Convective Cloud Clusters Research Articles

庆阳市连续2天短时暴雨特征及成因分析

利用Micaps常规资料以及区域气象站降水观测资料，对2016年7月9~11日庆阳市短时暴雨天气过程的环流背景、中尺度特征及物理指数进行了分析。结果表明：1) 连续2天短时暴雨发生在稳定的大陆高压脊中。低槽切变线形成中低层辐合，东部型南亚高压前部急流形成高空强辐散，高、低空系统耦合使暴雨区有较强上升运动。台风“尼伯特”和副热带高压作用下，庆阳市水汽辐合，湿层明显增厚，切变线触发高温高湿能量，造成暴雨天气。2) 地面中尺度辐合线是触发暴雨的重要因子，其与强降水有很好的对应关系。3) “7.9”过程为中小尺度对流云团在降水区的更替造成，中高层 “干侵入”、低层暖平流明显，降水强度大。“7.10”过程为中小尺度对流云团合并为中尺度对流云带经过降水区造成，强降水影响范围广，持续时间长。4) 通过分析两次过程, 发现高层显著冷平流，中低层暖平流环境，以及稳定增强的高湿高能和上干下湿的不稳定层结均有利于庆阳短时暴雨的产生。 Using the Micaps conventional data and regional meteorological stations precipitation observa-tions, the author analyzed circulation background, mesoscale characteristics and physical index of short-time rainstorm in Qingyang from 9 to 11 July 2016. The results are as follows: 1) The short-time rainstorm on 2 consecutive days occurred in the stable continental anticyclone. A convergence at mid-low level was formed by the trough shear line, and a strong divergence in the upper was formed by jet on the eastern of South Asia High, and an ascend movement was formed over the rainstorm area by coupling of high-low system. The water vapor convergence and wet layer obviously thickened in Qingyang under the interaction of Typhoon Nepartak and the subtropical high, and the shear line triggered high temperature and high humidity energy, and caused rainstorm weather. 2) The surface mesoscale convergence line was an important factor in triggering rainstorm that corresponded well with short-time strong rainfall. 3) “7.9” rainstorm was caused by mesoscale convective cloud clusters replacement in the precipitation area. Dry intrusion in middle-high level and warm advection at low level were obvious, so that stronger precipitation intensity formed. “7.10” rainstorm was caused by mesoscale convective cloud clusters that were merged into the mesoscale convective cloud zone through precipitation area, so that it had wider range of influence and longer time duration. 4) Through two analysis processes, the author found upper significant cold advection, warm advection at lower environment, stably enhanced high humidity energy, dry and caused unstable stratification were advantageous to short-time rainstorm in Qingyang.

引发甘肃白银市严重山洪地质灾害的暴雨过程分析

利用常规气象观测资料、自动站逐小时雨量、物理量、NCEP再分析资料和卫星云图等资料，对2011年5月28日发生在甘肃白银市的局地暴雨引发山洪天气过程进行诊断分析。结果表明：1)这次暴雨天气过程发生在西伯利亚冷涡东移的大尺度环流背景下，冷涡底部不断有分裂的冷槽携带干冷空气和西太平洋副热带高压西北侧的西南暖湿气流交汇，大气不稳定度增大，触发对流产生。2)700hpa低涡、切变线和850 hpa鞍形场环流配置，有利于耦合导致上升运动加强，触发不稳定能量的释放，成为暴雨天气的中尺度影响系统。3)散度场、涡度场、垂直速度场等物理量场的高低空配置和时空变化与暴雨过程的时空变化高度吻合。4)地面辐合线或干线是触发强对流天气的因子之一，对流单体一般生成于地面辐合线附近，在地面辐合线与干线结合处易于对流单体或云团的新生和发展。5)引发山洪的暴雨强度大，持续时间短，在卫星云图上表现为一个水平尺度200~400 km，云顶亮温最低为−80℃的中β尺度对流云团。6)山洪发生在地质条件脆弱区，不同的地质状况对应不同的灾害。 Based on the data of routine meteorological observation, hourly precipitation of automatic weather station and NCEP reanalysis data, the local rainstorm in Baiyin of Gansu province on 28 May 2011 was analyzed. The results show that this rainstorm occurred on large scale circulation background of the Siberia cold vortex moved eastward; the dry cold air which constantly split from the bottom of the cold vortex converged with southwest airflow of warm; the atmospheric instability increased that triggered convective weather. The meso-scale influential systems of this rainstorm were vortex and shear line of 700 hPa and saddle type of 850 hPa. This circulation pattern was beneficial to strengthen ascending movement and trigger off instability energy releasing. The allocation at different heights and temporal and spatial variation of physical field was consistent with the rainstorm. Surface convergence line was one of triggering factors contributed to convection. The convection cells were easily formed near the joint of surface convergence line and dry line. The characteristics of rainstorm which caused mountain torrent geology disaster were high intensity and short duration. There was a meso-β-scale convective cloud cluster in satellite cloud image. Its horizontal scale was 200 - 400 km and the minimum of TBB was −80˚C. The mountain torrent occurred in the geological conditions of vulnerable areas, different geological conditions can form different disasters.

Characteristics of cirrus clouds in the tropical lower stratosphere

A unique type of cloud in the tropical lower stratosphere, which we call “stratospheric cirrus”, is described in this study. Stratospheric cirrus clouds are generally detached from overshooting deep convection and are much smaller than subvisual cirrus often observed near the tropical tropopause. We analyzed two cases of stratospheric cirrus in the tropical and subtropical lower stratosphere captured by the space-borne lidar, Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). Both cases occurred 2–3 hours after the most active phase of the nearby convective cloud clusters. Case 1 has a double-layer structure above the cold point height (CPH); the CPH and two cloud top heights are, respectively, 17.8, 18.9, and 19.9km. Case 2 has a single cloud layer where CPH and the cloud top height are, respectively, 16.5 and 18.7km. The mode radius and ice water content of the stratospheric cirrus clouds are estimated to be 4–10μm and 0.2–0.8mg/m3 based on the radar-lidar method and consideration of the cloud particle terminal velocity. Comparisons with previous numerical model simulation studies suggest that the double-layer stratospheric cirrus clouds are likely from an overshooting plume, pushed up into the stratosphere in an overshoot when warm stratospheric air is inhomogeneously mixed with cold overshooting air. The single-layer stratospheric cirrus cloud is associated with some non-negligible wind shear, so it could be a jumping cirrus cloud, although we cannot rule out the possibility that it came from an overshooting plume because of the similarity in cloud characteristics and morphology between the two cases. Guided by the case studies, an automatic algorithm was developed to select stratospheric cirrus clouds for global survey and statistical analysis. A total of four years of CALIPSO and space-borne cloud radar (CloudSat) data were analyzed. Statistical analysis suggests that stratospheric cirrus clouds occur on the order of 3.0×103 times a year between 30 °S and 30 °N. Many of the stratospheric cirrus clouds are found in the pre-monsoon season in the South and Southeast Asia, where convection is deep and intense.

Possible causes of the variation in fractal dimension of the perimeter during the tropical cyclone Dan motion

We calculated the fractal dimensions D b of the perimeter of tropical cyclone (TC) Dan based on the satellite GMS-5 infrared sensor images from 1800 UTC, 1 October 1999 to 1200 UTC, 9 October 1999. The fractal dimensions D b were used to characterize objectively the temporal change of TC complex structure. Our results show that the change of fractal dimension during TC Dan motion can be divided into three stages. The statistically significant difference does not exist either between D m1 and D L or between D m3 and D L, but it exists between D m2 and D L, where D mi denotes the mean value of D b in i-th stage (i=1, 2 and 3); D L denotes Lovejoy’s fractal dimension calculated based on satellite and radar data within the size range (1−1.2×106 km2), which is used as a “normal value” of the fractal dimension of the cumulus cloud perimeter for the global tropical region. TC Dan turns to the north from the west abruptly at the end of the second stage. The emergence of the second stage with high fractal dimensions may be viewed as a possible premonition for the track turning. Our results also show that there are two kinds of processes resulting in the translation from the first stage to the second stage. One is the interaction of TC circulation and an adjacent small scale convective cloud cluster, causing to the complexity increase of a local segment of the perimeter. The other includes the fragmentation of a strong convective area within the TC inner region, the self-organization of the small strong convective cloud clusters, the emergence, development, and merger of the small scale non-convective holes, and the formation of a gap of the perimeter, causing to the complexity increase of the whole TC perimeter.

Impact of South China Sea Cold Surges and Typhoon Peipah on Initiating Cyclone Sidr in the Bay of Bengal

In the present study, an attempt was made to understand the role of South China Sea (SCS) convection associated with northerly cold surges and Typhoon Peipah in initiating Cyclone Sidr in the Bay of Bengal (BoB). The variation of air sea fluxes during the entire history of Cyclone Sidr tracking before its landfall over Bangladesh was also studied. The presence of cold surges in the north SCS associated with heavy rainfall episodes has been noticed at the southern Gulf of Tonkin coast prior to the formation of Typhoon Peipah. Subsequently, these surges migrated south, which resulted in intensification of a deep convection on reaching the Vietnamese coast. During the same period in the western Pacific, Typhoon Peipah developed, propagating in the westward direction and entering the SCS. Analysis of geostationary water vapour images, mean sea level pressure, and surface wind maps clearly depicted the transport of convective cloud clusters, moisture, and westward momentum from Typhoon Peipah to the deep convection cells over the SCS. Consequently, the existing deep convection over the Vietnamese coast resulted in a westward direction and entered the Gulf of Thailand and Andaman Sea. The availability of higher latent heat fluxes, warmer sea surface temperatures, and suitable atmospheric conditions over this region favoured the formation of a tropical depression in the Andaman Sea. This depression further intensified in the southeast BoB, resulting in the formation of Cyclone Sidr. NCEP/NCAR wind fields and air-sea fluxes revealed left asymmetry surface winds and higher latent heat flux on the left side of the track during the intensification phase of Sidr.

Origin of the pre-tropical storm Debby (2006) African easterly wave-mesoscale convective system

The origins of the pre-Debby (2006) mesoscale convective system (MCS) and African easterly wave (AEW) and their precursors were traced back to the southwest Arabian Peninsula, Asir Mountains (AS), and Ethiopian Highlands (EH) in the vicinity of the ITCZ using satellite imagery, GFS analysis data and ARW model. The sources of the convective cloud clusters and vorticity perturbations were attributed to the cyclonic convergence of northeasterly Shamal wind and the Somali jet, especially when the Mediterranean High shifted toward east and the Indian Ocean high strengthened and its associated Somali jet penetrated farther to the north. The cyclonic vorticity perturbations were strengthened by the vorticity stretching associated with convective cloud clusters in the genesis region—southwest Arabian Peninsula. A conceptual model was proposed to explain the genesis of convective cloud clusters and cyclonic vorticity perturbations preceding the pre-Debby (2006) AEW–MCS system.

A Clustering Approach to Compare Cloud Model Simulations to Satellite Observations

Abstract Cloud-resolving models (CRMs) offer an important pathway to interpret satellite observations of microphysical properties of storms. High-frequency microwave brightness temperatures (Tbs) respond to precipitating-sized ice particles and can therefore be compared with simulated Tbs at the same frequencies. By clustering the Tb vectors at these frequencies, the scene can be classified into distinct microphysical regimes (in other words, cloud types). A convective storm over the Amazon observed by the Tropical Rainfall Measuring Mission (TRMM) is simulated using the Regional Atmospheric Modeling System (RAMS) in a semi-ideal setting, and four regimes are defined within the scene using cluster analysis: the “clear sky/thin cirrus” cluster, the “cloudy” cluster, the “stratiform anvil” cluster, and the “convective” cluster. Cluster-by-cluster comparisons between the observations and the simulations disclose biases in the model that are consistent with an overproduction of supercooled water and an excess of large hail particles. While other problems cannot be completely ruled out, the method does provide some guidance to assess microphysical fidelity within each cluster or cloud type. Guided by the apparent model/observational discrepancies in the convective cloud cluster, the hail size parameter was adjusted in order to produce a greater number of smaller hail particles consistent with the observations. While the work cannot define microphysical errors in an unambiguously fashion, the cluster analysis is seen as useful to isolate individual microphysical inconsistencies that can then be addressed within each cluster of cloud type.

USING THE METEOSAT-9 IMAGES TO THE DETECTION OF DEEP CONVECTIVE SYSTEMS IN BRAZIL

The purpose of this article is to present a simple method of identification of deep convective clouds using water vapor (WV) and thermal infrared (IR) brightness temperature differences from the multispectral images of Spinning Enhanced Visible and Infra-Red Imager (SEVIRI) sensor. The use of this method is part of an international effort to calibrate the radiances of SEVIRI sensor for microphysical properties of deep convective systems. This approach was applied to the image from 08 September 2009 for the demonstration of its efficacy analysis. The results show that the difference values larger than –2º C for BT Differences (WV6.2 μm – WV7.3 μm) and + 50º C (IR3.9 μm –IR10.8 μm) were associated with areas of intense precipitation. The method can be easily implemented and effectively utilized in operational basis to monitor deep convective cloud clusters over Brazil.

Assessment of the WRF model in reproducing a flash-flood heavy rainfall event over Korea

This study examines the ability of the cloud-resolving weather research and forecasting (WRF) model to reproduce the convective cells associated with the flash-flooding heavy rainfall near Seoul, South Korea, on 12 July 2006. A triply nested WRF model with the highest resolution of 3-km horizontal grid spacing was integrated with conventional analysis data. The WRF model simulated the initiation of isolated thunderstorms, and the formation of a convective band, cloud cluster, and squall line at nearly the right time. The corresponding precipitation simulation was also reasonably reproduced in its distribution, although the amount was underestimated. A sensitivity experiment that excludes the orography over the peninsula revealed that orographic forcing over the peninsula is responsible for about 20% increase in precipitation over the heavy rainfall region. It was identified that in addition to the up-lifting local orographic forcing to the west of the mountain range in South Korea, anticyclonic circulation due to the presence of the Gaema Heights in North Korea contribute to the confinement of convective activities in the heavy rainfall region.

Relationships between lightning and properties of convective cloud clusters

Satellite observations of convective system properties and lightning flash rate are used to investigate the ability of potential lightning parameterizations to capture both the dominant land‐ocean contrast in lightning occurrence and regional differences between Africa, the Amazon and the islands of the maritime continent. As found in previous studies, the radar storm height is tightly correlated with the lightning flash rate. A roughly second order power‐law fit to the mean radar echo top height above the 0°C isotherm is shown to capture both regional and land‐ocean contrasts in lightning occurrence and flash rate using a single set of parameters. Recent developments should soon make it possible to implement a parameterization of this kind in global models. Parameterizations based on cloud top height, convective rain rate and convective rain fraction all require the use of separate fits over land and ocean and fail to capture observed differences between continental regions.

Impact of Doppler Radar Wind in Simulating the Intensity and Propagation of Rainbands Associated with Mesoscale Convective Complexes Using MM5-3DVAR System

Pre-monsoon rainfall around Kolkata (northeastern part of India) is mostly of convective origin as 80% of the seasonal rainfall is produced by Mesoscale Convective Systems (MCS). Accurate prediction of the intensity and structure of these convective cloud clusters becomes challenging, mostly because the convective clouds within these clusters are short lived and the inaccuracy in the models initial state to represent the mesoscale details of the true atmospheric state. Besides the role in observing the internal structure of the precipitating systems, Doppler Weather Radar (DWR) provides an important data source for mesoscale and microscale weather analysis and forecasting. An attempt has been made to initialize the storm-scale numerical model using retrieved wind fields from single Doppler radar. In the present study, Doppler wind velocities from the Kolkata Doppler weather radar are assimilated into a mesoscale model, MM5 model using the three-dimensional variational data assimilation (3DVAR) system for the prediction of intense convective events that occurred during 0600 UTC on 5 May and 0000 UTC on 7 May, 2005. In order to evaluate the impact of the DWR wind data in simulating these severe storms, three experiments were carried out. The results show that assimilation of Doppler radar wind data has a positive impact on the prediction of intensity, organization and propagation of rain bands associated with these mesoscale convective systems. The assimilation system has to be modified further to incorporate the radar reflectivity data so that simulation of the microphysical and thermodynamic structure of these convective storms can be improved.

The Role of Cloud Radiative Forcing in the Asian-Pacific Summer Monsoon

Convective-cloud clusters with strong precipitation occur frequently in most of the Asian-Pacific summer monsoon (APSM) regions such as the Bay of Bengal (BOB), South China Sea (SCS), and Tropical Western North Pacific (TWNP). Cloud radiative forcing (CRF) is important in these regions. The net CRF at the top of the atmosphere (TOA) has shown large cooling over these APSM regions. This is on account of the presence of large amounts of high clouds with large optical depth. Through data analysis, the summer convective precipitation in TWNP is as strong as that in the BOB. However, the average net CRF at the TOA in the BOB (~ -36 Wm -2 ) is twice as big as in the TWNP (~ -17 Wm -2 ). The spectral analysis of cloud optical depth shows that in the BOB, the highest power is in the intra-seasonal timescale, while in the TWNP, the leading spectral peaks are less than 10 days. The radiative cooling from net CRF at the TOA could be associated with lowfrequency oscillation. The difference between the APSM regions is related to their sub-stages separating from CRF in time evolution. In a convective system, convective clouds can detrain to form other high clouds. In the APSM regions, large areas of high-thin and high-thick clouds cause different CRF at the TOA. These two types of CRF relate to precipitation, atmospheric vertical motion, and cloud life cycles etc. and should be separated from the APSM time evolution. We divided the APSM precipitation into two categories. As in the heavy-precipitation stage, clouds with large optical depth shield solar radiation and cause local and instantaneous surface cooling. The outgoing longwave radiation (OLR) is generally lower than 210 (Wm -2 ). The net CRF at the TOA is large negatively.

A case study on a strong tropical disturbance and record heavy rainfall in Hat Yai, Thailand during the winter monsoon

The evolutionary process and structural characteristics of the atmospheric circulation and synoptic situation which caused the record heavy rainfall with a precipitation amount of 550 mm in Hat Yai, Thailand from 20 to 23 November 2000 is studied. In the study, the modern three dimensional observational data were collected as completely as possible, and detailed analyses were made. It is revealed that the cold surges of the Asian winter monsoon that originate from Siberia can arrive at the lower latitudes, including South Thailand, Malaysia, Indonesia, cause strong heavy rainfall there, and interact with weather systems in the near-equatorial regions of the Southern Hemisphere. This is strongly supported by Chinese scientist’s original finding in 1930s. The strong convective cloud clusters in the above areas are generated by the direct influence of the cold surges, and are related with the South China Sea disturbances in the lower troposphere. The maximum of the convergence of total moisture flux near South Thailand in the situation under study implies that the water vapour supply is abundant and very favorable to the occurrence of the heavy rainfall. The release of latent heat enhances the Hadley Circulation also. The feedback of the strong severe weather on climate indeed exists, and there are pronounced interactions between the multi-scale systems and between both hemispheres.

A diagnostic study of heavy rainfall in karachi due to merging of a mesoscale low and a diffused tropical depression during South Asian summer monsoon

This paper presents the results of a diagnostic study of a typical case of very heavy rainfall during the South Asian summer monsoon when a mesoscale low in a desert climate merged with a diffused tropical depression. The former low was located over Pakistan’s desert region and the latter depression originated over the Bay of Bengal. Surface and NCEP reanalysis data supported by satellite and radar images were incorporated in the diagnosis. The relationship between the heavy precipitation process and large-scale circulations such as monsoon trough, subtropical high, westerly jet, low level jet and water vapor transport were investigated to further understand the mechanism of this peculiar interaction. It was found that: (1) the mesoscale low developed as a result of cold air advection aloft from northern latitudes and strong convection over the region of humidity convergence on 24 July 2003 over the Indian Rajistan area. (2) On the same day, a low that formed over the Bay of Bengal was transformed into a monsoon depression and moved westward to the mesoscale low which existed over southwest India and the adjoining southeastern parts of Pakistan. (3) Initially, the mesoscale low received moisture supply from both the Bay of Bengal as well as the Arabian Sea, whereas the Bay of Bengal maintained the continuous supply of moisture to the monsoon depression. (4) After the depression crossed central India, the Bay’s moisture supply was cut off and the Arabian Sea became the only source of moisture to both the closely located systems. On 27 July, both of the systems merged together and the merger resulted in a heavy downpour in the Karachi metropolitan and in its surroundings. (5) With the intensification as well as the southeastward extension of the subtropical high and the shift of the monsoon trough axis from southwest-west to northeast-east, the monsoon depression moved southwestward. In this situation, there existed a very favourable condition for a merger of the two systems in the presence of cross-latitude influence. (6) A number of convective cloud clusters were developed and organized in the mesoscale low. Probably, interactions existed among the multi-scale systems.

NOTES AND CORRESPONDENCE Moistening Processes in the Upper Troposphere by Deep Convection: A Case Study over the Tropical Indian Ocean

Moistening processes in the upper troposphere are examined by relating upper tropospheric humidity (UTH) estimates from SSM/T-2 and Meteosat measurements to convective clouds over the Indian Ocean. Results are compared against NCAR/CCM3 outputs. The analysis separating the tropics into four cloud areas, i.e., deep convective, middle cloud, thin cirrus, and clear/low cloud area, indicates that the upper troposphere above the clear/low cloud area becomes drier (moister) in response to increases (decreases) in convective activity in the tropics. On the other hand, the area between deep convective cloud clusters, and clear/low cloud area, also appears to be moistened by thin cirrus originating from deep convective clouds. Although the CCM3 model reproduces the average number of cloud clusters found in the satellite observations, the model-derived UTH shows significantly different features, i.e., a drier upper troposphere within deep convective clusters, a much higher UTH over the clear/low cloud area, and a slight increase of UTH over the clear/low cloud area with respect to increases in the deep convective area, suggesting that the model needs more accurate physics for better description of the moistening/drying processes.

Radiative forcing of aerosols and gases over tropical ocean and its impact on the global atmospheric environment: GCM investigations during INDOEX campaigns

Radiative forcings due to aerosols and the pollutant gases accumulated as haze which are transported from nearby continent to the tropical ocean are essentially important elements of the world climate system. Vertical transport of aerosols and gaseous species takes place within the deep convective cloud clusters of the inter-tropical convergence zone and subsequently these are distributed by the upper atmospheric zonal wind flow, thereby have impact on the global atmospheric environment. The comprehensive global atmospheric models have shown capability of simulating the climate of the atmosphere with proper forcing. We have deduced the radiative forcing, optical depth and the global energy balance components by a global atmospheric model. The results are validated well with INDOEX and other available observational findings.

Global and Regional Diurnal Variations of Organized Convection

Abstract An automated objective classification procedure, the Convection Classification and Automated Tracking System (CCATS), is used to analyze the mean life cycles of organized convection in the global Tropics and midlatitudes (40°N–40°S). Five years (1989–93) of infrared satellite imagery are examined for the Pacific and Atlantic basins and one year (April 1988–March 1989) is studied for the Indian basin. Two main classes of organized convection (lifetime of 6 h or more) are tracked: MCT and CCC. MCT represent a combined dataset of tropical cyclones and mesoscale convective complexes (MCC). Convective cloud clusters (CCC) meet the same cold cloud-top temperature, time, and size criteria used to distinguish MCC, but fail to sustain the same high degree of symmetry for at least 6 h. That is, CCC represent more elongated systems, such as squall lines. The frequency of CCC exceeds that of MCT by a factor of 30 over both land and sea. MCT and CCC are each stratified to into 12 continental and oceanic regions and the diurnal variation of system characteristics in each geographic region are studied, leading to composite life cycle descriptions for each region. Oceanic CCC formed overnight and the shorter-lived, land-based CCC formed in the afternoon; apart from this time offset, oceanic and land-based CCC were found to have very similar life cycle evolution patterns. Continental MCT exhibit a rapid size expansion early; this is not part of the oceanic system life cycle. Apart from this growth spurt, the evolution of land and ocean MCT follows the same pattern of CCC with early symmetry, then size expansion until just before termination. Land-based MCT are longer lived and more symmetric than oceanic MCT.

Three Late Summer/Early Autumn Cases of Tropical–Extratropical Interactions Causing Precipitation in Northwest Africa

In contrast to the winter rain-dominated region along the Atlantic and Mediterranean coasts in northwest Africa, the semiarid to arid southern foothills of the Atlas Mountains receive significant contributions to their annual rainfall amounts from rainy episodes in late summer/early autumn. Three such cases (September 1988, September 1990, August–September 1999) are studied with respect to the sources and the vertical and horizontal transports of moisture, as well as local factors for precipitation generation. Besides station reports of precipitation, the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses and Meteosat water vapor images are considered. All three cases presented reveal similar tropical–extratropical interactions. Convective cloud clusters or squall lines over tropical West Africa and the adjacent tropical Atlantic Ocean, several of them associated with low-level African easterly waves, could be identified as midlevel moisture source regions by the use of trajectory analysis. The moisture is transported northward to the east of an mid- to upper-level subtropical trough, which extends anomalously deep into the Tropics. Most of the transport occurs above the dry Saharan planetary boundary layer. The moisture converges at midlevels (700–400 hPa) over northwestern Africa underneath a strong upper-level divergence center at the inflection point of the trough. The dynamically forced ascent in connection with orographic lifting at the Atlas Mountains in the southerly flow and surface heating over the elevated terrain triggers convective rainfalls, which occur preferably close to and downwind of the mountain chain. The three cases differ with respect to the synoptic evolution of the upper-level subtropical trough and the paths of the moisture export from the Tropics. At the end of the episode in September 1988, the tropical air over northwest Africa is displaced by polar air connected with some heavy rainfall events. The presented cases are compared to studies of tropical plumes and Soudano–Saharan depressions.

A comprehensive physical pattern of land-air dynamic and thermal structure on the Qinghai-Xizang Plateau

According to the boundary layer observations of three stations (Garze, Damxung and Qamdu) and relevant earth satellite, radiosonde and surface observations during the intensive observational period (IOP) of the second Tibetan (Qinghai-Xizang) Plateau Experiment of atmospheric science (TIPEX), the land-air physical process and dynamic model on the Tibetan Plateau were comprehensively analyzed in this study. The dynamic characteristics of boundary layer and the rules of turbulent motion on the plateau were illustrated. The characteristics of distributions of wind speed and direction with mutiple-layer structure and deep convective mixed layer on the plateau, the strong buoyancy effect in turbulent motion on the plateau on which the air density is obviously smaller than on the plain, and the Ekman spiral and its dynamic pump effect of the plateau deep boundary layer have been found. The local static distribution of water vapor and the horizontal advection of water vapor in the plateau boundary layer were studied. The abnomal thermodynamic structure on the plateau surface and boundary layer, including the plateau strong radiation phenomenon and strong heating source characteristics of the middle plateau, was also analyzed. The authors synthesized the above dynamic and thermodynamic structures of both surface and boundary layers on the plateau and posed the comprehensive physical model of the turbulence and convective mixture mechanism on the plateau boundary layer. The characteristics of formation, development and movement for convective cloud cluster over the plateau influencing floods in the Yangtze River area of China were studied. The conceptual model of dynamic and thermodynamic structures of turbulent motion and convective plume related to the frequent occurrence of “pop-corn-like” cloud system is given as well.

Quantitative flood forecasting using multisensor data and neural networks

Accurate quantitative forecasting of rainfall for basins with a short response time is essential to predict streamflow and flash floods. Previously, neural networks were used to develop a Quantitative Precipitation Forecasting model that highly improved forecasting skill at specific locations in Pennsylvania, using both Numerical Weather Prediction output and rainfall and radiosonde data. The objective of this study was to modify the existing artificial neural network model to include the evolving structure and frequency of intense weather systems in the mid-Atlantic region of the United States for improved flood forecasting. Besides using radiosonde and rainfall data, the model also used the satellite-derived characteristics of storm systems such as tropical cyclones, mesoscale convective complex systems, and convective cloud clusters as input. The convective classification and automated tracking system was used to identify and quantify storm properties such as life time, area, eccentricity, and track. As in standard expert prediction systems, the fundamental structure of the neural network model was learned from the hydroclimatology of the relationships among weather systems, rainfall production and streamflow response in the study area. Here, we present results from the application of the quantitative flood forecasting model in four watersheds on the leeward side of the Appalachian mountains in the mid-Atlantic region. The areal extent of the watersheds ranges from 750 to 8700 km 2. The reduction in the mean-squared error of the peak streamflow with respect to persistence was up to 60% for the 24 h lead-time forecasts. For the 18 h lead-time forecasts, the number of successful forecasts for streamflow peaks in the upper 5% percentile was consistently above 60%, and close to 80–90%.