Characterization and variability of Kiremt rainy season over Ethiopia

Summary Ethiopia has been ravaged by severe drought for many of the last 35 years, primarily due to the failure of its main (Kiremt) rainy season in boreal summer. Kiremt quality results from the timing of its onset and cessation and the frequency and duration of intervening dry-spells. To address these key aspects of Kiremt variability, we analyzed specially constructed sets of research quality, daily rainfall and rawinsonde data for the longest available periods (25–57 years).The analyses produced wide-ranging results of considerable value to Ethiopia. The long-term average spatial progression of the southwest-to-northeast Kiremt onset and its reverse cessation are documented, along with measures of their interannual variability. Treated on a similar geographical basis is the local vulnerability to Kiremt interruption by dry-spells. Rawinsonde data for central Ethiopia are analyzed to place these long-term mean surface Kiremt characteristics in the context of the annual cycles of tropospheric wind, temperature, and pressure.Investigation of the rich interannual Kiremt variation focuses on onset, cessation, and growing length (which excludes dry-spells) anomalies. The analyses begin with the compositing of indicative tropospheric profiles for sets of extremely dry and wet Kiremt seasons. This is followed by examination of 1961–99 time series of the above Kiremt parameters, which prompts case study investigations of the highly contrasting 1984 (very dry) and 1996 (much wetter) Kiremts in terms of both Ethiopian rainfall and the tropospheric circulation of the surrounding region. Finally, correlation analyses are used to investigate relations between the above key Kiremt parameters for the most drought-prone (northeastern) part of the Kiremt region and global tropical-subtropical sea surface temperature patterns, including the ENSO phenomenon.     

Intraseasonal variability in South America during the cold season

Intraseasonal (IS) variability in South America is analyzed during the cold season using 10–90 day bandpass filtered OLR anomalies (FOLR). IS variability explains a large percentage of variance with maximum values over Paraguay, northeastern Argentina, and southern Brazil. The leading pattern of FOLR, as isolated from an EOF analysis, (Cold Season IS pattern, CSIS), is characterized by a monopole centered over southeastern South America (SESA) with a northwest-southeast orientation. CSIS induces a large modulation on daily precipitation anomalies, especially on both wet spells and daily precipitation extremes, which are favored during positive (wet) CSIS phases. Large-Scale OLR anomalies over the tropical Indian and west Pacific Oceans associated with CSIS exhibit eastward propagation along tropical latitudes. In addition, circulation anomalies in the Southern Hemisphere reveal the presence of an anticyclonic anomaly over Antarctica with opposite-sign anomalies in middle latitudes 10 days before CSIS is maximum as well as evidence of Rossby wave-like patterns. Positive precipitation anomalies in SESA are favored during wet CSIS phases by the intensification of a cyclonic anomaly located further south, which is discernible over the southeastern Pacific for at least 14 days before CSIS peaks. The cyclonic anomaly evolution is accompanied by the intensification of an upstream anticyclonic anomaly, which remains quasi-stationary near the Antarctica Peninsula before the CSIS peak. We speculate that the stationary behavior of the anticyclonic center is favored by a hemispheric circulation anomaly pattern resembling that associated with a negative southern annular mode phase and a wavenumber 3–4 pattern at middle latitudes.     

江南雨季降水季节内演变及其年际、年代际变化特征

利用1961—2008年逐日降水资料,在对比我国东南部各地区气候态降水特征的基础上,着重探讨了江南地区(110~120°E、24~30°N)雨季降水的季节内变化特征及其年际、年代际变化规律。结果表明:1)江南雨季气候态降水的季节内变化具有明显的双峰型特征,两个峰值集中期分别是4、6月中旬前后。4月中下旬第一个降水峰值率先出现在江南地区,之后峰值降水南移,于6月上中旬华南地区达峰值集中期,之后强降水才逐渐北移,6月中下旬又回至江南地区,使江南地区降水达第二个峰值集中期。2)我国江南地区区域平均的双峰降水与4—6月的实际降水之间的相关系数达0.69,这表明双峰型降水确实反映了江南雨季降水的季节内演变特征。3)江南雨季降水双峰型的季节内变化特征具有明显的年际、年代际变化周期。年际变化周期为2~3 a,强信号主要集中在20世纪60年代后期到70年代中期以及80年代中期到21世纪初;年代际变化周期约为8~10 a,在整个时间域上都存在,最强信号集中在80年代初到90年代末期。4)年代际尺度上,江南雨季降水的季节内变化特征(双峰型态)具有隔代显著的特征,即20世纪60、80年代及21世纪初双峰型特征显著,而20世纪70、90年代双峰型特征不显著。     

华南季风槽暴雨特征分析

利用NCEP再分析资料、地面观测资料和GDAS资料,对2018年8月27日—9月1日广东受季风低压影响发生的超历史极值、持续性特大暴雨天气过程的水汽输送特征进行了详细分析,同时利用Hysplit后向轨迹模式对水汽来源进行了诊断分析。结果表明：持续性特大暴雨过程期间,我国华南沿海为北半球的水汽汇合区,水汽主要来源于印度洋,经印度半岛北上至青藏高原南部向东转进入华南上空;另一部分水汽来源于西北太平洋和南海地区,三支水汽汇聚于华南上空,建立了稳定、持续的水汽输送通道,使得此次特大暴雨过程范围广、持续时间长。降水发生前期水汽辐合中心位于华南东部沿海,29日开始逐渐向西移动,于夜间达到峰值,水汽辐合最为明显,31日夜间其中心进一步西移并趋于减弱;水汽通量势函数高值区的变化与此次过程中降水峰值的逐日变化对应良好。逐日水汽辐合表现出明显的日变化特点,白天水汽辐合减弱,夜间明显加强,此次持续性特大暴雨过程呈现出季风降水特征。华南区域南边界是主要的水汽输入边界,且水汽输入主要集中在低层,尤其是华南中东部南边界的水汽输入量持续较高;29日夜间开始华南区域南边界的水汽输入量明显增大,30日达到最大,与大范围大暴雨和特大暴雨的区域及时段基本吻合。

     

Caribbean Sea rainfall variability during the rainy season and relationship to the equatorial Pacific and tropical Atlantic SST

The present study investigates the Caribbean Sea rainfall variability during the early and late rainy seasons and its association with sea surface temperature (SST) and air?Csea interaction based on observational estimates, the NCEP Climate Forecast System (CFS) and Global Forecast System (GFS) simulations, and the CFS retrospective forecasts. Analysis of the observational estimates indicates that air?Csea interaction is important over the Caribbean Sea, whereas the atmospheric forcing of SST dominates over the Gulf of Mexico. The CFS simulation captures the basic elements of this observed air?Csea relationship. The GFS simulation produces spurious SST forcing of the atmosphere over the Gulf of Mexico largely due to prescribing SST. The CFS forecasts capture the air?Csea relationship in the late rainy season (August?COctober), but cannot reproduce the SST forcing of atmosphere over the Caribbean Sea in the early rainy season (May?CJuly). An empirical orthogonal function (EOF) analysis indicates that the leading modes of percent anomalies of the rainy season precipitation have the largest loading in the southern Caribbean Sea in observations. The model simulations and forecasts skillfully reproduce the spatial pattern, but not the temporal evolution. The Caribbean Sea rainfall variability in the early rainy season is mainly due to the tropical North Atlantic (TNA) SST anomalies in observations, is contributed by both the TNA and eastern equatorial Pacific (EEP) SST anomalies in the CFS simulation, and has an overly large impact from the EEP SST anomalies in the GFS simulation and the CFS forecasts. The observed Caribbean Sea rainfall variability in the late rainy season has a leading impact from the EEP SST anomalies, with a secondary contribution from the TNA SST anomalies. In comparison, the model simulations and forecasts overestimate the impacts of the EEP SST anomalies due to an earlier development and longer duration of the El Ni?o-Southern Oscillation in the CFS compared to observations.     

华北汛期大尺度降水条件的年代际变化

利用中国740站逐日降水资料和NCEP/NCAR逐日再分析资料,使用合成分析等方法,分析了华北汛期大尺度降水条件的年代际变化。结果表明：以1978年为界,华北汛期异常水汽先由南边界和西边界供应,后改变为由北边界和东边界供应;水汽收支由异常辐合和盈余,改变为辐散和亏损;先前能够到达华北北部甚至接近华北最北边界的暖湿气团,改变为后来只能抵达黄河南岸;并且沿着太行山走向的冷暖空气的相互作用也由强变弱;华北上空由异常上升运动,改变为异常下沉运动;区域平均的对流层涡度的垂直分布,由先前的两层结构（低层正涡度、高层负涡度）改变为后来的三层结构（对流层中低层负涡度、中高层正涡度和高层负涡度）,整层涡度效应值也由大变小。尽管华北区域平均的散度和垂直速度,在垂直方向上的结构没有发生明显的年代际变化,但是整层散度效应值和垂直速度值均由大变小。     

Regional variability of convection over northern India during the pre-monsoon season

In general, the overall differences in activity and timing of convection are a result of the influence of large-scale regional and synoptic flow patterns on the local mesoscale environment. The linkage between the space?Ctime variability of observed clouds and rainfall, with large-scale circulation patterns and mesoscale variables over north India during the pre-monsoon season (March to May) is the focus of this paper. We use harmonic analysis to identify the first hour of rainfall for 42 stations spread over the north Indian region during the pre-monsoon summer season (March to May), from 1980 to 2000. The variability is observed to be systematic, with large regions having similar timing for occurrence of rainfall. The stations located in the foothills of the Himalayas have a late night to early morning maximum of first hour rainfall. In the northwestern plains, the first hour of rainfall mostly starts in the early afternoon to evening hours. Further eastward, the rainfall occurs in the late evening hours. Overall, there is a gradient in the occurrence of first rainfall events from late afternoon hours in the southern sections of the north Indian region to nocturnal maxima in the higher altitude regions. Five of these stations, located in different regions of homogenous timing of rainfall occurrence, were selected to analyze in detail the variable trigger for convection. Our results indicate that convective episodes occur mostly in association with the passage of westerly troughs over this region. These upper atmosphere troughs enable moisture to flow from the surrounding oceanic regions to the dry inland regions and also provide some dynamic support to the episodes of convection. However, the actual occurrence of convection is triggered by local factors, giving rise to the mesoscale structure of the weather systems during this season. Specifically, over the plains of northwest India, convection is triggered in a moistened environment by diurnal solar heating. The late night to early morning convection over the foothills is triggered by the orography, when the moistened airflow is normally incident on the mountain slopes. Further eastward, the primary trigger for localized moist convection is downdrafts from south-eastward propagating convective systems that originate at a north?Csouth dry line over north India. These systems propagate with a speed of about 15?m?s^?1. The above results are supported by geostationary satellite brightness temperature data for March to May 2008.     

An analysis of onset date and rainy season duration over Zambia

Summary This study investigates the onset and cessation dates of the main summer rainy season over Zambia, their interannual variability, and potential relationships with ENSO and regional circulation anomalies. Focus is placed on onset and cessation dates because these rainy season characteristics are often of more relevance than seasonal rainfall totals to user groups such as farmers, water resource managers, health and tourism officials. It is found that there is substantial interannual variability in these parameters with some indications of a relationship between anomalies in onset date and those in Ni?o3.4 SST, particularly over the northern part of the country. A strong gradient exists between the south and the north in terms of rainfall amount, mean onset date and mean cessation date and all areas of the country experience significant variability. Analysis of circulation anomalies for early (late) onset seasons over northern Zambia shows that they are characterised by anomalous ridging (troughing) over and south of South Africa, a weaker (stronger) Angola heat low and enhanced (reduced) low level moisture flux into eastern Zambia from the Indian Ocean. The connection with ENSO during the onset season of austral spring appears to arise both through changes in the amount of subsidence over southern Africa as well as via the so-called Pacific South America pattern that extends across the South Pacific and South Atlantic towards southern Africa. Authors’ address: S. Hachigonta, C. J. C. Reason, M. Tadross, Department of Oceanography, University of Cape Town, Private Bag, Rondebosch 7701, South Africa.     

Determining rainy season onset and retreat over Nigeria from precipitation amount and number of rainy days

Summary This study assesses the relative efficiency of the use of rainfall amount and rainy days in the determination of rainfall onset and retreat dates in Nigeria based on rainfall data for the period 1961 to 2000. Daily rainfall data were sourced from the archives of the Nigerian Meteorological Services, Oshodi Lagos. The specific locations for which data were collected are: Ibadan, Ilorin, Kaduna and Kano. The method of percentage cumulative mean rainfall values was employed in the determination of the rainfall onset and retreat dates. The results obtained show that both rainfall amount and rainy days are equally effective in the determination of the mean rainfall onset and retreat dates in Nigeria. With regards to the rainfall onset and retreat dates of the individual years however, the method based on the rainy days is more effective than that based on rainfall amount, as the former yielded more realistic dates than the latter. It is thus recommended that studies investigating rainfall onset and retreat dates within a series of individual years in Nigeria, should be based on rainy days rather than rainfall amount.     

On the duration of the rainy season over different parts of India

Summary Applying a new criterion, based on the average monthly precipitation, the start and the cessation dates of rains over different parts of India have been determined using data from 466 widely-spread stations. The duration of rains or the rainy season is found to vary from nil to the entire year in some parts of the country. Based on the duration of the rainy season and the amount of precipitation six precipitation zones have been demarcated in the country and the utility of this demarcation for practical purposes is discussed. The relationship between the annual precipitation variability and the duration of the rainy season is also discussed.

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Zusammenfassung Unter Verwendung eines neuen Kriteriums, das auf der mittleren Monatssumme beruht, wurden Beginn und Ende der Regenzeit mit Hilfe der Daten von 466 über dem Land verteilten Stationen für verschiedenen Teile Indiens bestimmt. Die Extreme lagen bei keiner bis zu einer ganzjährigen Regenzeit für einzelne Landesteile. Über Dauer und Niederschlagsmenge der Regenzeit wurden sechs Niederschlagszonen unterschieden. Die Brauchbarkeit dieser Einteilung für praktische Anwendungen sowie die Beziehung zwischen der Schwankung der Niederschlagsmenge und der Dauer der Regenzeit werden besprochen.

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With 8 Figures     

A distinction between summer rainy season and summer monsoon season over the Central Highlands of Vietnam

Theoretical and Applied Climatology - The daily rainfall data at 13 stations over the Central Highlands (CH) Vietnam were collected for the period 1981–2014. Two different sets of criteria...     

Seasonal transition of summer rainy season over indochina and adjacent monsoon region

The mean onset and withdrawal of summer rainy season over the Indochina Peninsula were investigated using 5-day averaged rainfall data (1975-87). The mean seasonal transition process during onset and retreat phases in Indochina, India and the South China Sea is also examined using 5-day mean OLR (1975-87) and 850 hPa wind (1980-88) date. It was found that the onset of summer rainy season begins earlier in the inland region of Indochina (Thailand) in late April to early May than in the coastal region along the Bay of Bengal. This early onset of rainy sea-son is due to pre-monsoon rain under the mid-latitude westerly wind regime. The full summer monsoon circulation begins to establish in mid-May, causing active convective activity both over the west coast of Indochina and the cen-tral South China Sea. In case of withdrawal, the earliest retreat of summer rainy season is found in the central northern part of Indochina in late September. The wind field, on the other hand, already changes to easterlies in the northern South China Sea in early September. This easterly wind system covers the eastern part of Indochina where post-monsoon rain is still active. In late October, the wind field turns to winter time situation, but post monsoon rain still continues in the southern part of the Indochina Peninsula until late November     

Intraseasonal SST-precipitation relationship and its spatial variability over the tropical summer monsoon region

The SST-precipitation relationship in the intraseasonal variability (ISV) over the Asian monsoon region is examined using recent high quality satellite data and simulations from a state of the art coupled model, the climate forecast system version 2 (CFSv2). CFSv2 demonstrates high skill in reproducing the spatial distribution of the observed climatological mean summer monsoon precipitation along with its interannual variability, a task which has been a conundrum for many recent climate coupled models. The model also exhibits reasonable skill in simulating coherent northward propagating monsoon intraseasonal anomalies including SST and precipitation, which are generally consistent with observed ISV characteristics. Results from the observations and the model establish the existence of spatial variability in the atmospheric convective response to SST anomalies, over the Asian monsoon domain on intraseasonal timescales. The response is fast over the Arabian Sea, where precipitation lags SST by ~5 days; whereas it is slow over the Bay of Bengal and South China Sea, with a lag of ~12 days. The intraseasonal SST anomalies result in a similar atmospheric response across the basins, which consists of a destabilization of the bottom of the atmospheric column, as observed from the equivalent potential temperature anomalies near the surface. However, the presence of a relatively strong surface convergence over the Arabian Sea, due to the presence of a strong zonal gradient in SST, which accelerates the upward motion of the moist air, results in a relatively faster response in terms of the local precipitation anomalies over the Arabian Sea than over the Bay of Bengal and South China Sea. With respect to the observations, the ocean–atmosphere coupling is well simulated in the model, though with an overestimation of the intraseasonal SST anomalies, leading to an exaggerated SST-precipitation relationship. A detailed examination points to a systematic bias in the thickness of the mixed layer of the ocean model, which needs to be rectified. A too shallow (deep) mixed layer enhances (suppress) the amplitude of the intraseasonal SST anomalies, thereby amplifying (lessening) the ISV and the active-break phases of the monsoon in the model.     

青藏高原地表感热与华北夏季降水的相关分析

利用1956-2002年青藏高原地表感热状况,1957-2002年我国华北地区104站月降水资料,分析了青藏高原地表感热与华北夏季(7、8月)降水的可能联系.结果表明,青藏高原4月感热与华北地区夏季降水具有较显著的负相关,高关键区位于高原的北部.于是,进一步计算了高原关键区与华北各地区夏季降水的相关系数,影响最显著的地区主要位于内蒙古的东南部以及山东南部.最后通过计算得出华北南部与青藏高原北部地区有较好的相关性.     

Determination of the onset dates of the rainy season in central Amazon with equatorially antisymmetric outgoing longwave radiation

The present work provides a new methodology to determine onset dates of the rainy season (ONR) in central Amazon (CAM) using the antisymmetric in relation to the equator outgoing longwave radiation (OLR) (AOLR) data, for the 1979–2006 period. Spatial averages of the AOLR ( $\overline {AOLR} $ ) over the CAM for the ONR periods are obtained. These periods correspond to 25 pentads centered on the mean pentad for the ONR. The sign changes from positive to negative of the $\overline {AOLR} $ for the ONR periods indicate the transition from dry to wet season. Composites of several variables are done for pentads before and after the ONR dates. These composites show physically consistent features. The potential of the $\overline {AOLR} $ time series as an index for monitoring tasks is analyzed. The results here show that the $\overline {AOLR} $ for the ONR period captures the transition from dry to wet conditions in the CAM area during 2006. The advantages of this method are discussed. The new simple method proposed here seems to be efficient in determining the ONR in the CAM.     

Spatiotemporal variability of methane over the Amazon from satellite observations

The spatiotemporal variability of the greenhouse gas methane(CH_4) in the atmosphere over the Amazon is studied using data from the space-borne measurements of the Atmospheric Infrared Sounder on board NASA's AQUA satellite for the period 2003–12. The results show a pronounced variability of this gas over the Amazon Basin lowlands region, where wetland areas occur. CH_4 has a well-defined seasonal behavior, with a progressive increase of its concentration during the dry season, followed by a decrease during the wet season. Concerning this variability, the present study indicates the important role of ENSO in modulating the variability of CH_4 emissions over the northern Amazon, where this association seems to be mostly linked to changes in flooded areas in response to ENSO-related precipitation changes. In this region, a CH_4 decrease(increase) is due to the El Nino-related(La Ni ?na-related) dryness(wetness). On the other hand, an increase(decrease) in the biomass burning over the southeastern Amazon during very dry(wet) years explains the increase(decrease) in CH_4 emissions in this region. The present analysis identifies the two main areas of the Amazon, its northern and southeastern sectors, with remarkable interannual variations of CH_4. This result might be useful for future monitoring of the variations in the concentration of CH_4, the second-most important greenhouse gas, in this area.     

Numerical simulation of the diurnal cycle of rainfall in SW Amazon basin during the 1999 rainy season: the role of convective trigger function

In continental areas, the maximum rainfall simulated with the Brazilian developments on the Regional Atmospheric Modeling System (BRAMS) occurs around 4?h earlier than the one observed with rain gauges. This work presents the successful implementation of a new convective trigger function (CTF) in the convective parameterization scheme used in BRAMS that corrects this misfit between model and observations. The importance of the CTF formulation on the diurnal cycle of rainfall over the Amazon Basin is reflected by the following numbers: Over Rondonia (SW Amazonia), the original version of BRAMS simulates the maximum rainfall at 1400 UTC (1000 LST), with the new CTF maximum shifting to 1800?UTC (1400?LST), while the S-band radar rainfall maximum is at 1900?UTC (1500?LST). This is attributed to two factors: (1) the new CTF is now coupled to the sensible and latent heat fluxes at surface; (2) during the early morning, the convective available potential energy is reduced.     

2012年春季宜春地区降水期间高空风的频谱分析

利用地面和梯度塔的风连续观测数据进行的频谱分析已经开展很多,而受资料限制,高空风的频谱分析仍较欠缺。本文使用风廓线雷达获取的长时间序列连续测风数据,运用傅里叶变换的方法,计算了风的脉动谱密度。脉动谱能够反映不同频率的风速涨落对风动能的贡献。使用2012年4月江西宜春前汛期期间的高空风连续数据,结合地面降水资料进行了1 000～3 000 m高度区间的频谱分析,发现地处前汛期雨带上的宜春地区降水存在着两种不同时间周期的天气系统影响,脉动谱的分布表现出时间周期为5～7 d和2～3 d的峰值区。分别对两种不同时间周期的天气系统频谱进行了分析,并与平稳天气时的频谱进行比较。5～7 d周期峰区的脉动谱密度数值为2～3 d的4～5倍,脉动谱峰区在2 000～3 000 m高度上较强,峰值强度向下迅速降低;2～3 d周期的脉动谱峰区在低层比较明显,峰值强度较弱。风的脉动谱分布与地面降水的时间周期较为吻合。