low- plus middle-cloud cover. The monthly averaged diurnal variations in cloud fraction show marked seasonal variability in the amplitude and temporal structure. The mean annual cloud fraction is lower than the mean annual nighttime cloud fraction by about 9%. Annual and diurnal cycles of surface longwave and shortwave cloud radiative forcing were also estimated. The longwave cloud radiative forcing is about 45 W m−2 year-round, but, because of highly negative shortwave cloud radiative forcing, the net cloud radiative forcing is always negative with an annual mean of −50 W m−2.

The Effect of Imposed Drying on Parameterized Deep Convection

AbstractThis paper examines the influence of imposed drying, intended to represent horizontal advection of dry air, on parameterized deep convection interacting with large-scale dynamics in a single-column model framework. Two single-column models, one based on the NASA Goddard Earth Observing System general circulation model version 5 (GEOS5) and the other developed by Bony and Emanuel, are run in weak temperature gradient mode. Drying is imposed by relaxation of the specific humidity field toward zero within a specified vertical layer. The strength of the drying is controlled by specifying either the relaxation time scale or the vertically integrated drying tendency; results are insensitive to which specification is used.The two models reach very different solutions for the same boundary conditions and model configuration. Even when adjustments to the boundary conditions and model parameters are made to render the precipitation rates similar, large differences in the profiles of relative humidity and large-scale vertical velocity persist. In both models, however, drying in the middle troposphere is more effective, per kg m−2 s−1 (or W m−2) of imposed drying, in suppressing precipitation than is drying in the lower troposphere. Even when compared at equal relaxation time (corresponding to weaker net drying in the middle than lower troposphere), middle-tropospheric drying is comparably effective to lower-tropospheric drying. Upper-tropospheric drying has a relatively small effect on precipitation, although large drying in the upper troposphere cannot be imposed as a steady state because of the lack of moisture there. Consistent with the other model differences, the gross moist stabilities of the two models are quite different and vary somewhat differently as a function of imposed drying, but in both models the gross moist stability increases as the drying is increased when it is less than around 30 W m−2 and located in the middle troposphere. For lower-tropospheric drying, the gross moist stability either decreases with increased drying or increases more slowly than for middle-tropospheric drying.

The Moisture Mode in the Quasi-Equilibrium Tropical Circulation Model. Part II: Nonlinear Behavior on an Equatorial β Plane

Abstract Numerical calculations of a simplified quasi-equilibrium tropical circulation model (QTCM) on the equatorial β plane have been performed to explore the nonlinear regime of the moisture mode. Sensitivity tests have examined the effects of nonlinear advection and nonlinear wind-induced surface heat exchange (WISHE). Starting from a spatially homogeneous radiative–convective equilibrium with some background gustiness, the model develops quasi-stationary moisture modes, as expected from linear analysis. Upon nonlinear saturation due to thermodynamic limiting processes, a different regime emerges. A classical Gill model augmented with a prognostic humidity variable is found to be able to capture the nonlinear dynamics of the moisture mode; the time evolution solely resides in the humidity variable, and it is possible to understand its dynamics by examining the humidity budget. A scaling analysis shows that the approximation with a Gill model is valid for disturbances whose time scale is much longer than the damping time scale of equatorial waves. When nonlinear WISHE is included, a large-scale disturbance of wavenumber 1 grows and moves westward because evaporation is enhanced to the west of increased convection. Turning on nonlinear advection leads to disturbances of wavenumber 10 that translate eastward via advection of dry air by Rossby gyres. Combining nonlinear WISHE and nonlinear advection leads to gross moist instability and prohibits long-term numerical integration, which suggests that QTCM must be refined to fully describe the nonlinear dynamics of the moisture mode.

The development of extremely dry surface air due to vertical exchanges under the exit region of a jet streak

Observations and numerical model simulations indicate that anomalous surface drying is strongly forced by mass and momentum adjustments under the right exit region of a polar jet streak overtaking and modifying a weak surface cold front. This drying event, which was associated with a wildfire in south-central New Jersey, is related to multi-scale atmospheric forcing that began far upstream and was likely not coupled to the classic cold conveyor belt model, as described by Carlson (Mon Wea Rev 108:1498–1509, 1980). The analyses indicate that both deep tropospheric circulations and boundary layer dry air advection occur in tandem to create a favorable environment for two closely associated extreme surface drying events. The first drying event occurs when lower tropospheric air is transported downwards in the descending branch of the low-level cold front’s thermally direct circulation. This low-level circulation, which is vertically separated from the upper-level jet, is still enhanced by the hydrostatic pressure rises under the velocity convergence in the equatorward exit region of the polar jet. The upper-level convergence phases with the low-level cold air advection to intensify the low-level isallobaric wind. Dry air increases at low levels in conjunction with the isallobaric divergence behind the low-level cold front. Sinking air occurs within the 100-hPa layer centered just above 900 hPa as a result of the isallobaric divergence. The shallow descending circulation within the upstream side of this low-level front produces the first injection of dry air into the surface layer independently of deep-boundary layer mixing. Surface moisture divergence and shallow sinking sustain the dry cold front in the boundary layer. The descending air in the upper tropospheric jet circulation does not immediately couple to the deepening boundary layer accompanying the cold front’s circulation. A second drying event occurs at the surface shortly thereafter, when surface heating deepens the well-mixed layer, such that the boundary layer behind the low-level front (i.e., within its trailing descending air) is linked to the dry air under the jet’s equatorward exit region descending branch. Thus, the first drying event is created by the low-level direct circulation in a region of isallobaric divergence, while the second drying event is created by the coupling between the upper level indirect circulation and the deepening convective boundary layer. The two dry events combine to create a favorable environment for an isolated wildfire as both dry air and increasing surface winds develop during maximum surface heating.

An analysis of the vertical structure of the atmosphere and the upper‐level meteorology and their impact on surface ozone levels in Houston, Texas

Despite emission reductions, Houston continues to be designated as a nonattainment area for ozone (O3) by the Environmental Protection Agency. Upper‐level synoptic maps and information about the vertical structure of the lower troposphere obtained by in situ measurements were analyzed to characterize ozone exceedances in which peak 8‐h average concentration exceeded 85 ppb during the Texas Air Quality Study‐II in August–September 2006. Cluster analysis of meteorological conditions showed that the highest background surface O3 concentrations occurred under northerly or easterly flow regimes at 850 hPa, coinciding with the advection of dry continental air. Exceedance days in September 2006 occurred almost exclusively in postfrontal environments. These frontal passages are associated with shifts in wind direction and may lead to increases in background O3 from 30 ppbv (marine) to 60–70 ppbv (continental) throughout the lower troposphere. Several factors are identified to be important for 8‐h average ozone peaks in Houston under well‐developed land‐sea‐bay breeze conditions, including (1) the presence of easterly winds advecting industrial emissions from the Ship Channel, and (2) the presence of persistent large‐scale northerly flows aloft advecting elevated continental background ozone levels that are eventually entrained into lower layers through the growth of the convective planetary boundary layer.

Northern hemisphere storm tracks during the last glacial maximum in the PMIP2 ocean-atmosphere coupled models: energetic study, seasonal cycle, precipitation

Mid-latitude eddies are an important component of the climatic system due to their role in transporting heat, moisture and momentum from the tropics to the poles, and also for the precipitation associated with their fronts, especially in winter. We study northern hemisphere storm-tracks at the Last Glacial Maximum (LGM) and their influence on precipitation using ocean-atmosphere general circulation model (OAGCM) simulations from the second phase of the Paleoclimate Modelling Intercomparison Project (PMIP2). The difference with PMIP1 results in terms of sea-surface temperature forcing, fundamental for storm-track dynamics, is large, especially in the eastern North Atlantic where sea-ice extends less to the south in OAGCMs compared to atmospheric-only GCMs. Our analyses of the physics of the eddies are based on the equations of eddy energetics. All models simulate a consistent southeastward shift of the North Pacific storm-track in winter, related to a similar displacement of the jet stream, partly forced by the eddies themselves. Precipitation anomalies are consistent with storm-track changes, with a southeastward displacement of the North Pacific precipitation pattern. The common features of North Atlantic changes in the LGM simulations consist of a thinning of the storm-track in its western part and an amplification of synoptic activity to the southeast, in the region between the Azores Islands and the Iberian Peninsula, which reflects on precipitation. This southeastward extension is related to a similar displacement of the jet, partly forced by the eddies. In the western North Atlantic, the synoptic activity anomalies are at first order related to baroclinic generation term anomalies, but the mean-flow baroclinicity increase due to the presence of the Laurentide ice-sheet is partly balanced by a loss of eddy efficiency to convert energy from the mean flow. Moisture availability in this region is greatly reduced due to more advection of dry polar air by stationary waves, leading to less synoptic-scale latent heat release and hence less precipitation also. In terms of seasonality, the stormy season is shifted later in the year by a few days to a month depending on the season and the model considered. This shift does not directly reflect on the first-order seasonal cycle of precipitation, which also depends on other mechanisms, especially in summer.

The Dryline on 22 May 2002 during IHOP: Ground-Radar and In Situ Data Analyses of the Dryline and Boundary Layer Evolution

Abstract On the afternoon and evening of 22 May 2002, high-resolution observations of the boundary layer (BL) and a dryline were obtained in the eastern Oklahoma and Texas panhandles during the International H2O Project. Using overdetermined multiple-Doppler radar syntheses in concert with a Lagrangian analysis of water vapor and temperature fields, the 3D kinematic and thermodynamic structure of the dryline and surrounding BL have been analyzed over a nearly 2-h period. The dryline is resolved as a strong (2–4 g kg−1 km−1) gradient of water vapor mixing ratio that resides in a nearly north–south-oriented zone of convergence. Maintained through frontogenesis, the dryline is also located within a gradient of virtual potential temperature, which induces a persistent, solenoidally forced secondary circulation. Initially quasi-stationary, the dryline retrogrades to the west during early evening and displays complicated substructures including small wavelike perturbations that travel from south to north at nearly the speed of the mean BL flow. A second, minor dryline has similar characteristics to the first, but has weaker gradients and circulations. The BL adjacent to the dryline exhibits complicated structures, consisting of combinations of open cells, horizontal convective rolls, and transverse rolls. Strong convergence and vertical motion at the dryline act to lift moisture, and high-based cumulus clouds are observed in the analysis domain. Although the top of the analysis domain is below the lifted condensation level height, vertical extrapolation of the moisture fields generally agrees with cloud locations. Mesoscale vortices that move along the dryline induce a transient eastward dryline motion due to the eastward advection of dry air following misocyclone passage. Refractivity-based moisture and differential reflectivity analyses are used to help interpret the Lagrangian analyses.

The Indian drought of 2002—a sub‐seasonal phenomenon?

AbstractA major drought occurred over India in the year 2002 with a seasonal rainfall deficit of 21.5%, a result of 56% below normal rainfall in the month of July. The largest anomalies occurred in the western parts of India, when an Indian monsoon field experiment was in progress there. The present study is based primarily on data collected from a research ship that was deployed 100–250 km off the west coast of India for the experiment. Surface and upper air observations made over the eastern Arabian Sea during July 2002 are presented. Sea‐surface conditions were favourable for supporting deep convection over the study area. Strong atmospheric inversions around 800 hPa prevented the growth of cumulonimbus during the first half and towards the end of July. A second inversion above the melting level was also prominently present. The strength and persistence of the inversions are unusual. Back‐trajectory analysis reveals a major change in the low‐level circulation during July 2002 with frequent advection of dry air from over the deserts around the eastern Arabian Sea instead of marine air from across the equator. Copyright © 2006 Royal Meteorological Society

The Interaction of Clouds and Dry Air in the Eastern Tropical Pacific

Abstract Cloud radar observations of eastern Pacific intertropical convergence zone cloud vertical structure are interpreted in light of soundings, 100-km-scale divergence profiles calculated from precipitation radar Doppler velocities, and surface rain gauge data, from a ship at 10°N, 95°W during the 2001 East Pacific Investigation of Climate (EPIC) experiment. The transition from convective to stratiform rain is clear in all four datasets, indicating a coherence from local to 100-km scale. A novel finding is dry air intrusions at altitudes of 6–8 km, often undercutting upper-level ice clouds. Two distinct dry air source regions are identified. One is a relatively dry area overlying the cooler waters of the Costa Rica oceanic thermocline dome, centered approximately 400 km east-northeast of the ship site. The other is the even drier near-equatorial subsidence zone south of 6°–7°N. The former source is somewhat peculiar to this specific ship location, so that the ship sample is not entirely representative of the region. The 20–25 September period is studied in detail, as it depicts two influences of the dry air on cloud vertical structure. One is the modulation of small-scale surface-based convection, evident as a weakening and narrowing of cloud radar reflectivity features. The other springs from intense sublimation cooling as differential advection brought snowing anvil clouds over the dry layers. During one half-day period of strong sublimation, the cooling rate is inferred to be several tens of degrees per day over a 100-hPa layer, based on a heat budget estimate at 100-km scale involving the horizontal wind divergence data. This is consistent with fluxing ice water contents of 0.05–0.10 g m−3 derived from the cloud radar reflectivities. The temperature profile shows the dynamically expected response to this cooling, a positive–negative–positive temperature anomaly pattern centered on the sublimating layer. A buoyancy-sorting diagnostic model of convection indicates that these upper-troposphere temperature anomalies can cause premature detrainment of updrafts into the lower part of the cloudy layer, a feedback that may actively maintain these long-lasting dense anvils. Middle-troposphere southerly dry air inflow is also evident in large-scale analysis. Given the proximity of the dry equatorial subsidence zone to the eastern tropical Pacific, the differential advection of dry and cloudy air, the ensuing sublimation, and its dynamical aftereffects may play a role in establishing the region’s climate, although the extent of their significance needs to be further established.

Evaluation of the air‐sea bulk formula and sea‐surface temperature variability from observations

Eddy‐correlation fluxes are compared to air‐sea fluxes predicted by a widely used bulk flux formulation without wave‐state effects. Systematic discrepancies are found. For example, the model approximately equates the roughness lengths for heat and moisture; however, the observed roughness length for heat (zoh) exceeds that for moisture (zoq) by an order of magnitude or more, except in the strongest wind‐speed conditions. This is apparently due to the dynamic nature of temperature, which dominates buoyancy generation of turbulence in these data sets. The observed correlation between temperature and vertical velocity fluctuations generally exceeds that for moisture. For 10‐m wind speeds above a threshold value of 12 m s−1, zoq exceeds zoh apparently owing to enhanced moisture flux associated with the onset of wave breaking coupled with advection of cold dry air from land. In near‐collapsed turbulence, the observed momentum flux is smaller than predicted, and there is no clear indication of a smooth flow viscous regime. The scatter between observed and bulk fluxes generally decreases with averaging the observed fluxes over greater length scales even with variations in sea‐surface temperature (SST). The reduction in random flux sampling errors more than compensates for capturing increased surface heterogeneity with increasing averaging scale. Since similarity theory does not apply to heterogeneous surfaces, the bulk model does break down in the extreme case where the averaging window includes a sharp SST front. The response of the flow to changes in SST is presented for different amplitudes of SST variability. The change in vertical structure and acceleration of the low‐level wind over warm pools is discussed.

Moisture variability over the South American Altiplano during the South American Low Level Jet Experiment (SALLJEX) observing season

During the austral summer, precipitation over the high‐altitude plateau of the South American Altiplano exhibits a marked intraseasonal variability which has been associated with alternating moist and dry conditions observed at surface stations near the Altiplano western cordillera. In this study the characteristics of humid (wet) and dry (dry) episodes observed at a station on the western rim of the Altiplano during the summer of 2002–2003 are examined using observational data from the South American Low Level Jet Experiment (SALLJEX) and satellite‐based estimates of precipitable water (PW) from Global Positioning System, Moderate Resolution Imaging Spectrometer, and Passive Microwave Radiometer instruments. We find that on wet days, mean moist conditions (PW greater than 10 mm) prevail across the entire plateau. This moisture is apparently brought to the Altiplano by sustained westward advection of air at similar altitude above the continental basin and propagates across the Altiplano to moisten the atmosphere over the Pacific Ocean a considerable distance (>1000 km) west of the plateau. In contrast, during dry events, significant drying of the boundary layer is observed mainly along the western mountain range and over the southern Altiplano. There is no evidence of strong midlevel advection of dry air across the northern Altiplano, and moisture remains constant and high over the northern basin and along the Altiplano eastern ridge. Consequently, precipitation and convective cloudiness show the strongest relation to moisture along the western cordillera of the Altiplano. Significant diurnal variability of moisture is found to occur most strongly on the outer slopes of the Altiplano. There is little indication that daytime upslope flow on the eastern slope of the Altiplano plays a major role in the transport of moisture to the Altiplano during wet episodes.

Southerly Surges on Submonthly Time Scales over the Eastern Indian Ocean during the Southern Hemisphere Winter

Abstract Meridional wind surges from the extratropics into the Tropics strongly regulate tropical convective activity. This paper confirms that extratropical forcing manifested as a meridional surge does modulate the tropical atmosphere over the eastern Indian Ocean, and it describes the tropical–extratropical connection in the region. Surges in the lower atmosphere on submonthly (6–25 days) time scales over the eastern Indian Ocean were examined in tandem with associated tropical convection and large-scale atmospheric fields during the Southern Hemisphere (SH) winter (June–August). Data used in this study are NCEP-2 reanalyses and daily NOAA/Climate Diagnostics Center (CDC) outgoing longwave radiation (OLR) data for 23 yr, from 1979 to 2001. A low-level surge index was calculated using the 850-hPa meridional wind component (υ) averaged over a region where sub–monthly scale υ variance shows a local maximum (17.5°–2.5°S, 87.5°–97.5°E). The surge index defines 62 different surge events. Composites of various components were generated based on the index to define relationships between surge events and large-scale fields. Low-level southerly surges over the eastern Indian Ocean originate from midlatitude Rossby waves with strong baroclinic development in the entrance region of a subtropical jet core off Australia’s west coast. Strengthened low-level wind surges cause cross-equatorial flow stretching from the subtropical eastern Indian Ocean to the southern Bay of Bengal. Surges are accompanied by the advection of cold, dry air from midlatitudes into the Tropics. A cold and dry front develops at the leading surge edge during the surge period. Two to four days later, as the surge peaks, negative OLR anomalies develop near the key region. The OLR anomalies indicate a local blow up of convection over the tropical eastern Indian Ocean. Convection reflects increased instability in the surge region, which is caused by low-level dry air advection and near-surface moistening that is forced by enhanced sea surface evaporation associated with the surge. The southerly surge on submonthly time scales is an important bridge linking the Tropics and midlatitudes over the Indian Ocean.

On the weakening of Hurricane Lili, October 2002

This paper addresses the weakening of Hurricane Lili of October 2002 just before it made landfall in Louisiana. This hurricane weakened from a category 4 storm on October 3, 2002 at 0000 UTC to a category 1 storm on October 3, 2002 at 1300 UTC. This sudden drop in intensity has been a subject of considerable interest. In this paper we explore a forecast model diagnostic approach that explores the contribution to the hurricane intensity changes arising from a number of dynamical and physical possibilities. Running several versions of a global model at very high resolution, the relative contribution to the intensity drop of Lili arising from cooler sea surface temperatures, dry air advection into the storm, advective non-linear dynamics, non-advective dynamics, and shallow and deep cumulus convection was examined. This line of inquiry led to the conclusion that dry air advection from the north into the storm and the slightly cold sea surface temperatures were not the primary contribution to the observed pressure rise by 22 hPa. The primary contribution to the pressure rise was found to be the ‘rest of dynamics’ (the non-advective dynamics). The shallow convection contributed slightly to an overall cooling, i.e. a weakening of the warm core of Lili. The effects of deep cumulus convection appeared to be opposite, i.e. towards maintaining a strong storm. A primary term in the ‘rest of dynamics’, the advection of Earth’s angular momentum into the storm, is identified as a major contributor for the intensity change in the analysis. This feature resembles an intrusion of dry air into the core of the storm. This intrusion contributes to a reduction of spin and an overall rapid weakening of the hurricane. The angular momentum partitioning appears quite revealing on the sudden demise of Lili.

The Africa–South America Intercontinental Teleconnection

Abstract The influence of heating over Africa on the South American precipitation climatology, and the influence of South America on Africa, is examined through the application of GCM simulations with idealized boundary conditions and perpetual solstice (January and July) conditions. The presence of Africa is associated with a pronounced (up to 4 mm day−1) decrease in precipitation in Brazil's Nordeste region during austral summer. Low-level moisture divergence and dry-air advection associated with the downbranch of a Walker circulation induced by heating over southern Africa is amplified over the Nordeste due to the response of the land surface. The response is much smaller during austral winter due to differences in the heat source over Africa and a reduced sensitivity in the surface heat balance over tropical South America. Forcing from South America in January shifts the position of the South Indian convergence zone (SICZ) to the southwest over southern Africa in association with the formation of the ...

Large-Scale Meteorology and Deep Convection during TRMM KWAJEX*

An overview of the large-scale behavior of the atmosphere during the Tropical Rainfall Measuring Mission (TRMM) Kwajalein Experiment (KWAJEX) is presented. Sounding and ground radar data collected during KWAJEX, and several routinely available datasets including the Geostationary Meteorological Satellite (GMS), NOAA outgoing longwave radiation (OLR), the Special Sensor Microwave Imager (SSM/I), and ECMWF operational analyses are used. One focus is on the dynamical characterization of synoptic-scale systems in the western/central tropical Pacific during KWAJEX, particularly those that produced the largest rainfall at Kwajalein. Another is the local relationships observed on daily time scales among various thermodynamic variables and areal average rain rate. These relationships provide evidence regarding the degree and kind of local thermodynamic control of convection. Although convection in the Marshall Islands and surrounding regions often appears chaotic when viewed in satellite imagery, the largest rain events at Kwajalein during the experiment were clearly associated with large-scale envelopes of convection, which propagated coherently over several days and thousands of kilometers, had clear signals in the lower-level large-scale wind field, and are classifiable in terms of known wave modes. Spectral filtering identifies mixed Rossby–gravity (MRG) and Kelvin waves prominently in the OLR data. “Tropical depression–type” disturbances are also evident. In some cases multiple wave types may be associated with a single event. Three brief case studies involving different wave types are presented. Daily-mean sounding data averaged over the five sounding sites show evidence of shallow convective adjustment, in that near-surface moist static energy variations correlate closely with lower-tropospheric temperature. Evidence of thermodynamic control of deep convection on daily time scales is weaker. Upper-tropospheric temperature is weakly correlated with near-surface moist static energy. There are correlations of relative humidity (RH) with deep convection. Significant area-averaged rainfall occurs only above a lower-tropospheric RH threshold of near 80%. Above this threshold there is a weak but significant correlation of further lower-tropospheric RH increases with enhanced rain rate. Upper-tropospheric RH increases more consistently with rain rate. Lag correlations suggest that higher lower-tropospheric RH favors subsequent convection while higher upper-tropospheric RH is a result of previous or current convection. Convective available potential energy and surface wind speed have weak negative and positive relationships to rain rate, respectively. A strong relationship between surface wind speed (a proxy for latent heat flux) and rain rate has been recently observed in the eastern Pacific. It is suggested that in the KWAJEX region, this relationship is weaker because there are strong zonal gradients of vertically integrated water vapor. The strongest surface winds tend to be easterlies, so that strong surface fluxes are accompanied by strong dry-air advection from the east of Kwajalein. These two effects are of opposite sign in the moist static energy budget, reducing the tendency for strong surface fluxes to promote rainfall.

Connections between the summer east African and Indian rainfall regimes

Rainfall over parts of east Africa and India is observed to be positively correlated and associated with variations in the low‐level height field over the northern Indian Ocean and the Arabian Peninsula. A regional climate model is used to understand the mechanisms of this covariability. SST anomalies in the Arabian Sea are imposed to generate anomalies to analyze. Variations in the monsoon trough, which is located over east Africa extending eastward to southeastern Asia, regulate the connection between east Africa and India and mediate the precipitation covariability in the model. When the monsoon trough is weak, northern Ethiopia and western India are dry, in association with a weaker Somali jet, but rainfall is enhanced over southern Ethiopia. Over east Africa, rainfall decreases over northern Ethiopia are related to increased dry air advection from the north. The atmospheric response when the monsoon trough is strong is not the opposite of the weak trough simulation as conditions are wetter over West India, but drier over all of Ethiopia. Hydrodynamical processes including latent heating are more influential in the vicinity of the western Indian Ocean when the monsoon is perturbed by warm Arabian Sea SSTAs.

The 500 and 1000 hPa weather type circulations and their relationship with some extreme climatic conditions over southern South America

AbstractDifferent circulation types for southern South America are derived from the circulation anomaly maps at 500 hPa corresponding to a 9 year period. The series of daily maps was obtained from the ECWMF reanalysis. These data are particularly useful, since real data, especially the radiosonde network, are sparse in this region. The properties to be studied are selected in such a way that they describe different flow conditions over the region, in order to obtain a statistical diagnosis useful for modelling an objective forecast.The circulation types were obtained by a correlation–map‐based pattern classification technique. Lund's method is used in this paper to identify the most frequent circulation patterns.The classification method was applied to daily Z anomalies at 500 hPa over the whole record (1980–88). This allows one to analyse the evolution and presence of a particular type over different months and years. The most outstanding circulation‐type structures, represented by only eight types, explain about 63% of the total number of cases in the sample. The most frequent type, Type 1 (Z), shows an almost zonal circulation with a strong meridional gradient, associated with a trough in the west of the region. Type 2 (LCE) is represented by a low‐pressure system in the centre of the region (approximately at 37° S, 65° W). In general, this type is related to the occurrence of blocking situations in the South Atlantic Ocean and the passage of cold fronts over the region. Types 3 (SW) and 4 (WNW) show a SW and WNW atmospheric circulation over the whole region with a meridional gradient lower than in Type 1. Type 5 (TNS) shows an NW–SE trough axis over the continent. Type 6 (WW) has an intense SW flow over the southern part of the continent. Type 7 (NW) shows a very deep trough to the west of the continent, located over the Pacific Ocean at 80° W and 34° S. The continent is affected by a NW atmospheric circulation. Type 8 (R) represents a ridge over the continent with an intense NW flow in the south. The frequency of Types 1 to 8 is about 63%. None of the circulation types mentioned accounts for more than 12% of the sample. There is a considerable variability in monthly circulation type distributions; the first six types only account for approximately 50% of the variability. This suggests that the use of probability models related to circulation type occurrence in the middle troposphere should be analysed on a monthly basis.The results attempt to summarize the region's synoptic regime through a small number of circulation types at 500 hPa and the corresponding 1000 hPa maps. The frequency distribution of each type during warm, neutral and cold phases in the equatorial Pacific Ocean was analysed. These results partially explain the best dynamic conditions (cyclonic vorticity), for the Argentine humid and semi‐humid region, southern Brazil, Paraguay and Uruguay, that lead to the harvest precipitation events in spring (October–December 1982) with SOI < 0. In winter these circulation types are more important during cold phases with advection of cold and dry air (July–September 1982).The circulation types at 500 hPa and the corresponding maps at 1000 hPa associated with mean maximum and minimum monthly temperature and precipitation in the region are studied. The monthly maximum and minimum precipitation recorded in stations near the Uruguay River basin in southeastern South America and the mean monthly maximum and minimum temperatures from three stations in Argentina during 1980–88 are used in the analysis. Some of the daily patterns are strongly associated with the occurrence of maximum and minimum monthly temperature and precipitation in different places of the country. In reference to the maximum and minimum precipitation records, the behaviour of the variables under study is observed to be physically consistent with the associated circulation patterns. Copyright © 2003 Royal Meteorological Society