On the diurnal cycle of rainfall and convection over Lake Victoria and its catchment. Part 1: Rainfall and Mesoscale Convective Systems

2021 ◽  
Author(s):  
Sharon E. Nicholson ◽  
Adam T. Hartman ◽  
Douglas A. Klotter
Keyword(s):  
Diurnal Cycle ◽  
Lake Victoria ◽  
Mesoscale Convective Systems ◽  
Convective Systems ◽  
Early Evening ◽  
Lake Effect ◽  
Mesoscale Convective ◽  
Dry Seasons

AbstractThis article examines the diurnal cycle of lake-effect rains over Lake Victoria and of rainfall in the surrounding catchment. The analysis focuses on four months, which represent the two wet seasons (April and November) and the two dry seasons (February and July). Lake-effect rains are strongest in April, weakest in July. In all cases there is a nocturnal rainfall maximum over the lake and a daytime maximum over the catchment, with the transition between rainfall over the lake and over the catchment occurring between 1200 and 1500 LST. During the night the surrounding catchment is mostly dry. Conversely, little to no rain falls over the lake during the afternoon and early evening. In most cases the maximum over the lake occurs at either 0600 or 0900 LST and the maximum over the catchment occurs around 1500 to 1800 LST. The diurnal cycle of Mesoscale Convective Systems (MCSs) parallels that of over-lake rainfall. MCS initiation generally begins over the catchment around 1500 LST and increases at 1800 LST. MCS initiation over the lake begins around 0300 LST and continues until 1200 LST. While some MCSs originate over the highlands to the east of the lake, most originate in situ over the lake. Maximum MCS activity over the lake occurs at 0600 LST and is associated with the systems that initiate in situ.

Lake-effect rains over Lake Victoria and their association with Mesoscale Convective Systems

2021 ◽  
Author(s):  
Sharon E. Nicholson ◽  
Douglas Klotter ◽  
Adam T. Hartman
Keyword(s):  
Large Scale ◽  
Lake Victoria ◽  
Ground Truth ◽  
Mesoscale Convective Systems ◽  
Convective Systems ◽  
Lake Effect ◽  
Mesoscale Convective ◽  
Strong Convection ◽  
Short Rains ◽  
Lake Catchment

AbstractThis article examined rainfall enhancement over Lake Victoria. Estimates of over-lake rainfall were compared with rainfall in the surrounding lake catchment. Four satellite products were initially tested against estimates based on gauges or water balance models. These included TRMM 3B43, IMERG V06 Final Run (IMERG-F), CHIRPS2, and PERSIANN-CDR. There was agreement among the satellite products for catchment rainfall but a large disparity among them for over-lake rainfall. IMERG-F was clearly an outlier, exceeding the estimate from TRMM 3B43 by 36%. The overestimation by IMERG-F was likely related to passive microwave assessments of strong convection, such as prevails over Lake Victoria. Overall, TRMM 3B43 showed the best agreement with the "ground truth" and was used in further analyses. Over-lake rainfall was found to be enhanced compared to catchment rainfall in all months. During the March-to-May long rains the enhancement varied between 40% and 50%. During the October-to-December short rains the enhancement varied between 33% and 44%. Even during the two dry seasons the enhancement was at least 20% and over 50% in some months. While the magnitude of enhancement varied from month to month, the seasonal cycle was essentially the same for over-lake and catchment rainfall, suggesting that the dominant influence on over-lake rainfall is the large-scale environment. The association with Mesoscale Convective Systems (MCSs) was also evaluated. The similarity of the spatial patterns of rainfall and MCS count each month suggested that these produced a major share of rainfall over the lake. Similarity in interannual variability further supported this conclusion.

Evaluating Diurnal and Semi-Diurnal Cycle of Precipitation in CMIP6 Models Using Satellite- and Ground-Based Observations

2021 ◽  
pp. 1-56
Author(s):  
Shuaiqi Tang ◽  
Peter Gleckler ◽  
Shaocheng Xie ◽  
Jiwoo Lee ◽  
Min-Seop Ahn ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
Climate Models ◽  
Early Morning ◽  
Mesoscale Convective Systems ◽  
Convective Systems ◽  
Ensemble Mean ◽  
Mesoscale Convective ◽  
Diurnal Cycle Of Precipitation

AbstractThe diurnal and semi-diurnal cycle of precipitation simulated from CMIP6 models during 1996-2005 are evaluated globally between 60°S and 60°N, as well as at ten selected locations representing three categories of diurnal cycle of precipitation: (1) afternoon precipitation over land, (2) early morning precipitation over ocean, and (3) nocturnal precipitation over land. Three satellite-based and two ground-based rainfall products are used to evaluate the climate models. Globally, the ensemble mean of CMIP6 models shows a diurnal phase of 3 to 4 hours earlier over land and 1 to 2 hours earlier over ocean, when compared with the latest satellite products. These biases are in line with what were found in previous versions of climate models but reduced compared to the CMIP5 ensemble mean. Analysis at the selected locations complimented with in-situ measurements further reinforces these results. Several CMIP6 models have shown a significant improvement in the diurnal cycle of precipitation compared to their CMIP5 counterparts, notably on delaying afternoon precipitation over land. This can be attributed to the use of more sophisticated convective parameterizations. Most models are still unable to capture the nocturnal peak associated with elevated convection and propagating mesoscale convective systems, with a few exceptions that allow convection to be initiated above the boundary layer to capture nocturnal elevated convection. We also quantify an encouraging consistency between the satellite- and ground-based precipitation measurements despite differing spatiotemporal resolutions and sampling periods, which provides confidence in using them to evaluate the diurnal and semi-diurnal cycle of precipitation in climate models.

scholarly journals Mesoscale Convective Systems over Western Equatorial Africa and Their Relationship to Large-Scale Circulation

2009 ◽  
Vol 137 (4) ◽  
pp. 1272-1294 ◽  
Author(s):  
Brian Jackson ◽  
Sharon E. Nicholson ◽  
Douglas Klotter
Keyword(s):  
Diurnal Cycle ◽  
Rainy Season ◽  
Large Scale ◽  
Critical Role ◽  
Mesoscale Convective Systems ◽  
Convective Systems ◽  
Continental Scale ◽  
Equatorial Africa ◽  
Mesoscale Convective ◽  
The Right

Abstract This study examines mesoscale convective systems (MCSs) over western equatorial Africa using data from the Tropical Rainfall Measuring Mission (TRMM) satellite. This region experiences some of the world’s most intense thunderstorms and highest lightning frequency, but has low rainfall relative to other equatorial regions. The analyses of MCS activity include the frequency of occurrence, diurnal and annual cycles, and associated volumetric and convective rainfall. Also evaluated is the lightning activity associated with the MCSs. Emphasis is placed on the diurnal cycle and on the continental-scale motion fields in this region. The diurnal cycle shows a maximum in MCS count around 1500–1800 LT, a morning minimum, and substantial activity during the night; there is little seasonal variation in the diurnal cycle, suggesting stationary influences such as orography. Our analysis shows four maxima in MCS activity, three of which are related to local geography (two orographic and one over Lake Victoria). The fourth coincides with a midtropospheric convergence maximum in the right entrance quadrant of the African easterly jet of the Southern Hemisphere (AEJ-S). This maximum is substantially stronger in the September–November rainy season, when the jet is well developed, than in the March–May rainy season, when the jet is absent. Lightning frequency and flashes per MCS are also greatest during September–November; maxima occur in the right entrance quadrant of the AEJ-S. The lightning maximum is somewhat south of the MCS maximum and coincides with the low-lying areas of central Africa. Overall, the results of this study suggest that large-scale topography plays a critical role in the spatial and diurnal patterns of convection, lightning, and rainfall in this region. More speculative is the role of the AEJ-S, but this preliminary analysis suggests that it does play a role in the anomalous intensity of convection in western equatorial Africa.

scholarly journals In Situ Initiation of East Pacific Easterly Waves in a Regional Model

2017 ◽  
Vol 74 (2) ◽  
pp. 333-351 ◽  
Author(s):  
Adam V. Rydbeck ◽  
Eric D. Maloney ◽  
Ghassan J. Alaka
Keyword(s):  
Wrf Model ◽  
Warm Pool ◽  
Early Morning ◽  
Mesoscale Convective Systems ◽  
Weather Research And Forecasting ◽  
Easterly Waves ◽  
Convective Systems ◽  
East Pacific ◽  
Mesoscale Convective

Abstract The in situ generation of easterly waves (EWs) in the east Pacific (EPAC) is investigated using the Weather Research and Forecasting (WRF) Model. The sensitivity of the model to the suppression of EW forcing by locally generated convective disturbances is examined. Specifically, local forcing of EWs is removed by reducing the terrain height in portions of Central and South America to suppress robust sources of diurnal convective variability, most notably in the Panama Bight. High terrain contributes to the initiation of mesoscale convective systems in the early morning that propagate westward into the EPAC warm pool. When such mesoscale convective systems are suppressed in the model, EW variance is significantly reduced. This result suggests that EPAC EWs can be generated locally in association with higher-frequency convective disturbances, and these disturbances are determined to be an important source of EPAC EW variability. However, EPAC EW variability is not completely eliminated in such sensitivity experiments, indicating the importance for other sources of EW forcing, namely, EWs propagating into the EPAC from West Africa. Examination of the EW vorticity budget in the model suggests that nascent waves are zonally elongated and amplified by horizontal advection and vertical stretching of vorticity. Changes in the mean state between the control run and simulation with reduced terrain height also complicate interpretation of the results.

A five-year climatological lightning characteristics of linear mesoscale convective systems over North China

2021 ◽  
Vol 256 ◽  
pp. 105580
Author(s):  
Dongxia Liu ◽  
Mengyu Sun ◽  
Debin Su ◽  
Wenjing Xu ◽  
Han Yu ◽  
...  
Keyword(s):  
North China ◽  
Mesoscale Convective Systems ◽  
Convective Systems ◽  
Mesoscale Convective

scholarly journals An Observational Study of Mesoscale Convective Systems with Heavy Rainfall over the Korean Peninsula

2006 ◽  
Vol 21 (2) ◽  
pp. 125-148 ◽  
Author(s):  
Hyung Woo Kim ◽  
Dong Kyou Lee
Keyword(s):  
Observational Study ◽  
Korean Peninsula ◽  
Wind Shear ◽  
Heavy Rainfall ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Mesoscale Convective Systems ◽  
Low Level ◽  
Convective Systems ◽  
Mesoscale Convective

Abstract A heavy rainfall event induced by mesoscale convective systems (MCSs) occurred over the middle Korean Peninsula from 25 to 27 July 1996. This heavy rainfall caused a large loss of life and property damage as a result of flash floods and landslides. An observational study was conducted using Weather Surveillance Radar-1988 Doppler (WSR-88D) data from 0930 UTC 26 July to 0303 UTC 27 July 1996. Dominant synoptic features in this case had many similarities to those in previous studies, such as the presence of a quasi-stationary frontal system, a weak upper-level trough, sufficient moisture transportation by a low-level jet from a tropical storm landfall, strong potential and convective instability, and strong vertical wind shear. The thermodynamic characteristics and wind shear presented favorable conditions for a heavy rainfall occurrence. The early convective cells in the MCSs initiated over the coastal area, facilitated by the mesoscale boundaries of the land–sea contrast, rain–no rain regions, saturated–unsaturated soils, and steep horizontal pressure and thermal gradients. Two MCSs passed through the heavy rainfall regions during the investigation period. The first MCS initiated at 1000 UTC 26 July and had the characteristics of a supercell storm with small amounts of precipitation, the appearance of a mesocyclone with tilting storm, a rear-inflow jet at the midlevel of the storm, and fast forward propagation. The second MCS initiated over the upstream area of the first MCS at 1800 UTC 26 July and had the characteristics of a multicell storm, such as a broken areal-type squall line, slow or quasi-stationary backward propagation, heavy rainfall in a concentrated area due to the merging of the convective storms, and a stagnated cluster system. These systems merged and stagnated because their movement was blocked by the Taebaek Mountain Range, and they continued to develop because of the vertical wind shear resulting from a low-level easterly inflow.

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