Synoptic Hours Research Articles

The Nigeria climate zones: Variability, trends and analysis from era-interim data (2010-2015)

This study analyses diurnal weather patterns in the lower atmosphere across four climate domains in Nigeria from Era-Interim data for synoptic hours of 0000, 0600, 1200 and 1800 hours from 2010-2015. The climate domain includes: Port Harcourt (tropical maritime), Enugu (bi-modal tropical continental), Jos (montane), Kano (mono-modal tropical continental) and Maiduguri (semi-arid). Findings revealed that air temperature across the areas indicate strong latitudinal positions with drier and hotter weather pattern northwards. Average air temperature across diurnal hours were 23.2-26oC, 23-29oC, 26-32oC, 26-34oC and 26.6-34 oC for Port Harcourt, Enugu, Jos, Kano and Maiduguri. Average solar radiation peaked at 1200 noon with values 508 W/m2, 639 W/m2, 830 W/m2, 905 W/m2, 832 W/m2 for the areas. Average wind speed increases northwards, i.e. 0.9-1.3m/s, 2.0-2.5m/s, 2.1-2.5m/s, 2.4-3.0m/s and 2.9-3.5m/s respectively. The wind variation is due to the surface characteristics across the areas. Southwesterly winds dominated coastal zone while southeasterly/northeasterly winds dominated the arid zone except during periods of peak rainy season. Average cloud cover for the coastal and northern domains in oktas was: 6-7 and 2-4. Relative humidity shows that prevailing atmospheric conditions at any given time are stronger at the areas close to the sources of influence and the position of Intertropical discontinuity (ITD). Humidity values were: 67-93% and 31-71% for coastal and northern domains. Rainfall amount reduces northwards i.e. 200-46 mm. Rainfall pattern is significantly influenced by the air masses source regions. These regions include closeness to massive water bodies, proximity to Sahara Desert and effects of mountainous barriers.

OCCURRENCES OF LOW CLOUDS OVER THE WESTERN MARGINS OF THE CONGO BASIN (SEPTEMBER TO DECEMBER 2021)

This study aims to analyze the occurrence of low cumuliform clouds over the western margins of the Congo Basin from September to December 2021. The imagery of the Cloud Top Height product of Meteosat Second Generation set on synoptic hours (i.e. 00 hours, 06 hours, 12 hours and 18 hours) have been retained for this study. Through these data, we have developed an observation grid of 1° over the entire Congolese territory to make an inventory of low clouds identified at each 1°. The results obtained show that the low clouds appear most often between 00 hours and 12 hours. And the spatial distribution of the occurrences of clouds attests that the low cloud cover remains more pronounced in the Southern, Southwestern and Northern parts of the country.

Generation of Non-Linear Technique Based 6 Hourly Wind Reanalysis Products Using SCATSAT-1 and Numerical Weather Prediction Model Outputs

We combined observations of ocean surface winds from Indian SCATterometer SATellite-1 (SCATSAT-1) with a background wind field from a numerical weather prediction (NWP) model available at National Centre for Medium-Range Weather Forecast (NCMRWF) to generate a 6-hourly gridded hybrid wind product. A distinctive feature of the study is to produce a global gridded wind field from SCATSAT-1 scatterometer passes with spatio-temporal data gaps at regular synoptic hours relevant for forcing models and other NWP studies. We are following the concept from the modern particle filter technique, which does not represent the model probability density function (PDF) as Gaussian. We generated the 6-hourly hybrid winds for 2018 and validated them using the wind speed from daily gridded level-4 SCATSAT-1 winds (L4AW), Cross Calibrated Multi-Platform (CCMP) dataset and global buoy data from National Data Buoy Centre (NDBC). The results suggest the potential of the technique to produce scatterometer winds at the desired temporal frequency with significantly less noise and bias along the swath. The study shows that the generated hybrid winds are of prime quality compared with the already existing daily products available from Indian Space Research Organization (ISRO).

Isı Dalgası Oluşumlarının Zamansal Analizi: Kano Metropolisi, Nijerya

Heatwaves have become one of the biggest ecological threats to the survival of people in thebiosphere. For the reason, in this study is aimed that is analyzed the temporal occurrences of heatwaves in the Kano between 2009 to 2018 so as and that it devised a protocol against for this menace. Daily maximum temperature is measured at the synoptic hours during the period of extreme heat (hot anddry). With the se temperature values were used to takeout the period of heatwaves using percentile thresholds of 95th. In 2016 it experienced high number of heatwaves occurrences with consecutive 26 days of 40.8°C. The heatwaves is occurred frequently in April and May. This situation, it implies that is identifying as the periods when the occurrence of heatwaves is intense in Kano in April and May.

Increasing the skill of short-term wind speed ensemble forecasts combining forecasts and observations via a new dynamic calibration

All numerical weather prediction models used for the wind industry need to produce their forecasts starting from the main synoptic hours 00, 06, 12, and 18 UTC, once the analysis becomes available. The 6-h latency time between two consecutive model runs calls for strategies to fill the gap by providing new accurate predictions having, at least, hourly frequency. This is done to accommodate the request of frequent, accurate and fresh information from traders and system regulators to continuously adapt their work strategies. Here, we propose a strategy where quasi-real time observed wind speed and weather model predictions are combined by means of a novel Ensemble Model Output Statistics (EMOS) strategy. The success of our strategy is measured by comparisons against observed wind speed from SYNOP stations over Italy in the years 2018 and 2019.

Hourly mapping of surface air temperature by blending geostationary datasets from the two-satellite system of GOES-R series

Spatio-temporally continuous surface air temperature (SAT) is a valuable input for many research fields such as environmental studies and hydrological models. Substantial efforts have been focused on mapping daily SAT using land surface temperature (LST) derived from polar-orbiting satellites such as NASA’s Terra and Aqua. However, reconstruction of SAT at very high temporal scales such as sub-daily or hourly was carried out in very few studies, most of which are based on the Meteosat Second Generation (MSG) satellites for small-scale areas with limited stations. In this study, we developed estimation schemes based on random forest for mapping hourly SAT over a large-scale region (CONUS) by blending LST datasets from the two-satellite system of the Geostationary Operational Environmental Satellite-R (GOES-R) Series with the aim of reconstructing the hourly SAT maps with spatially uniform resolutions of about 2 km over CONUS. Estimation schemes with different variables were compared to evaluate the contributions of reanalysis variables and the hour of day (HOD) variable to model predicative performance, and the impacts of station density on model performance were also explored. We found that the model scheme with reanalysis variables and the scheme with HOD achieved higher predicative performance with the mean RMSE of 2.0 K and 2.5 K, respectively. Systematic predictive errors across the synoptic hours of a diurnal cycle are significantly reduced when considering HOD in the modeling of hourly SAT. The model scheme considering both reanalysis variables and HOD achieved the highest performance with a mean RMSE of 1.9 K. We observed that the influence of station density on model performance is related to types of cross-validation and model schemes, and that the uncertainty levels associated with the evaluation of predicative performance for the model schemes decrease with station density, which increases our confidence in the evaluation results for the schemes. Therefore, the mapping accuracy of hourly SAT over the large-scale area CONUS could be conservatively estimated to be 1.9–2.5 K using site-based cross-validation. We expect that this study will facilitate the further mapping of SAT at high-temporal scales over large-scale areas based on abundant and sustained observations from global systems of meteorological geostationary satellites.

Planetary Layer Lapse Rate Comparison of Tropical,Montane and Hot Semi-Arid Climates of Nigeria

This study assessed the pattern of planetary layer lapse rate across the major climate belts of Nigeria. Six years’ data (2010-2015) for air temperature values between 1000 mbar and 850 mbar atmospheric pressure levels were acquired from Era-Interim Re-analysis data centre. The data was retrieved at 6-hourly synoptic hours: 0000H, 0600H, 1200H and 1800H at 0.125o grid resolution. Results showed that the lower tropospheric layers throughout the various climate belts has a positive lapse rate. Findings also revealed that the average annual lapse rate condition were: Tropical wet zone (Port Harcourt) -5.6 oC/km; Bi-modal Tropical continental zone (Enugu) 5.8 oC/km; Montane zone (Jos) -6.5 oC/km; Mono-modal Tropical continental zone (Kano) -6.6 oC/km; and Hot semi-arid zone (Maiduguri) -6.6 oC/km. This average values presents the lapse rates to be close to the Saturated Adiabatic Lapse Rate (SALR). Average diurnal results for the climate belts showed that lapse rate is higher during the afternoon and transition periods than the rest periods and increases from the coastal areas northward. The seasonal periods of highest lapse rates during the day time are from December – May (i.e. 5.8-9.5 oC/km) with slight decrease from June – November. The positive lapse rate of range -1.8 – -5.9 oC/km observed during the period of dawn across the entire region showed that infrared radiation was still being released and modified by less energetic mechanical turbulence that characterizes the surface layer across Nigeria. This also indicates that global warming is real and in substantial effect. The study findings imply that conditional instability prevailed over the entire region, therefore, the planetary layer environment will be of slow to moderate dispersive potential and will require forceful mechanism to lift emissions are introduced into it. It is recommended that industrial stacks are above 50 m to enhance the dispersion of emissions aloft.

Análise da regularidade da velocidade do vento no Nordeste do Brasil através da Sample Entropy

O uso de fontes renováveis tem crescido mundialmente e, no Brasil o incentivo ao uso de energia eólica tem sido ampliado de forma expressiva. O maior potencial eólico do país concentra-se no Nordeste (NE). O objetivo deste trabalho é medir o grau de regularidade ou a previsibilidade da velocidade do vento sobre o NE do Brasil através do método de entropia Sample Entropy. As análises foram realizadas a partir das séries históricas da reanálise ERA-40, disposta sobre uma grade mundial com resolução espacial de 2,5°× 2,5° em registros realizados a 10 m do solo durante as 00, 06, 12 e 18h durante o período de 1957 a 2001. Além das quatro horas sinóticas, foram também geradas séries diárias, mensais e sazonais. Os resultados mostraram que as regiões de maior velocidade do vento também apresentam melhor previsibilidades para as séries. Além de indicar que áreas distintas com comportamento das médias da velocidade do vento semelhantes podem apresentar previsibilidade diferentes.

Characterization of Surface Layer Turbulence across a West African Tropical Climate Belt

This study surveyed the levels of boundary layer surface turbulence across a West African climate region. Five years (2011-2015) temperature and wind speed data at synoptic hours 0000 Hr, 0600 Hr, 1200 Hr and 1800 Hr within 0.125° grid resolution was sourced from Era-Interim Reanalysis platform at 1000 mbar pressure level. Using the Richardson (Ri) number technique, results showed that mechanical turbulence of Ri range 0.04 - 0.57 dominates across the surface layer for study locations of Port Harcourt, Enugu, Jos, Kano and Maiduguri than thermal turbulence. However, the least turbulent area was the coastal zone of surveyed region. Results also indicated that the vertical height (L) at which thermal turbulence replaces mechanical turbulence across study locations ranged from 20 - 250 m with lowest replacement levels (20 - 50 m) occurring mainly in the coastal area of Port Harcourt during periods of dawn. The most turbulent periods in the southern coastal location of study region were during key rainy periods from June-August while that for the rest far northern inland areas occur during the dry season/early rainy periods i.e. November-May. The implication of the lower surface turbulence/replacement level within coastal domains most especially during periods of dawn is that emission releases near surface layer will be dispersed after initial rise due to buoyancy at horizontal levels thereby increasing ground level pollutants concentration across sensitive receptors that are close to emission source. At heights of thermal turbulence replacement, emission releases will be transported vertically and then dispersed further away from emission sources, thus impacting sensitive receptors at farther distances. This is the atmospheric boundary layer dynamics that makes ground level pollution worse in the coastal city of Port Harcourt in recent times during periods of dawn. Efforts must be made by concerned Stakeholders towards ensuring that emissions are reduced during the periods of dawn within and around coastal environments.

КОРЕЛЯЦІЙНИЙ ЗВ’ЯЗОК МІЖ МЕТЕОРОЛОГІЧНИМИ ВЕЛИЧИНАМИ ПРИ ЕКСТРЕМАЛЬНИХ ЗНАЧЕННЯХ МАКСИМАЛЬНОЇ ТЕМПЕРАТУРИ ПОВІТРЯ

The object of the study is the maximum daily air temperature during the months of the year over 1991-2016 by the data of 186 meteorological stations of Ukraine. Extreme values of the maximum daily temperature equal to or exceeded their 95th (Tmax95p and above, ºС) percentile were taken as extreme. The article sets the dates (137 cases) of extreme values of maximum air temperature on more than 60 % of the territory. For these dates, 13 meteorological parameters were selected: average, minimum, and maximum air temperatures; average, minimum and maximum relative humidity; station and sea-level pressure; average, maximum (from 8 synoptic hours) wind speed; rainfall; height of snow cover. The purpose of this work is to determine the correlation coefficient (K), in particular, statistically significant (K≤-0.6, K≥0.6), on these dates between selected meteorological parameters at 186 meteorological stations of Ukraine for 1991-2013. The density of the cases of statistically significant dependence between the meteorological parameters in extremely warm days in separate seasons is determined. In extremely warm days, meteorological parameters and areas with statistically significant correlations at K≤-0.6 were detected: T and F (focally in southern and some western regions with significant density) − in winter; T and F (with the highest density ubiquitous or almost ubiquitous), P and V (in a large number of regions, usually west or right-bank, but with less frequency) − in the transition seasons, and in the autumn between − T and F (in the south with smaller density) and P and F (in some areas of the north, northwest, west, lower east). In all seasons, such a correlation between other meteorological parameters had a focal distribution, usually with a smaller density. In these days, a focal distribution with a small frequency of dependencies at K≥0.6 was found between the meteorological parameters detected (F and V in transition seasons, T and F in winter), except for similar ones. However, such dependence is observed between T and V in some regions in winter and autumn and in some areas of south, southeast, east with a smaller density. The study of the maximum daily temperature is relevant, because from the level of natural hydrometeorological phenomena it is accompanied by dangerous phenomena, negatively affecting the weather dependent industries.

Improved Empirical Coefficients for Estimating Water Vapor Weighted Mean Temperature over Europe for GNSS Applications

Development of the so-called global navigation satellite system (GNSS) meteorology is based on the possibility of determining a precipitable water vapor (PWV) from a GNSS zenith wet delay (ZWD). Conversion of ZWD to the PWV requires application of water vapor weighted mean temperature (Tm) measurements, which can be done using a surface temperature (Ts) and its linear dependency to the Tm. In this study we analyzed up to 24 years (1994–2018) of data from 49 radio-sounding (RS) stations over Europe to determine reliable coefficients of the Tm-Ts relationship. Their accuracy was verified using 109 RS stations. The analysis showed that for most of the stations, there are visible differences between coefficients estimated for the time of day and night. Consequently, the ETm4 model containing coefficients determined four times a day is presented. For hours other than the primary synoptic hours, linear interpolation was used. However, since this approach was not enough in some cases, we applied the dependence of Tm-Ts coefficients on the time of day using a polynomial (ETmPoly model). This resulted in accuracy at the level of 2.8 ± 0.3 K. We also conducted an analysis of the impact of this model on the PWV GNSS. Analysis showed that differences in PWV reached 0.8 mm compared to other commonly used models.

Satellite-based monitoring of recent heavy flooding over north-eastern states of India in July 2019

South-west monsoon of 2019 advanced towards northern parts of India during the first week of July. Heavy-to-very-heavy rainfall events during July 05–16 resulted in severe flooding over Bihar and Assam and caused huge damage in the form of life and property. More than 90 lakhs people have been affected, and about 100 causalities have been reported in Bihar and Assam due to severe flooding. Results presented in this short communication highlight the importance of space-borne near real-time monitoring of flood events. It is reported that few districts of Assam and Bihar received cumulative rainfall in excess of 1200 mm from multiple heavy rainy events during July 05–16, 2019. Heaviest rainfall events were reported on July 06, 11 and 13. Few districts of Assam and Bihar experienced daily rainfall in excess of 300 to 400 mm during these heavy rainy days with peak hourly rainfall exceeding 50 mm at few synoptic hours. Discharge from Kosi and Brahmaputra rivers combined with heavy cumulative rainfall from multiple rainy episodes during July 05–16 resulted in severe flooding over Assam and Bihar causing losses to life and property.

Analysis of Lower Tropospheric Lapse Rate Trend over Port Harcourt Coastal City, Nigeria

Understanding the spatial distribution of temperature, especially the relationship between temperature and altitude, is essential for understanding both climatological and hydrological processes and their variabilities. This is because those processes are sensitive to air temperature, especially in sub humid tropical regions, where air temperature influences the movements of pollutants and controls exchanges of energy and water fluxes between land and atmosphere particularly within the lower troposphere. This study examined the trend of lower tropospheric lapse rate in the coastal area of Port Harcourt, Nigeria. Six years’ data (2010-2015) for temperature between 1000 mbar and 850 mbar pressure levels was retrieved from era-interim re-analysis platform for the analysis. The data was acquired at 6-hourly synoptic hours: 0000H, 0600H, 1200H and 1800H at 0.125° grid resolution. Findings from the computed environmental lapse rate (ELR) show that conditional instability with an annual lapse rate of 5.5°C/km persists at the area from January to December. It was revealed that the months of December and January constituted the highest ELR trends of 6.5°C/km and 5.9°C/km respectively. This indicates that the month of December assumes a normal tropospheric lapse rate trend. The average range of lapse rate trend in the area which is close to the moist adiabatic lapse rate (MALR) of 5.0°C/km than the dry adiabatic lapse rate (DALR) shows that the study atmospheric environment is rich in water vapour. The 6-hourly synoptic analysis of the ELR pattern shows that lapse rate range between 1°C/km - 6.4°C/km and 6.5°C/km - 10°C/km dominates throughout the year at 0000 - 0600 Hrs and 1200 - 1800 Hrs respectively. This demonstrates a higher and lower lapse rate trend during the day and night periods respectively. Relating study findings to the potential of air to disperse emissions in the area suggests that air emissions will be conveyed through far and near distances across the boundary layers due to the moderate dispersive potential of air regarding the closeness of the average ELR to the MALR. Policies that will ensure that pollutants are dispersed aloft especially emission stacks above 50 m is advocated in the city of Port Harcourt.

Winter Wind Regimes over Israel Using Self-Organizing Maps

AbstractThe aim of this study is to objectively define and automatically reconstruct surface wind regimes over Israel. Unlike other previous studies that subjectively examined case studies or applied the semiobjective synoptic classification (SOC), this study shows the ability of the method of self-organizing maps (SOM) to directly define well-known wind regimes at the synoptic hours (0000, 0600, 1200, and 1800 UTC) during the winter. This ability sets the groundwork for future automatic climatological analysis and applications. The investigation is performed by analyzing surface wind measurements from 53 Israel Meteorological Service stations. The relation between the synoptic variables and the wind regimes is revealed from the averages of ECMWF interim reanalysis (ERA-Interim) variables at each SOM wind regime. The inspection of wind regimes and their average pressure anomalies has shown that wind regimes relate to the gradient of the pressure anomalies rather than to the specific isobaric pattern. Two main wind regimes—strong western and strong eastern—are well known over this region. During daytime, SOM classification identifies these two regimes while SOC reveals only strong western regimes since SOC considers depth of the pressure gradients only in the case of low pressure centers. In accordance with previous studies, two main groups—winter low and high pressure centers and/or Red Sea troughs—are related to the strong westerly or easterly wind regimes with low diurnal variability and high daily persistence. Regimes under weak pressure gradients have higher diurnal variability, relatively lower steadiness, and weaker speed. Their daily persistence is not necessarily low.

Geo-spatial distribution of cloud cover and influence of cloud induced attenuation and noise temperature on satellite signal propagation over Nigeria

The study of the influence of cloud cover on satellite propagation links is becoming more demanding due to the requirement of larger bandwidth for different satellite applications. Cloud attenuation is one of the major factors to consider for optimum performance of Ka/V and other higher frequency bands. In this paper, the geo-spatial distribution of cloud coverage over some chosen stations in Nigeria has been considered. The substantial scale spatial dispersion of cloud cover based on synoptic meteorological data and the possible impact on satellite communication links at higher frequency bands was also investigated. The investigation was based on 5years (2008–2012) achieved cloud cover data collected by the Nigerian Meteorological Agency (NIMET) Federal Ministry of Aviation, Oshodi Lagos over four synoptic hours of the day covering day and night. The performances of satellite signals as they traverse through the cloud and cloud noise temperature at different seasons and over different hours of days at Ku/W-bands frequency are also examined. The overall result shows that the additional total atmospheric noise temperature due to the clear air effect and the noise temperature from the cloud reduces the signal-to-noise ratio of the satellite receiver systems, leading to more signal loss and if not adequately taken care of may lead to significant outage. The present results will be useful for Earth-space link budgeting, especially for the proposed multi-sensors communication satellite systems in Nigeria.

An Assessment of Atmospheric Boundary Layer Turbulence in Maiduguri, Nigeria

This study examined the level of turbulence in the atmospheric boundary layer in Maiduguri, north-eastern Nigeria. Five years (2011-2015) temperature and wind speed data at 1000 mbar pressure level retrieved from Era-Interim Reanalysis Platform was used. These data were gotten at 6-hourly synoptic hours: 0000H, 0600H, 1200H and 1800H at 0.125° grid resolution. The gradient Richardson (Rig) number method was utilised in analysing turbulence across three layers: 10 - 50 m (surface layer); 50 - 100 m (mid layer) and 100 - 1300 m (upper layer). Findings shows that the surface layer is always in a turbulent state as over 95% of Rig values were below Richardson Critical (Ric) value of 0.25 with range 0.02 - 0.94. However, all values across the hours were below the Richardson Termination (RT) value of 1. Laminar conditions exist at the mid layer across the hours as 99.9% of Rig values ranging 0.88 - 8.02 were greater than RT of 1. Rig values for the upper layer were largely negative and ranged between -78.71 to -724.14. This indicates robust turbulent conditions. Turbulence generated through forced and free ascents prevailed at the surface layer and upper layer respectively. This shows that wind shear is dominant at the surface while thermal buoyancy prevails at the upper level. The months of February and September at 1200 and 1800 hours respectively are the periods of maximum (about 134 m) and minimum (below 15 m) heights were free convection destabilises forced convection in the study area. Relating findings to emission dispersion suggests that air pollutants will be transported across far and near distances at the upper layer and surface layers respectively. This is due to the stable nature of the mid layer that will limits vertical emission dispersion. Policy makers should ensure that potential emission sources stacks are above 50 m to ensure pollutants are dispersed aloft in the area.

TENDÊNCIA DOS ÍNDICES DE INSTABILIDADE NA AMÉRICA DO SUL

The instability indices evaluate the vertical structure of the atmosphere related to temperature and air humidity aiming to identify instability or stability. Where exists instability, these provide an indication of the intensity of precipitation systems. The K index indicates the potential for storms based on the lapse rate and humidity in the lower troposphere and in the extension of the wet layer. The Total Totals (TT) index combines temperature and moisture at low levels with the temperature on middle levels. Due to the importance of these indices in weather forecast and the absence of studies of the tendency of these parameters in South America, this research aimed to determine the K and TT indices for the synoptic hours make a climatology and calculate the seasonal trend of these indices. Results showed that on the continent higher values of the indices K and TT are obtained in the summer, mainly due to seasonal variation of temperature, in addition to rainfall associated with the monsoon particularly in southeastern Brazil. There is an increasing trend in the values of both indices in the northern Amazon and decreasing trend in the northwest of Argentina and southwest Bolivia.

Circulation types classification for hourly precipitation events in Lublin (East Poland)

AbstractThe paper presents an objective classification of circulation types for East-Central Europe with application to hourly precipitation events in Lublin (East Poland) from May to September. The development of the classification utilized sub-daily sea-level pressure values (at the main standard synoptic hours,

Climatology of low-level temperature inversions at the Prague-Libuš aerological station

Temperature inversions (TIs) have been closely studied for a variety of reasons. Here, we present an analysis of low-level TIs, based on a dataset of atmospheric soundings from the aerological station at Prague-Libus (Czech Republic). Measurements have been carried out here since 1974, at the four main synoptic hours. First, the homogeneity of both the radiosonde data and sounding-derived time series was investigated. Since the homogeneity of the derived time series was affected by changes in the observation routine, only the period of 1992–2012 was further analysed. Second, diurnal and annual regimes of surface-based (SBTIs) and elevated (ETIs) temperature inversions were described. Maximum frequency of the SBTIs occurred at 00 UTC and in September and October. During autumn, persistent SBTIs also often occurred, which could be linked to decreased cyclonicity. Third, changes in TIs were evaluated, and the strongest trends were compared with the nearby station at Kummersbruck (Germany). In all seasons except spring, nocturnal SBTIs decreased almost identically at both stations. Morning SBTIs decreased as well; however, the trend was considerably stronger at Prague; and it was also mirrored by an increase in ETIs. Since only a part of these trends could be explained by changes in the large-scale circulation, other factors (low tropospheric warming, suburbanisation) are hypothesized to have affected the trends.

Atmospheric divergence over equatorial East Africa and its influence on distribution of rainfall

Spatial and temporal variability of divergence is analysed over equatorial East Africa. According to the analysis, seasonal march of upper level divergence is bimodal. There is peak upper level divergence during March‐April‐May and October‐November‐December. Analysis of upper level divergence at synoptic hours reveals that peak divergence occurs in the afternoon over the inland areas. Over the coastal areas, peak upper level divergence occurs at night. Spectral analysis results indicate that there is existence of oscillations in upper level divergence. These oscillations vary in terms of their periodicities and the amount of variance that they account for in the observed divergence.