Westerly Regime Research Articles

Longer Duration of the Weak Stratospheric Vortex During Extreme El Niño Events Linked to Spring Eurasian Coldness

AbstractThe stratosphere is a key link between El Niño and Eurasian surface climate in winter. Instead of the amplitude of the stratospheric response discussed by many previous studies, we focus on the persistence of wintertime extreme El Niño impacts through the stratospheric pathway to midlatitude Eurasia in spring. A novel approach is used by running WACCM4 with El Niño forcing imposed only during winter to isolate the stratospheric role in the following spring. We show that a descending signal with a strong deceleration of zonal wind throughout the atmosphere could reach the surface and persist in spring during extreme El Niño events, whereas during moderate El Niño events the signal is generally confined to the stratosphere. Two different mechanisms are involved in maintaining the tropospheric signal of extreme El Niño. Under the westerly regime, the planetary Rossby waves from the wave centers over East Asia and the North Pacific transmit the signals from the stratosphere to the troposphere; and under the easterly regime, strong tropospheric eddy feedback over the midlatitude Atlantic favors the persistence of tropospheric responses during extreme El Niño events. The prolonged stratospheric pathway of extreme El Niño events amplifies and extends the equatorward shift of the midlatitude jet, allowing southward intrusions of cold polar air into Eurasia. Such circulation changes induced by a prolonged stratospheric pathway can cause springtime coldness and daily cold extremes in midlatitude Eurasia.

Quasi-biennial oscillation of the equatorial total ozone: A seasonal dependence and forecast for 2019–2021

The interannual variability of the total ozone (TOZ) in the equatorial region (5°S – 5°N) has been studied using the satellite daily and monthly data in 1978–2018. The coupling of the equatorial TOZ changes to the quasi-biennial oscillation (QBO) of the zonal wind in the equatorial stratosphere has long been known. However, the detailed analysis of the daily deseasonalized TOZ data over the course of 15 complete wind QBO cycles convincingly showed some new important features of the ozone QBO (ΔTOZ). The key points of the equatorial ΔTOZ variations are clearly linked with the certain time moments during the downward propagation from ~10 to ~70 hPa of the easterly and westerly regimes of the wind QBO. The equatorial ΔTOZ distinctly maximizes when the westerly regime reaches ~70 hPa and the easterly regime appears at ~10 hPa, which occur simultaneously. The times of minima of the equatorial ΔTOZ are less obvious due to the westerly regime appearance at ~10 hPa on the background of the unfinished descent of the easterly regime. Furthermore, the seasonal regularities of the wind QBO causes the seasonal dependence of the ozone QBO in the equatorial region. Maxima of the equatorial ΔTOZ are definitely observed near solstice months (June or December). This result contradicts to the general belief about lack of the seasonal synchronization of the equatorial ozone QBO in contrast to the seasonally synchronized extratropical ozone. However, the coincidence of the equatorial ΔTOZ maxima with solstices and, consequently, their recurrence every 2 or 2.5 years allow a prediction of the equatorial TOZ, based on the long-term forecast of the wind QBO. The ozone QBO forecast for subsequent about two years (2019–2021) is presented.

Atmospheric boundary layer height and surface parameters: Trends and relationships over the west coast of India

Trends and features of Atmospheric Boundary Layer (ABL) over southern part of the west coast of India has been studied using MERRA datasets for 1980–2018 during different seasons incorporating the marine and continental influences. A significant decrease in the ABL height (ABLH) has been observed in the westerly regimes of the west coast. The shallow ABL is expected to cause amplified response in the atmosphere and enhance the formation of extreme events over the region. It is also observed that its variation is related with other surface layer parameters such as specific humidity, temperature, surface zonal wind, Richardson number, CAPE (Convective Available Potential Energy) and sensible & latent heat fluxes. A significant increasing (decreasing) trend in specific humidity (surface zonal wind) is observed in pre-monsoon, monsoon and post monsoon seasons and this trend has significant correlation with the ABL height. The CAPE has a significant increasing trend in daytime of pre-monsoon season, which can lead to formation of intense thunderstorms. A significant reduction in the surface dynamic instability due to decrease in surface westerly zonal wind is observed and it has a significant role in the decreasing trend of ABL height. Surface sensible heat flux possesses a significant decreasing (increasing) trend during the pre-monsoon and post monsoon seasons (monsoon season). Surface latent heat flux shows a decreasing trend during the monsoon which may be attributed to the combined effect of decrease in ABL height and trapping of moisture at lower levels together with an increase in surface temperature.

Morphology of the Wavenumber 1 and Wavenumber 2 Stratospheric Kelvin Waves Using the Long-Term Era-Interim Reanalysis Dataset

The atmospheric Kelvin wave has been widely studied due to its importance in atmospheric dynamics. Since a long-term climatological study is absent in the literature, we have employed the two-dimensional fast Fourier transform (2D-FFT) method for the 40-year long-term reanalysis of the dataset, ERA-Interim, to investigate the properties of Kelvin waves with wavenumbers 1 (E1) and 2 (E2) at 6–24 days wave periods over the equatorial region of ±10° latitude between a 15 and 45 km altitude during the period 1979–2019. The spatio-temporal variations of the E1 and E2 wave amplitudes were compared to the information of stratospheric quasi-biennial oscillation (QBO), and the wave amplitudes were found to have an inter-QBO cycle variation that was related to sea surface temperature and convections, as well as an intra-QBO cycle variation that was caused by interactions between the waves and stratospheric mean flows. Also, the E1 waves with 6–10 day periods and the E2 waves with 6 days period were observed to penetrate the westerly regime of QBO, which has a thickness less than the vertical wavelengths of those waves, and the waves could further propagate upward to higher altitudes. In a case study of the period 2006–2013, the wave amplitudes showed a good correlation with the Niño 3.4 index, outgoing longwave radiation (OLR), and precipitation during 2006–2013, though this was not the case for the full time series. The present paper is the first report on the 40-year climatology of Kelvin waves, and the morphology of Kelvin waves will help us diagnose the anomalies of wave activity and QBO in the future.

How Momentum Coupling Affects SST Variance and Large-Scale Pacific Climate Variability in CESM

The contribution of buoyancy (thermal + freshwater fluxes) versus momentum (wind driven) coupling to SST variance in climate models is a longstanding question. Addressing this question has proven difficult because a gap in the model hierarchy exists between the fully coupled (momentum + buoyancy + ocean dynamics) and slab–mixed layer ocean coupled (thermal with no ocean dynamics) versions. The missing piece is a thermally coupled configuration that permits anomalous ocean heat transport convergence decoupled from the anomalous wind stress. A mechanically decoupled model configuration is provided to fill this gap and diagnose the impact of momentum coupling on SST variance in NCAR CESM. A major finding is that subtropical SST variance increases when momentum coupling is disengaged. An “opposing flux hypothesis” may explain why the subtropics (midlatitudes) experience increased (reduced) variance without momentum coupling. In a subtropical easterly wind regime, Ekman fluxes [Formula: see text] oppose thermal fluxes [Formula: see text], such that when the air and sea are mechanically decoupled [Formula: see text], [Formula: see text] variance increases. As a result, SST variance increases. In a midlatitude westerly regime where [Formula: see text] and [Formula: see text] typically reinforce each other, SST variance is reduced. Changes in mean surface winds with climate change could impact the [Formula: see text] and [Formula: see text] covariance relationships. A by-product of mechanically decoupling the model is the absence of ENSO variability. The Pacific decadal oscillation operates without momentum coupling or tropical forcing, although the pattern is modified with enhanced (reduced) variability in the subtropics (midlatitudes). Results show that Ekman fluxes are an important component to tropical, subtropical, and midlatitude SST variance.

Synoptic characteristics, atmospheric controls, and long-term changes of heat waves over the Indochina Peninsula

The characteristics of heat wave (HW) events over the Indochina Peninsula are studied using the Climate Forecast System Reanalysis data for 1979–2010. HWs in the dry and wet seasons of Indochina are considered separately, and their typical synoptic behavior is examined in detail. Our results show that HWs in both seasons are accompanied by suppressed precipitation, and the region lies under an anomalously dry, hot, and subsiding atmospheric column. Further diagnoses reveal that HWs in the dry and wet seasons are linked to the weakening of the East Asian winter monsoon (EAWM) and the South Asian summer monsoon (SASM) circulations, respectively. On the one hand, HWs in the dry season are coincident with high-temperature anomalies over East Asia and accompanied by an anomalous cyclone over eastern China. The anomalous southwesterly flow over East Asia indicates a weakening of the climatological northeasterly circulation associated with the EAWM. On the other hand, HWs in the wet season are coincident with hot and dry anomalies in South Asia (i.e., Indian subcontinent). The anomalous easterly flow in that region opposes the climatological westerly regime of the SASM. Moreover, we found that the statistics for the frequency, duration, and amplitude of HWs for both seasons exhibit prominent intensifying trends, and the intensifying trends in the wet season are mostly about two to three times stronger than the dry season. These intensifying HWs activities are likely attributable to the weakening trend of monsoon circulations in recent decades.

Contrasting Features of Monsoon Precipitation Around the Meghalaya Plateau Under Westerly and Easterly Regimes

AbstractPrecipitation features around the Meghalaya Plateau, northeast India, during summer are investigated using a 17 year (1998–2014) high‐spatial‐resolution Tropical Rainfall Measuring Mission precipitation radar data set. Precipitation around the plateau fell into two distinct regimes based on the low‐level wind direction that fluctuates on intraseasonal time scales over Bangladesh, windward of the plateau: a westerly regime (WR) and an easterly regime (ER). Under the WR, strong low‐level onshore southwesterlies across Bangladesh encounter the plateau, and localized strong low‐level southerlies running parallel to the Arakan Mountains (i.e., the barrier jet) also blow toward the plateau, concentrating convective unstable air onto its southern slopes. The low‐level wind fields and large‐scale upper level divergent fields promote frequent and intense orographic rainfall along the southern slopes due to forced uplift, generating high precipitation. In contrast, under the ER, strong southeasterlies that blow along the Gangetic Plain without encountering the plateau and subsidence inhibit upward motion around the plateau, resulting in low precipitation. Diurnal variations in precipitation significantly affect the daily precipitation around the plateau under both regimes. High rainfall frequency persists over the southern slopes between 2100 and 1200 LT of the next day under the WR, whereas modest rainfall frequency occurs between 0000 and 0600 LT under the ER, with a daytime minimum and nocturnal maximum in both regimes. The atmospheric boundary layer processes over Bangladesh regulate the wind speed and vertical structure of the low‐level wind toward the plateau, with deceleration during daytime and acceleration at night (i.e., nocturnal jet) that result in the nocturnal rainfall maximum.

A new classification of large-scale climate regimes around the Tibetan Plateau based on seasonal circulation patterns

This study aims to develop a large-scale climate classification for investigating the characteristics of the climate regimes around the Tibetan Plateau based on seasonal precipitation, moisture transport and moisture divergence using in situ observations and ERA40 reanalysis data. The results indicate that the climate can be attributed to four regimes around the Plateau. They situate in East Asia, South Asia, Central Asia and the semi-arid zone in northern Central Asia throughout the dryland of northwestern China, in addition to the Köppen climate classification. There are different collocations of seasonal temperature and precipitation: 1) in phase for the East and South Asia monsoon regimes, 2) anti-phase for the Central Asia regime, 3) out-of-phase for the westerly regime. The seasonal precipitation concentrations are coupled with moisture divergence, i.e., moisture convergence coincides with the Asian monsoon zone and divergence appears over the Mediterranean-like arid climate region and westerly controlled area in the warm season, while it reverses course in the cold season. In addition, moisture divergence is associated with meridional moisture transport. The northward/southward moisture transport corresponds to moisture convergence/divergence, indicating that the wet and dry seasons are, to a great extent, dominated by meridional moisture transport in these regions. The climate mean southward transport results in the dry-cold season of the Asian monsoon zone and the dry-warm season, leading to desertification or land degradation in Central Asia and the westerly regime zone. The mean-wind moisture transport (MMT) is the major contributor to total moisture transport, while persistent northward transient eddy moisture transport (TEMT) plays a key role in dry season precipitation, especially in the Asian monsoon zone. The persistent TEMT divergence is an additional mechanism of the out-of-phase collocation in the westerly regime zone. In addition, the climate-mean MMT and TEMT are associated with the atmospheric stationary wave and storm track, which results from the uplift of orography and land-sea thermal contrast. Therefore, the paleoclimate changes in mid-latitude arid-semi-arid regions are linked to the different phases of uplift of mountains and plate motion tied to the evolution of the Mediterranean.

Subseasonal variability of mesoscale convective systems over the tropical northeastern Pacific

Mesoscale Convective Systems (MCS) properties during large‐scale easterly and westerly regimes of the low‐level winds over the tropical northeastern Pacific (TEP) are analysed during boreal summers of years 2012–2014. East of 120°W, two regions located north and south of 10°N present different sensitivity of the MCS properties to the zonal wind regime. Long‐lasting and large MCS are favoured during the westerly regime over the northern region, while more numerous MCS, but with a steady lifetime distribution, are triggered during the easterly regime over the southern part of the Intertropical Convergence Zone (ITCZ). For long‐lasting MCS (lifetime longer than 10 h), the moderate low‐level wind shear conditions during the westerly regime are associated with larger MCS covering longer distances. The MCS cold cloud cover and the MCS lifetime tend to locally increase with the moisture of the mid‐troposphere. Conversely, the TEP area west of 120°W becomes less favourable to MCS development during the low‐level westerly regime, in accordance with the diminution of rainfall observed locally.

Trends of daily peak wind gusts in Spain and Portugal, 1961–2014

AbstractGiven the inconsistencies of wind gust trends under the widespread decline in near‐surface wind speed (stilling), our study aimed to assess trends of observed daily peak wind gusts (DPWG) across Spain and Portugal for 1961–2014 by analyzing trends of (i) the frequency (90th percentile) and (ii) the magnitude (wind speed maxima) of DPWG. Wind gust series were homogenized on a daily basis, using MM5‐simulated series as reference, resulting in 80 suitable station‐based data sets. The average DPWG 90th percentile frequency declined by −1.49 d decade−1(p < 0.05) annually. This showed marked seasonal differences: decreasing in winter (−0.75 d decade−1;p < 0.05) and increasing in summer (+0.18 d decade−1;p > 0.10). A negligible trend was calculated for the annual magnitude of DPWG (−0.005 m s−1decade−1;p > 0.10), with distinct seasonality: declining in winter (−0.168 m s−1decade−1;p < 0.10) and increasing in summer (+0.130 m s−1decade−1;p < 0.05). Combined, these results reveal less frequent and declining DPWG during the cold semester (November–April) and more frequent and increasing DPWG during the warm semester (May–October). Large‐scale atmospheric changes such as the North Atlantic Oscillation Index (negative correlations ~−0.4–−0.6;p < 0.05) and the Jenkinson and Collison scheme (positive correlations mainly with Westerly regime: ~+0.5–0.6;p < 0.05) partly account for the decadal fluctuations of both frequency and magnitude of DPWG, particularly in winter. However, the North Atlantic Oscillation index‐DPWG relationships are smaller in spring, summer, and autumn (~−0.1–−0.2;p > 0.10), especially for the frequency, suggesting the role of local‐to‐mesoscale drivers.

Radar-Derived Statistics of Convective Storms in Southeast Queensland

AbstractThe aim of this study is to examine the statistics of convective storms and their concomitant changes with thermodynamic variability. The thermodynamic variability is analyzed by performing a cluster analysis on variables derived from radiosonde releases at Brisbane Airport in Australia. Three objectively defined regimes are found: a dry, stable regime with mainly westerly surface winds, a moist northerly regime, and a moist trade wind regime. S-band radar data are analyzed and storms are identified using objective tracking software [Thunderstorm Identification, Tracking, Analysis, and Nowcasting (TITAN)]. Storm statistics are then investigated, stratified by the regime subperiods. Convective storms are found to form and maintain along elevated topography. Probability distributions of convective storm size and rain rate are found to follow lognormal distributions with differing mean and variance among the regimes. There was some evidence of trimodal storm-top heights, located at the trade inversion (1.5–2 km), freezing level (3.6–4 km), and near 6 km, but it was dependent on the presence of the trade inversion. On average, storm volume and height are smallest in the trade regime and rain rate is largest in the westerly regime. However, westerly regime storms occur less frequently and have shorter lifetimes, which were attributed to the enhanced stability and decreased humidity profiles. Furthermore, time series of diurnal rain rate exhibited early morning and midafternoon maxima for the northerly and trade regimes but were absent for the westerly regime. The observations indicate that westerly regime storms are primarily driven by large-scale forcing, whereas northerly and trade wind regime storms are more responsive to surface characteristics.

The UK’s wet and stormy winter of 2013/2014

The <scp>UK</scp>’s wet and stormy winter of 2013/2014

On the Effects of Large-Scale Environment and Surface Types on Convective Cloud Characteristics over Darwin, Australia

Abstract Two seasons of Darwin, Australia, C-band polarimetric (CPOL) research radar, radiosoundings, and lightning data are examined to study the relative influence of the large-scale atmospheric regimes and the underlying surface types on tropical convective cloud properties and their diurnal evolution. The authors find that in the “deep westerly” regime, which corresponds to the monsoon period, the convective cloud occurrence rate is highest, consistent with its highest relative humidity. However, these convective clouds have relatively low cloud-top heights, smaller-than-average cell volumes, and are electrically least active. In this regime, the cloud cell volume does not vary significantly across different underlying surfaces and afternoon convective activity is suppressed. Thus, the picture emerging is that the convective cloud activity in the deep westerly regime is primarily regulated by the large-scale conditions. The remaining regimes (“easterly,” “shallow westerly,” and “moist easterly”) also demonstrate strong dependence on the large-scale forcing and a secondary dependence on the underlying surface type. The easterly regime has a small convective cloud occurrence rate and low cloud heights but higher lightning counts per convective cloud. The other two regimes have moderate convective cloud occurrence rates and larger cloud sizes. The easterly, shallow westerly, and moist easterly regimes exhibit a strong, clearly defined semidiurnal convective cloud occurrence pattern, with peaks in the early morning and afternoon periods. The cell onset times in these three regimes depend on the combination of local time and the underlying surface.

The Role of Air–Sea Interaction for Prediction of Australian Summer Monsoon Rainfall

Abstract Forecast skill for seasonal mean rainfall across northern Australia is lower during the summer monsoon than in the premonsoon transition season based on 25 years of hindcasts using the Predictive Ocean Atmosphere Model for Australia (POAMA) coupled model seasonal forecast system. The authors argue that this partly reflects an intrinsic property of the monsoonal system, whereby seasonally varying air–sea interaction in the seas around northern Australia promotes predictability in the premonsoon season and demotes predictability after monsoon onset. Trade easterlies during the premonsoon season support a positive feedback between surface winds, SST, and rainfall, which results in stronger and more persistent SST anomalies to the north of Australia that compliment the remote forcing of Australian rainfall from El Niño in the Pacific. After onset of the Australian summer monsoon, this local feedback is not supported in the monsoonal westerly regime, resulting in weaker SST anomalies to the north of Australia and with lower persistence than in the premonsoon season. Importantly, the seasonality of this air–sea interaction is captured in the POAMA forecast model. Furthermore, analysis of perfect model forecasts and forecasts generated by prescribing observed SST results in largely the same conclusion (i.e., significantly lower actual and potential forecast skill during the monsoon), thereby supporting the notion that air–sea interaction contributes to intrinsically lower predictability of rainfall during the monsoon.

Ciclo diário da precipitação estimada através de um radar banda S e pelo algoritmo 3B42_V6 do projeto TRMM durante a estação chuvosa de 1999 no sudoeste da Amazônia

Neste artigo apresenta-se uma avaliação da precipitação estimada através do radar S-POL (banda S) e pelo algoritmo 3B42_V6 do satélite TRMM, durante o experimento do período chuvoso de 1999 do projeto TRMM-LBA na Bacia Amazônica, enfocando-se a variabilidade diária. Para tal, utilizam-se técnicas estatísticas baseadas em tabela de contingência e índices, bem como a análise harmônica. Demonstra-se que precipitação determinada com o radar é coerente com as observações de rede de pluviômetros. O ciclo diário exibe um máximo principal no final da tarde, e um pico secundário à noite, que é associado à propagação de Linhas de Instabilidades no interior da Bacia Amazônica. O algoritmo 3B42_V6 reproduz características importantes do ciclo diário de precipitação: (i) o máximo intenso às 2100 UTC (1700 HL); (ii) precipitação noturna mais intensa durante o regime de Oeste. Uma importante aplicação deste estudo é o uso dos resultados para avaliação de modelos numéricos de simulação do tempo e do clima, principalmente no que concerne à representação da convecção sobre a região.

Diurnally driven scaling properties of Amazonian rainfall fields: Fourier spectra and order‐q statistical moments

The influence of the diurnal cycle on spatial scaling properties of Amazonian rainfall fields is investigated using data gathered during the January–February 1999 Wet Season Atmospheric Meso‐scale Campaign in the state of Rondonia (Brazil, SW Amazonia). Most intense precipitation events with large spatial coverage occur during early afternoon. Amplitudes of average and maximum intensity diurnal cycles are higher during the easterly than during the westerly atmospheric regime. The diurnal cycle of average rainfall occupancy exhibits a significantly larger amplitude during the westerly regime. Storms exhibit power law Fourier spectra, E(k) = ck−β, with two scaling regimes characterized by different scaling exponents (β1 and β2), separated at a critical distance, which depends on the spatial extent of rainfall organization. Inversely correlated diurnal cycles for β1 and β2 reflect rainfall organization patterns at different spatial scales through the 24‐h period. The break occurs at smaller (larger) spatial scales during the morning (afternoon‐evening). Average values of c and β exhibit inversely related diurnal cycles, and different behavior during either atmospheric regime. Order‐q statistical moments indicate multiscaling of rainfall fields. Departures from simple scaling are also driven by the diurnal cycle, reflecting differences in convective activity and the spatial organization of rainfall throughout the 24‐h cycle. Departures from simple scaling are dependent on the moment order q. Clear‐cut differences between the estimated order‐q statistical moments appear during both atmospheric regimes. These results shed light toward linking physical processes with statistics in Amazonian storms.

Intraseasonal Variability in Diurnal Rainfall over New Guinea and the Surrounding Oceans during Austral Summer

Abstract High-resolution Tropical Rainfall Measuring Mission (TRMM) rainfall data for six wet seasons (December–March) were used to investigate the time and space structure of the diurnal cycle of rainfall over and around New Guinea, a major island of the Maritime Continent. The diurnal cycle shows a systematic modulation associated with intraseasonal variability in the large-scale circulation pattern, with regimes associated with low-level easterlies or westerlies over the island. Lower-tropospheric easterly (westerly) wind components correspond to periods of inactive (active) convection over the islands that are associated with the passage of intraseasonal atmospheric disturbances such as the Madden–Julian oscillation (MJO). A striking feature is the diurnal rainfall that develops over the central mountain ranges in the evening and propagates toward the southwest (northeast) of the island with an inferred phase speed of about 2–3 m s−1 under low-level easterly (westerly) flow. In the case of the easterly regime, diurnal rainfall is strongly concentrated over the southwestern part of the island, inhibited from spreading offshore southwest of New Guinea. Under the westerly regime, in contrast, the rainfall area spread far and wide along the low-level westerlies from the island toward the Pacific Ocean. Significant offshore rainfall propagation extending from the island appears during the night over the north-northeastern coast and moves with a phase speed of about 7–8 m s−1, reaching the open ocean the following day. Possible processes for controlling the variability in diurnal rainfall through the interaction between large-scale circulation and previously denoted complex local circulation over the island are discussed.

Improving Simulations of Convective Systems from TRMM LBA: Easterly and Westerly Regimes

Abstract The 3D Goddard Cumulus Ensemble model is used to simulate two convective events observed during the Tropical Rainfall Measuring Mission Large-Scale Biosphere–Atmosphere (TRMM LBA) experiment in Brazil. These two events epitomized the type of convective systems that formed in two distinctly different environments observed during TRMM LBA. The 26 January 1999 squall line formed within a sheared low-level easterly wind flow. On 23 February 1999, convection developed in weak low-level westerly flow, resulting in weakly organized, less intense convection. Initial simulations captured the basic organization and intensity of each event. However, improvements to the model resolution and microphysics produced better simulations as compared to observations. More realistic diurnal convective growth was achieved by lowering the horizontal grid spacing from 1000 to 250 m. This produced a gradual transition from shallow to deep convection that occurred over a span of hours as opposed to an abrupt appearance of deep convection. Eliminating the dry growth of graupel in the bulk microphysics scheme effectively removed the unrealistic presence of high-density ice in the simulated anvil. However, comparisons with radar reflectivity data using contoured-frequency-with-altitude diagrams (CFADs) revealed that the resulting snow contents were too large. The excessive snow was reduced primarily by lowering the collection efficiency of cloud water by snow and resulted in further agreement with the radar observations. The transfer of cloud-sized particles to precipitation-sized ice appears to be too efficient in the original scheme. Overall, these changes to the microphysics lead to more realistic precipitation ice contents in the model. However, artifacts due to the inability of the one-moment scheme to allow for size sorting, such as excessive low-level rain evaporation, were also found but could not be resolved without moving to a two-moment or bin scheme. As a result, model rainfall histograms underestimated the occurrence of high rain rates compared to radar-based histograms. Nevertheless, the improved precipitation-sized ice signature in the model simulations should lead to better latent heating retrievals as a result of both better convective–stratiform separation within the model as well as more physically realistic hydrometeor structures for radiance calculations.

Time–Space Characteristics of Diurnal Rainfall over Borneo and Surrounding Oceans as Observed by TRMM-PR

Abstract Five years of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data were used to investigate the time and space characteristics of the diurnal cycle of rainfall over and around Borneo, an island in the Maritime Continent. The diurnal cycle shows a systematic modulation that is associated with intraseasonal variability in the large-scale circulation pattern, with regimes associated with low-level easterlies or westerlies over the island. The lower-tropospheric westerly (easterly) components correspond to periods of active (inactive) convection over the island that are associated with the passage of intraseasonal atmospheric disturbances related to the Madden–Julian oscillation. A striking feature is that rainfall activity propagates to the leeward side of the island between midnight and morning. The inferred phase speed of the propagation is about 3 m s−1 in the easterly regime and 7 m s−1 in the westerly regime. Propagation occurs over the entire island, causing a leeward enhancement of rainfall. The vertical structure of the developed convection/rainfall system differs remarkably between the two regimes. In the easterly regime, stratiform rains are widespread over the island at midnight, whereas in the westerly regime, local convective rainfall dominates. Over offshore regions, convective rainfall initially dominates then gradually decreases in both regimes, while the storms develop into deeper convective systems in the easterly regime. Aside from leeward rainfall propagation, shallow storms develop over the South China Sea region during the westerly regime, resulting in heavy precipitation from midnight through morning.

Momentum transport processes in the stratiform regions of mesoscale convective systems over the western Pacific warm pool

AbstractMomentum transport by the stratiform components of mesoscale convective systems (MCSs) during the Tropical Ocean–Global Atmosphere Coupled Ocean–Atmosphere Response Experiment in December 1992 is investigated using a cloud‐resolving model. The mesoscale momentum transport by the stratiform regions of MCSs is examined in two distinct large‐scale flow regimes associated with the intraseasonal oscillation over the western Pacific warm pool. Model simulations for 14 December 1992 characterize the ‘westerly onset’ period, which has relatively weak low‐level westerlies with easterlies above. Simulations for 23–24 December represent the ‘strong westerly’ regime, when westerlies extend from the upper troposphere to the surface, with a jet 2–3 km above the surface. In the westerly onset simulation, the extensive stratiform region of a MCS contained a broad region of descent that transported easterly momentum associated with the mid‐level easterly jet downward. Thus, the stratiform regions acted as a negative feedback to decrease the large‐scale mean westerly momentum developing at low levels. In the strong westerly regime, the mesoscale downward air motion in the stratiform regions of large MCSs transported westerly momentum downward and thus acted as a positive feedback, strengthening the already strong westerly momentum at low levels. Momentum fluxes by the mesoscale stratiform region downdraughts are shown to have a systematic and measurable impact on the large‐scale momentum budget. Copyright © 2006 Royal Meteorological Society.