Radar-Observed Characteristics of Precipitating Systems during NAME 2004

Abstract A multiradar network, operated in the southern Gulf of California (GoC) region during the 2004 North American Monsoon Experiment, is used to analyze the spatial and temporal variabilities of local precipitation. Based on the initial findings of this analysis, it is found that terrain played a key role in this variability, as the diurnal cycle was dominated by convective triggering during the afternoon over the peaks and foothills of the Sierra Madre Occidental (SMO). Precipitating systems grew upscale and moved WNW toward the gulf. Distinct precipitation regimes within the monsoon are identified. The first, regime A, corresponded to enhanced precipitation over the southern portions of the coast and GoC, typically during the overnight and early morning hours. This was due to precipitating systems surviving the westward trip (∼7 m s−1; 3–4 m s−1 in excess of steering winds) from the SMO after sunset, likely because of enhanced environmental wind shear as diagnosed from local soundings. The second, regime B, corresponded to the significant northward/along-coast movement of systems (∼10 m s−1; 4–5 m s−1 in excess of steering winds) and often overlapped with regime A. The weak propagation is explainable by shallow–weak cold pools. Reanalysis data suggest that tropical easterly waves were associated with the occurrence of disturbed regimes. Gulf surges occurred during a small subset of these regimes, so they played a minor role during 2004. Mesoscale convective systems and other organized systems were responsible for most of the rainfall in this region, particularly during the disturbed regimes.

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

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-16-0124.1 ◽

2017 ◽

Vol 74 (2) ◽

pp. 333-351 ◽

Cited By ~ 5

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.

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Three MCS Cases Occurring in Different Synoptic Environments in the Sub-Sahelian Wet Zone during the 2002 West African Monsoon

Journal of the Atmospheric Sciences ◽

10.1175/jas3757.1 ◽

2006 ◽

Vol 63 (9) ◽

pp. 2369-2382 ◽

Cited By ~ 11

Author(s):

Jon M. Schrage ◽

Andreas H. Fink ◽

Volker Ermert ◽

Epiphane D. Ahlonsou

Keyword(s):

West Africa ◽

West African Monsoon ◽

Lower Troposphere ◽

Integrated Approach ◽

West African ◽

Easterly Waves ◽

Convective Systems ◽

Efficient Management ◽

Mesoscale Convective ◽

Wet Zone

Abstract Three mesoscale convective systems (MCSs) occurring in the sub-Sahelian wet zone of West Africa are examined using observations from the 2002 Integrated Approach to the Efficient Management of Scarce Water Resources in West Africa (IMPETUS) field campaign, the European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, and Meteosat infrared imagery. These datasets enable the analysis of the synoptic-scale environment in which the MCSs were embedded, along with a high-resolution monitoring of surface parameters during the systems’ passages. The available data imply that cases I and II were of a squall-type nature. Case I propagated into a moderately sheared and rather moist lower and middle troposphere over the Upper Ouémé Valley (UOV). In contrast, case II was associated with a well-sheared and dry lower troposphere and a large, moist instability. In either case, behind the convective cluster a westward-propagating cyclonic vorticity maximum that was likely captured by the ECMWF analysis as a result of the special upper-air station at Parakou (Benin). In case I, the fast-moving vorticity signal slowed down over the Guinean Highlands where convection dissipated. Farther downstream, it might have played a role in the consolidation of an African easterly waves (AEW) trough over the West African coast and the eastern Atlantic. Case III proved to be a more stationary pattern of convection associated with a vortex in the monsoon flow. It also exhibited a moist and low shear environment.

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The Diurnal and Microphysical Characteristics of MJO Rain Events during DYNAMO

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-18-0316.1 ◽

2019 ◽

Vol 2019 (1) ◽

pp. 67-80 ◽

Cited By ~ 1

Author(s):

Angela K. Rowe ◽

Robert A. Houze ◽

Stacy Brodzik ◽

Manuel D. Zuluaga

Keyword(s):

Deep Convection ◽

Intraseasonal Variability ◽

Early Morning ◽

Synoptic Scale ◽

Convective Systems ◽

Mesoscale Convective ◽

The Mean ◽

Microphysical Processes ◽

Rain Events ◽

Afternoon Peak

Abstract The Madden–Julian oscillation (MJO) dominates the intraseasonal variability of cloud populations of the tropical Indian and Pacific Oceans. Suppressed MJO periods consist primarily of shallow and isolated deep convection. During the transition to an active MJO, the shallow and isolated deep clouds grow upscale into the overnight hours. During active MJO periods, mesoscale convective systems occur mostly during 2–4-day bursts of rainfall activity with a statistically significant early morning peak. Yet when these rain events are separated into individual active periods, some periods do not follow the mean pattern, with the November events in particular exhibiting an afternoon peak. The radar-observed microphysical processes producing the precipitation during the major rain events of active MJO periods evolve in connection with synoptic-scale wave passages with varying influences of diurnal forcing. MJO studies that do not account for the intermittency of rainfall during active MJO phases through averaging over multiple events can lead to the misimpression that the primary rain-producing clouds of the MJO are modulated solely by the diurnal cycle.

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The Atmospheric Controls of Extreme Convective Events over the Southern Arabian Peninsula during the Transition Seasons

10.5194/egusphere-egu21-3234 ◽

2021 ◽

Author(s):

Narendra Reddy Nelli ◽

Diana Francis ◽

Ricardo Fonseca ◽

Rachid Abida ◽

Michael Weston ◽

...

Keyword(s):

Large Scale ◽

Arabian Peninsula ◽

Arabian Gulf ◽

Reanalysis Data ◽

Convective Systems ◽

Stratospheric Temperature ◽

Moisture Advection ◽

Mesoscale Convective ◽

Upper Level ◽

Warming World

In this paper, the processes behind severe convective events over the Arabian Peninsula during spring and autumn seasons and their local-scale impacts are investigated using reanalysis data, satellite-derived and observational products. The focus on the transition seasons is justified as Mesoscale Convective Systems (MCSs) are more common at that time of the year, in particular in the months of March and April. The analysis of 48 events from 2000 to 2019 revealed that they are triggered by low-level wind convergence and moisture advection from the Arabian Sea, Arabian Gulf and/or Red Sea. An equatorward displacement and strengthening of the subtropical jet also precondition the environment, as does the presence of a mid-level trough. The latter is generally part of a large-scale pattern of anomalies that are equivalent barotropic in nature, and therefore likely a response to tropical or subtropical forcing. At more local-scales, a drying of the mid-troposphere between 850 and 250 hPa typically by 50%, a reduction of the upper-level winds by about 5 m s-1, and an increase in the upper-tropospheric and lower-stratospheric temperature on averaged by 2-3 K, are typically observed during a MCS event. Over the 20-year period, a statistically significant increase in the MCSs&#8217; spatial extent, intensity and duration over the UAE and surrounding region has been found, suggesting that such extreme events may be even more impactful in a hypothetical warming world. The rainfall they generate, on the other hand, shows an increase that is not statistically significant.

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The vertical moisture structure and precipitation intensity distributions associated with tropical convective systems

10.5194/egusphere-egu2020-21506 ◽

2020 ◽

Author(s):

Kathleen Schiro ◽

Sylvia Sullivan ◽

Jiabo Yin ◽

Pierre Gentine

Keyword(s):

Large Scale ◽

Southern Oscillation ◽

Probability Distributions ◽

Reanalysis Data ◽

Precipitation Intensity ◽

Maximum Precipitation ◽

Convective Systems ◽

Mesoscale Convective ◽

Environmental Relative Humidity ◽

Relative Gains

In the tropics, the majority of high-intensity precipitation comes from the organization of multiple convective cells into mesoscale convective systems (MCS). Here, we use a synthesis of multi-decade satellite and reanalysis data to investigate relationships between the column water vapor content (CWVC), instability (CAPE), and precipitation from MCS. We find a linear relationship between MCS maximum precipitation intensity and CAPE and a sharp, nonlinear relationship between this maximum precipitation intensity and CWVC. The latter suggests that a deep layer of inflow to the MCS dominates buoyancy and precipitation production. From these multidecade data, we can also illustrate robust shifts in the probability distributions of precipitation intensity with the El Ni&#241;o Southern Oscillation. El Ni&#241;o-La Ni&#241;a relative gains and losses in precipitation intensity can be understood with a vertical momentum budget and the role of environmental relative humidity and large-scale circulation therein. Understanding the associated vertical moisture structure and instability is essential to better predict future variability in tropical precipitation.

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The Diurnal Cycle of Rainfall and Convective Intensity according to Three Years of TRMM Measurements

Journal of Climate ◽

10.1175/1520-0442-16.10.1456 ◽

2003 ◽

Vol 16 (10) ◽

pp. 1456-1475 ◽

Cited By ~ 463

Author(s):

Stephen W. Nesbitt ◽

Edward J. Zipser

Keyword(s):

Diurnal Cycle ◽

Tropical Rainfall Measuring Mission ◽

Early Morning ◽

Convective Systems ◽

Rain Gauges ◽

Mesoscale Convective ◽

Late Evening ◽

Microwave Imager ◽

Trmm Microwave Imager ◽

Rain Rates

Abstract The Tropical Rainfall Measuring Mission (TRMM) satellite measurements from the precipitation radar and TRMM microwave imager have been combined to yield a comprehensive 3-yr database of precipitation features (PFs) throughout the global Tropics (±36° latitude). The PFs retrieved using this algorithm (which number nearly six million Tropicswide) have been sorted by size and intensity ranging from small shallow features greater than 75 km2 in area to large mesoscale convective systems (MCSs) according to their radar and ice scattering characteristics. This study presents a comprehensive analysis of the diurnal cycle of the observed precipitation features' rainfall amount, precipitation feature frequency, rainfall intensity, convective–stratiform rainfall portioning, and remotely sensed convective intensity, sampled Tropicswide from space. The observations are sorted regionally to examine the stark differences in the diurnal cycle of rainfall and convective intensity over land and ocean areas. Over the oceans, the diurnal cycle of rainfall has small amplitude, with the maximum contribution to rainfall coming from MCSs in the early morning. This increased contribution is due to an increased number of MCSs in the nighttime hours, not increasing MCS areas or conditional rain rates, in agreement with previous works. Rainfall from sub-MCS features over the ocean has little appreciable diurnal cycle of rainfall or convective intensity. Land areas have a much larger rainfall cycle than over the ocean, with a marked minimum in the midmorning hours and a maximum in the afternoon, slowly decreasing through midnight. Non-MCS features have a significant peak in afternoon instantaneous conditional rain rates (the mean rain rate in raining pixels), and convective intensities, which differs from previous studies using rain rates derived from hourly rain gauges. This is attributed to enhancement by afternoon heating. MCSs over land have a convective intensity peak in the late afternoon, however all land regions have MCS rainfall peaks that occur in the late evening through midnight due to their longer life cycle. The diurnal cycle of overland MCS rainfall and convective intensity varies significantly among land regions, attributed to MCS sensitivity to the varying environmental conditions in which they occur.

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Elucidating the Life Cycle of Warm-Season Mesoscale Convective Systems in Eastern China from the Himawari-8 Geostationary Satellite

Remote Sensing ◽

10.3390/rs12142307 ◽

2020 ◽

Vol 12 (14) ◽

pp. 2307

Author(s):

Dandan Chen ◽

Jianping Guo ◽

Dan Yao ◽

Zhe Feng ◽

Yanluan Lin

Keyword(s):

Life Cycle ◽

Large Scale ◽

Eastern China ◽

Warm Season ◽

Northern China ◽

Early Morning ◽

Geostationary Satellite ◽

Mesoscale Convective Systems ◽

Convective Systems ◽

Mesoscale Convective

The life cycle of mesoscale convective systems (MCSs) in eastern China is yet to be fully understood, mainly due to the lack of observations of high spatio-temporal resolution and objective methods. Here, we quantitatively analyze the properties of warm-season (from April to September of 2016) MCSs during their lifetimes using the Himawari-8 geostationary satellite, combined with ground-based radars and gauge measurements. Generally, the occurrence of satellite derived MCSs has a noon peak over the land and an early morning peak over the ocean, which is several hours earlier than the precipitation peak. The developing and dissipative stages are significantly longer as total durations of MCSs increase. Aided by three-dimensional radar mosaics, we find the fraction of convective cores over northern China is much lower when compared with those in central United States, indicating that the precipitation produced by broad stratiform clouds may be more important for northern China. When there exists a large amount of stratiform precipitation, it releases a large amount of latent heat and promotes the large-scale circulations, which favors the maintenance of MCSs. These findings provide quantitative results about the life cycle of warm-season MCSs in eastern China based on multiple data sources and large numbers of samples.

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Observations of Coastally Transitioning West African Mesoscale Convective Systems during NAMMA

International Journal of Geophysics ◽

10.1155/2012/438706 ◽

2012 ◽

Vol 2012 ◽

pp. 1-25

Author(s):

Bradley W. Klotz ◽

Paul Kucera

Keyword(s):

West African ◽

Radar Reflectivity ◽

Mesoscale Convective Systems ◽

Environmental Stability ◽

Tropical Cyclogenesis ◽

Easterly Waves ◽

Convective Systems ◽

Maximum Reflectivity ◽

Multidisciplinary Analysis ◽

Mesoscale Convective

Observations from the NASA 10 cm polarimetric Doppler weather radar (NPOL) were used to examine structure, development, and oceanic transition of West African Mesoscale Convective Systems (MCSs) during the NASA African Monsoon Multidisciplinary Analysis (NAMMA) to determine possible indicators leading to downstream tropical cyclogenesis. Characteristics examined from the NPOL data include echo-top heights, maximum radar reflectivity, height of maximum radar reflectivity, and convective and stratiform coverage areas. Atmospheric radiosondes launched during NAMMA were used to investigate environmental stability characteristics that the MCSs encountered while over land and ocean, respectively. Strengths of African Easterly Waves (AEWs) were examined along with the MCSs in order to improve the analysis of MCS characteristics. Mean structural and environmental characteristics were calculated for systems that produced TCs and for those that did not in order to determine differences between the two types. Echo-top heights were similar between the two types, but maximum reflectivity and height and coverage of intense convection (>50 dBZ) are all larger than for the TC producing cases. Striking differences in environmental conditions related to future TC formation include stronger African Easterly Jet, increased moisture especially at middle and upper levels, and increased stability as the MCSs coastally transition.

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Tropical Atlantic Hurricanes, Easterly Waves, and West African Mesoscale Convective Systems

Advances in Meteorology ◽

10.1155/2010/284503 ◽

2010 ◽

Vol 2010 ◽

pp. 1-13 ◽

Cited By ~ 6

Author(s):

Yves K. Kouadio ◽

Luiz A. T. Machado ◽

Jacques Servain

Keyword(s):

West Africa ◽

West African ◽

Mesoscale Convective Systems ◽

Tropical Atlantic ◽

Atmospheric Conditions ◽

Easterly Waves ◽

Convective Systems ◽

Mesoscale Convective ◽

Sst Anomaly ◽

Atlantic Hurricanes

The relationship between tropical Atlantic hurricanes (Hs), atmospheric easterly waves (AEWs), and West African mesoscale convective systems (MCSs) is investigated. It points out atmospheric conditions over West Africa before hurricane formation. The analysis was performed for two periods, June–November in 2004 and 2005, during which 12 hurricanes (seven in 2004, five in 2005) were selected. Using the AEW signature in the 700 hPa vorticity, a backward trajectory was performed to the African coast, starting from the date and position of each hurricane, when and where it was catalogued as a tropical depression. At this step, using the Meteosat-7 satellite dataset, we selected all the MCSs around this time and region, and tracked them from their initiation until their dissipation. This procedure allowed us to relate each of the selected Hs with AEWs and a succession of MCSs that occurred a few times over West Africa before initiation of the hurricane. Finally, a dipole in sea surface temperature (SST) was observed with a positive SST anomaly within the region of H generation and a negative SST anomaly within the Gulf of Guinea. This SST anomaly dipole could contribute to enhance the continental convergence associated with the monsoon that impacts on the West African MCSs formation.

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Favorable monsoon environment over eastern Africa for subsequent tropical cyclogenesis of African easterly waves

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-20-0339.1 ◽

2021 ◽

Author(s):

Kelly M. Núñez Ocasio ◽

Alan Brammer ◽

Jenni L. Evans ◽

George S. Young ◽

Zachary L. Moon

Keyword(s):

Large Scale ◽

West African Monsoon ◽

Eastern Africa ◽

Tropical Cyclogenesis ◽

African Easterly Waves ◽

Easterly Waves ◽

Convective Systems ◽

Mesoscale Convective ◽

Vertically Aligned ◽

Somali Jet

AbstractEastern Africa is a common region of African easterly wave (AEW) onset and AEW early-life. How the large-scale environment over east Africa relates to the likelihood of an AEW subsequently undergoing tropical cyclogenesis in a climatology has not been documented. This study addresses the following hypothesis: AEWs that undergo tropical cyclogenesis (i.e., developing AEWs) initiate and propagate under a more favorable monsoon large-scale environment over eastern Africa when compared to non-developing AEWs. Using a 21-year August-to-September (1990-2010) climatology of AEWs, differences in the large-scale environment between developers and non-developers are identified and are propose to be used as key predictors of subsequent tropical cyclone formation and could informtropical cyclogenesis prediction. TC precursors when compared to non-developing AEWs experience: an anomalously active West African Monsoon, stronger northerly flow, more intense zonal Somali jet, anomalous convergence over the Marrah Mountains (region of AEW forcing), and a more intense and elongated African easterly jet (AEJ). These large-scale conditions are linked to near-trough attributes of developing AEWs which favor more moisture ingestion, vertically aligned circulation, a stronger initial 850-hPa vortex, deeper wave pouch, and arguably more AEW and Mesoscale convective systems interactions. AEWs that initiate over eastern Africa and cross the west coast of Africa are more likely to undergo tropical cyclogenesis than those initiating over central or west Africa. Developing AEWs are more likely to be southern-track AEWs than non-developing AEWs.

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