Variability of rainfall characteristics over the South Coast region of South Africa

Thirty years of daily rainfall data are analysed for the South Coast region of South Africa, a region which experiences substantial rainfall variability and frequent severe drought and flood events, but whose climate variability has not been much researched. It is found that El Niño–Southern Oscillation (ENSO) exerts an influence since most wet years correspond to mature phase La Niña years. ENSO also influences South Coast rainfall via increases in the number of cut-off lows in southern South Africa during mature phase La Niña years. A statistically significant correlation between the Niño 3.4 index and monthly rainfall totals, and between this index and the frequency of wet days, exists for two summer months and also for June. There are also changes in the heavy rainfall day frequencies from one decade to another. Examination of NCEP re-analyses indicates that wet (dry) years result from an equatorward (poleward) shift in the subtropical jet, cyclonic (anticyclonic) pressure anomalies over the South Atlantic and South Africa, and increased (decreased) density of mid-latitude cyclonic systems.     

Attributes of cut-off low induced rainfall over the Eastern Cape Province of South Africa

Cut-off low (COL) weather systems that are associated with rainfall over the Eastern Cape are considered in this study. COLs are objectively identified and tracked over a 31-year period. Daily rainfall data of 22 evenly distributed stations over the Eastern Cape are utilized. Only COLs with a minimum spatial distribution, defined as more than a third of the rainfall stations that need to report rainfall on at least 1 day of a COL event, are considered for analysis of rainfall attributes. These attributes include the occurrence of COL rain days of different magnitudes, the distribution of the depth and temperature of the COL centres for the rain days of different magnitudes, the associated spatial distribution of rainfall as well as the associated atmospheric circulation. The frequency of COLs over the Eastern Cape has a winter maximum and a summer minimum. COL rain days of small, medium and large magnitudes occur most frequently during the winter, while small- and medium-magnitude COL rain days experience peaks in autumn and spring, respectively. The low-level flow, and in particular the position of the low/trough, seems to be the determinant factor in the occurrence, magnitude and spatial extent of COL-induced rainfall.     

Classification of persistent heavy rainfall events over South China and associated moisture source analysis

Persistent heavy rainfall events (PHREs) over South China during 1981–2014 were selected and classified by an objective method, based on the daily precipitation data at 752 stations in China. The circulation characteristics, as well as the dry-cold air and moisture sources of each type of PHREs were examined. The main results are as follows. A total of 32 non-typhoon influenced PHREs in South China were identified over the study period. By correlation analysis, the PHREs are divided into three types: SC-A type, with its main rainbelt located in the coastal areas and the northeast of Guangdong Province; SC-B type, with its main rainbelt between Guangdong Province and Guangxi Region; and SC-C type, with its main rainbelt located in the north of Guangxi Region. For the SC-A events, dry-cold air flew to South China under the steering effect of troughs in the middle troposphere which originated from the Ural Mountains and West Siberia Plain; whereas, the SC-C events were not influenced by the cold air from high latitudes. There were three water vapor pathways from low-latitude areas for both the SC-A and SC-C PHREs. The tropical Indian Ocean was the main water vapor source for these two PHRE types, while the South China Sea also contributed to the SC-C PHREs. In addition, the SC-A events were also influenced by moist and cold air originating from the Yellow Sea. Generally, the SC-C PHREs belonged to a warm-sector rainfall type, whose precipitation areas were dominated by southwesterly wind, and the convergence in wind speed was the main reason for precipitation.     

华南夏季12-30 d持续性强降水的低频特征分析

利用1982-2011年夏季（5-8月）中国气象观测站点逐日降水资料、NCEP/NCAR逐日再分析资料、NOAA逐日向外长波辐射和海表温度资料集,通过选取低频降水事件的方法,分析了华南夏季12-30 d持续性强降水事件的基本特征,然后利用位相合成法对持续性强降水期间伴随的低频大气环流型以及低频信号的来源和传播情况进行研究,同时也分析了低频海-气耦合过程对持续性强降水的影响。结果表明：（1）华南夏季降水具有显著的12-30 d低频振荡特征,持续性强降水事件在6月发生次数最多,低频降水期间的雨带自东南向西北传播。（2）在持续性强降水发生期间,华南及邻近海域低层受强大的低频气旋式环流控制,低频上升运动显著,而中国南海-菲律宾海一带则是强的低频反气旋式环流,其西侧向北的低频水汽输送不断将中国南海的水汽送至华南及邻近海域进行辐合上升。低层的低频信号来源于热带西太平洋和中国南海-菲律宾海一带低频振荡的西北向传播,同时伴随着西太平洋副热带高压明显的西伸东退过程。（3）在高层,华南北侧（22°-45°N,95°-130°E）区域强大的低频气旋式环流和孟加拉湾-中国南海一带的低频反气旋式环流相互配合,使华南高层处于强大的辐散环境中,从而加强了华南低层的辐合与低频上升运动,造成持续性强降水的增强。高层的低频信号来源于低频罗斯贝波列的东南向传播。（4）低频大气环流异常通过云辐射和热通量过程改变低频海表温度异常,而由大气强迫的低频海表温度异常通过影响低层大气的稳定性来对大气施加明显的反馈作用,该海-气耦合过程有利于大气低层低频信号向华南地区传播,从而影响了华南持续性强降水的发生、发展与结束。     

A diagnostic study of two heavy rainfall events in South China

South China is prone to heavy rainfall which may occur both in the pre-monsoon and the monsoon season. The responsible synoptic systems and the water-vapor sources, however, can be substantially different for different seasons. In this study, we aim to develop conceptual models for typical heavy rainfall events in South China through diagnostic case studies. A number of events have been analyzed using the NCEP/NCAR data, but the discussions here are focused on two representative events, one for the pre-monsoon season and the other for the monsoon season. Both events are found to be associated with extensive moist convective instability in the lower part of the troposphere. For the pre-monsoon case the instability was much weaker and the uplift of the warm moist air was provided by the cold air intrusion related to a weak cold front. The moist potential vorticity theory can be used to explain the increase of vorticity and vertical velocity in the lower part of the troposphere. For the monsoon event, the lower troposphere was extensively and strongly unstable and the upward motion was provided by the shear of a southwesterly low-level jet (LLJ) which appeared to be driven, at least partially driven, by the upper-level jet. In both events, LLJ played a major role in not only providing the dynamic conditions but also the supply of water vapor for heavy rainfall in South China. The diagnostic results presented in this study provide a useful guidance for future numerical simulations.     

用TRMM资料研究1998年长江流域暴雨

热带测雨卫星ＴＲＭＭ资料于１９９８年６月起向公众用户开放。本文使用了１９９８年７月２０日世界时２１：４０分的ＴＲＭＭ资料对长江流域暴雨作了初步研究。ＴＲＭＭ导出的降水产品亦与雨量计，地面雷达观测以及数值模拟结果进行了比较。由此得出，ＴＲＭＭ资料非常适合于暴雨监测研究。此外，它们还有其他广泛得用途。     

SST variability in the South Indian Ocean and associated circulation and rainfall patterns over Southern Africa

Summary A general circulation model is used to study the response of the atmosphere to an idealised sea surface temperature (SST) anomaly pattern (warm throughout the southern midlatitudes, cool in the tropics) in the South Indian Ocean region. The anomaly imposed on monthly SST climatology captures the essence of patterns observed in the South Indian Ocean during both ENSO events and multidecadal epochs, and facilitates diagnosis of the model response. A previous study with this anomaly imposed in the model examined differences in the response between that on the seasonal scale (favours enhancement of the original SST anomaly) and that on the decadal scale (favours damping of the anomaly). The current study extends that work firstly by comparing the response on the intraseasonal, seasonal and interannual scales, and secondly, by assessing the changes in the circulation and rainfall over the adjoining African landmass.It is found that the atmospheric response is favourable for enhancement of the original SST anomaly on scales up to, and including, annual. However, as the scale becomes interannual (i.e., 15–21 months after imposition of the anomaly), the model response suggests that damping of the original SST anomaly becomes likely. Compared to the shorter scale response, the perturbation pressure and wind distribution on the interannual scale is shifted poleward, and is more reminiscent of the decadal response. Winds are now stronger over the warm anomaly in the southern midlatitudes suggesting enhanced surface fluxes, upper ocean mixing, and consequently, a damping of the anomaly.Examination of the circulation and rainfall patterns indicates that there are significant anomalies over large parts of southern Africa during the spring, summer and autumn seasons for both short (intraseasonal to interannual) and decadal scales. It appears that rainfall anomalies are associated with changes in the advection of moist tropical air from the Indian Ocean and its related convergence over southern Africa. Over eastern equatorial Africa, the austral autumn season (the main wet season) showed rainfall increases on all time scales, while parts of central to eastern subtropical southern Africa were dry. The signals during summer were more varied. Spring showed generally dry conditions over the eastern half of southern Africa on both short and decadal time scales, with wet areas confined to the west. In all cases, the magnitude of the rainfall anomalies accumulated over a 90 day season were of the order of 90–180 mm, and therefore represent a significant fraction of the annual total of many areas. It appears that relatively modest SST anomalies in the South Indian Ocean can lead to sizeable rainfall anomalies in the model. Although precipitation in general circulation models tends to be less accurately simulated than many other variables, the model results, together with previous observational work, emphasize the need for ongoing monitoring of SST in this region.With 14 Figures     

A note on: Estimation of rainfall due to squall lines over West Africa using meteosat imagery

Summary The results of the first step of a project to develop a method to estimate precipitation over the Soudano-Sahelian belt of West Africa are reported.The study has been performed over the period from 10 June to 9 July 1986 using hourly METEOSAT infrared images. 122 individual cloud clusters associated with squall lines or tropical storms have been tracked. For each event, the time variations of a convection index giving the volume of cloud cooler than –40°C has been determined every hour. The convection index exhibits a strong diurnal cycle. From daily rainfall amounts obtained at about 300 stations, and assuming a time apportion of rainfall within a cloud cluster, the time variations of the hourly total rainfall produced by the cluster can be determined and represents the precipitation index. Because of insufficient rainfall, the precipitation index has been determined for only 17 events. For 2/3 of the 17 cases, there is a significant correlation between the two indices. For each of the 17 events, precipitation has been regressed on the associated convective index and relative time variations. In that case, the results indicate that a convective index representing the life history of the cloud cluster can be calibrated with corresponding raingage measurements provided raingage data are available. Then, estimation of rainfall due to that event over data void regions can be obtained. However, this study shows that no universal relationship exists between precipitation: no rainfall can be estimated if there is not enough raingage measurements to construct a precipitation index for a tracked cloud cluster. This represents a limitation to the method.With 9 Figures     

Performance assessment of three convective parameterization schemes in WRF for downscaling summer rainfall over South Africa

Austral summer rainfall over the period 1991/1992 to 2010/2011 was dynamically downscaled by the weather research and forecasting (WRF) model at 9 km resolution for South Africa. Lateral boundary conditions for WRF were provided from the European Centre for medium-range weather (ECMWF) reanalysis (ERA) interim data. The model biases for the rainfall were evaluated over the South Africa as a whole and its nine provinces separately by employing three different convective parameterization schemes, namely the (1) Kain–Fritsch (KF), (2) Betts–Miller–Janjic (BMJ) and (3) Grell–Devenyi ensemble (GDE) schemes. All three schemes have generated positive rainfall biases over South Africa, with the KF scheme producing the largest biases and mean absolute errors. Only the BMJ scheme could reproduce the intensity of rainfall anomalies, and also exhibited the highest correlation with observed interannual summer rainfall variability. In the KF scheme, a significantly high amount of moisture was transported from the tropics into South Africa. The vertical thermodynamic profiles show that the KF scheme has caused low level moisture convergence, due to the highly unstable atmosphere, and hence contributed to the widespread positive biases of rainfall. The negative bias in moisture, along with a stable atmosphere and negative biases of vertical velocity simulated by the GDE scheme resulted in negative rainfall biases, especially over the Limpopo Province. In terms of rain rate, the KF scheme generated the lowest number of low rain rates and the maximum number of moderate to high rain rates associated with more convective unstable environment. KF and GDE schemes overestimated the convective rain and underestimated the stratiform rain. However, the simulated convective and stratiform rain with BMJ scheme is in more agreement with the observations. This study also documents the performance of regional model in downscaling the large scale climate mode such as El Niño Southern Oscillation (ENSO) and subtropical dipole modes. The correlations between the simulated area averaged rainfalls over South Africa and Nino3.4 index were ?0.66, ?0.69 and ?0.49 with KF, BMJ and GDE scheme respectively as compared to the observed correlation of ?0.57. The model could reproduce the observed ENSO-South Africa rainfall relationship and could successfully simulate three wet (dry) years that are associated with La Niña (El Niño) and the BMJ scheme is closest to the observed variability. Also, the model showed good skill in simulating the excess rainfall over South Africa that is associated with positive subtropical Indian Ocean Dipole for the DJF season 2005/2006.     

Mesoscale Analysis of a Heavy Rainfall Event over Hong Kong During a Pre-rainy Season in South China

During the Heavy Rainfall Experiment in South China （HUAMEX） of 1998, a record heavy rainfall event occurred in the delta of the Pearl River during the 24 hours from 1200 UTC 8 June to 1200 UTC 9 June, 1998, and a 24-hour precipitation maximum of 574 mm was reported in Hong Kong. In this paper, some mesoscale characteristics of this heavy rainfall event are studied using data from satellites, Doppler radar, wind profilers, and automatic meteorological stations collected during HUAMEX. The following conclusions are drawn： （1） During this heavy rainfall event, there existed a favorable large-scale environment, that included a front with weak baroclinity in the heavy rain area and with an upward motion branch ahead of the front. （2） Unlike most extratropical or subtropical systems, the closed low in the geopotential height field does not exited. The obvious feature was that a southerly branch trough in the westerlies existed and Hong Kong was located ahead of the trough. （3） The rainfall areas were located in the warm sector ahead of the front, rather than in the frontal zone, which is one of the characteristics of heavy rainfalls during the pre-rainy season of South China. A southerly warm and moist current contributed to the heavy rainfall formation, including the transportation of rich water vapor and the creation of strong horizontal wind convergence. （4） The observations show that the heavy rainfall in Hong Kong was directly caused by a series of meso β systems rather than a mesoscale convective complex （MCC）. These meso β systems moved with the steering current in the lower-mid troposphere, their life cycles were 3-6 hours, and their horizontal sizes were 10-100 km. （5） The disturbances in the lower and mid troposphere, especially that in the planetary boundary layer （PBL） were very shallow. However, they are a possible trigger mechanism for the occurrence and development of the mesoscale convective systems and related heavy rainfalls. Finally, a conceptual model of the heav     

Simulation of the annual and diurnal cycles of rainfall over South Africa by a regional climate model

The capability of a current state-of-the-art regional climate model for simulating the diurnal and annual cycles of rainfall over a complex subtropical region is documented here. Hourly rainfall is simulated over Southern Africa for 1998–2006 by the non-hydrostatic model weather research and forecasting (WRF), and compared to a network of 103 stations covering South Africa. We used five simulations, four of which consist of different parameterizations for atmospheric convection at a 0.5 × 0.5° resolution, performed to test the physic-dependency of the results. The fifth experiment uses explicit convection over tropical South Africa at a 1/30° resolution. WRF simulates realistic mean rainfall fields, albeit wet biases over tropical Africa. The model mean biases are strongly modulated by the convective scheme used for the simulations. The annual cycle of rainfall is well simulated over South Africa, mostly influenced by tropical summer rainfall except in the Western Cape region experiencing winter rainfall. The diurnal cycle shows a timing bias, with atmospheric convection occurring too early in the afternoon, and causing too abundant rainfall. This result, particularly true in summer over the northeastern part of the country, is weakly physic-dependent. Cloud-resolving simulations do not clearly reduce the diurnal cycle biases. In the end, the rainfall overestimations appear to be mostly imputable to the afternoon hours of the austral summer rainy season, i.e., the periods during which convective activity is intense over the region.     

Diagnosing GCM errors over West Africa using relaxation experiments. Part I: summer monsoon climatology and interannual variability

The CNRM atmospheric general circulation model Arpege-Climat is relaxed towards atmospheric reanalyses outside the 10°S?C32°N 30°W?C50°E domain in order to disentangle the regional versus large-scale sources of climatological biases and interannual variability of the West African monsoon (WAM). On the one hand, the main climatological features of the monsoon, including the spatial distribution of summer precipitation, are only weakly improved by the nudging, thereby suggesting the regional origin of the Arpege-Climat biases. On the other hand, the nudging technique is relatively efficient to control the interannual variability of the WAM dynamics, though the impact on rainfall variability is less clear. Additional sensitivity experiments focusing on the strong 1994 summer monsoon suggest that the weak sensitivity of the model biases is not an artifact of the nudging design, but the evidence that regional physical processes are the main limiting factors for a realistic simulation of monsoon circulation and precipitation in the Arpege-Climat model. Sensitivity experiments to soil moisture boundary conditions are also conducted and highlight the relevance of land?Catmosphere coupling for the amplification of precipitation biases. Nevertheless, the land surface hydrology is not the main explanation for the model errors that are rather due to deficiencies in the atmospheric physics. The intraseasonal timescale and the model internal variability are discussed in a companion paper.     

Analysis of spatial and temporal extreme monsoonal rainfall over South Asia using complex networks

We present a detailed analysis of summer monsoon rainfall over the Indian peninsular using nonlinear spatial correlations. This analysis is carried out employing the tools of complex networks and a measure of nonlinear correlation for point processes such as rainfall, called event synchronization. This study provides valuable insights into the spatial organization, scales, and structure of the 90th and 94th percentile rainfall events during the Indian summer monsoon (June–September). We furthermore analyse the influence of different critical synoptic atmospheric systems and the impact of the steep Himalayan topography on rainfall patterns. The presented method not only helps us in visualising the structure of the extreme-event rainfall fields, but also identifies the water vapor pathways and decadal-scale moisture sinks over the region. Furthermore a simple scheme based on complex networks is presented to decipher the spatial intricacies and temporal evolution of monsoonal rainfall patterns over the last 6 decades.