Types Of Mesoscale Convective Systems Research Articles

Contribution of Mesoscale Convective Systems to Floods in the East Asian Summer Monsoon Region

AbstractThe quantitative relationship between Mesoscale Convective Systems (MCSs) and floods over East Asia has not been established. In this study, MCSs are clustered into four types with Self‐Organizing Map approach. Floods in June‐August of 2000–2021 are linked with different types of MCS by automated algorithms we constructed. We find that among the major floods (potential flood peak periods), 91% (87%) are related to MCS, 65% (78%) are dominated by MCS, and 38% (20%) are dominated by multi‐types of MCS. Types 1 and 2 MCS have higher flood‐inducing efficiencies than common MCS (Type‐4). Type‐1 MCS, characterized by the least number (2% of the total number), the largest precipitation volume, longest lifetime, slowest moving, strongest precipitation, can most efficiently produce floods. Type‐2 MCS, characterized by the second largest precipitation volume, more numerous than Type‐1 particularly over land, can induce floods not only relatively efficiently but also more frequently than Type‐1.

Monitoring Mesoscale Convective System Using Swin-Unet Network Based on Daytime True Color Composite Images of Fengyun-4B

The monitoring of mesoscale convective systems (MCS) is typically based on satellite infrared data. Currently, there is limited research on the identification of MCS using true color composite cloud imagery. In this study, an MCS dataset was created based on the true color composite cloud imagery from the Fengyun-4B geostationary meteorological satellite. An MCS true color composite cloud imagery identification model was developed based on the Swin-Unet network. The MCS dataset was categorized into continental MCS and oceanic MCS, and the model’s performance in identifying these two different types of MCS was examined. Experimental results indicated that the model achieved a recall rate of 83.3% in identifying continental MCS and 86.1% in identifying oceanic MCS, with a better performance in monitoring oceanic MCS. These results suggest that using true color composite cloud imagery for MCS monitoring is feasible, and the Swin-Unet network outperforms traditional convolutional neural networks. Meanwhile, we find that the frequency and distribution range of oceanic MCS is larger than that of continental MCS, and the area is larger and some parts of it are stronger. This study provides a novel approach for satellite remote-sensing-based MCS monitoring.

A statistical analysis of the distribution of convective windstorms and their radar characteristics in Hubei Province, China. Part 2

A statistical analysis of the distribution of convective windstorms and their radar characteristics in Hubei Province, China. Part 2

Statistical Characteristics of Mesoscale Convective Systems Initiated over the Tibetan Plateau in Summer by Fengyun Satellite and Precipitation Estimates

In order to investigate the key characteristics of mesoscale convective systems (MCSs) initiated over the Tibetan Plateau (TP) in recent years and the main differences in circulation and environmental factors between different types of MCSs, an automatic MCS identification and tracking method was applied based on the data from China’s Fengyun satellite and precipitation estimates. In total, 8820 MCSs were found to have been initiated over the TP during the summers from 2013 to 2019, and a total of 9.3% of them were able to move eastward out of the TP (EO). The number of MCSs showed a monthly variation, with a maximum in July and a minimum in June, while most EOs occurred in June. Compared with other types of MCSs, EOs usually had a lower cloud-top temperature, a greater rainfall intensity, a longer life duration, more rapid development, larger areas of rainfall and convective clouds, longer tracks and a wider influence range, indicating that EOs are more vigorous than the other types of MCSs. The movement of MCSs is mainly due to the mid- to high-level dynamic conditions, and moisture is an essential factor in their development and maintenance.

Statistical analysis of ice microphysical properties in tropical mesoscale convective systems derived from cloud radar and in situ microphysical observations

Abstract. This study presents a statistical analysis of the properties of ice hydrometeors in tropical mesoscale convective systems observed during four different aircraft campaigns. Among the instruments on board the aircraft, we focus on the synergy of a 94 GHz cloud radar and two optical array probes (OAP; measuring hydrometeor sizes from 10 µm to about 1 cm). For two campaigns, an accurate simultaneous measurement of the ice water content is available, while for the two others, ice water content is retrieved from the synergy of the radar reflectivity measurements and hydrometeor size and morphological retrievals from OAP probes. The statistics of ice hydrometeor properties are calculated as a function of radar reflectivity factor measurement percentiles and temperature. Hence, mesoscale convective systems (MCS) microphysical properties (ice water content, visible extinction, mass–size relationship coefficients, total concentrations, and second and third moments of hydrometeor size distribution) are sorted in temperature (and thus altitude) zones, and each individual campaign is subsequently analyzed with respect to median microphysical properties of the merged dataset (merging all four campaign datasets). The study demonstrates that ice water content (IWC), visible extinction, total crystal concentration, and the second and third moments of hydrometeor size distributions are similar in all four types of MCS for IWC larger than 0.1 g m−3. Finally, two parameterizations are developed for deep convective systems. The first concerns the calculation of the visible extinction as a function of temperature and ice water content. The second concerns the calculation of hydrometeor size distributions as a function of ice water content and temperature that can be used in numerical weather prediction.

Investigation of Mesoscale Convective Systems over Indonesian Maritime Continent using Geostationary Meteorological Satellite

Variation of Mesoscale Convective Systems (MCSs) over Indonesian Maritime Continent (IMC) for 5 years in 2010-2014 were investigated using implementation of graph theory with hourly product from geostationary meteorological satellite infrared brightness temperature and Tropical Rainfall Measuring Mission (TRMM) precipitation data. Variations of MCSs are analyzed based on six criteria of convective cloud organization derived from area, duration and eccentricity characteristics. The result shows that monthly average of six types of MCSs in the IMC region forms a bimodal pattern with two peaks at March to May (MAM) and September to November (SON). A statistical analysis exhibit a maximum frequency occurrence of six types of MCSs in the region of equatorial rainfall type, which amount of events up to 40%, where the greatest contribution concentrated in the Indian Ocean, Sumatra, Kalimantan, Sulawesi, and Papua. Diurnal pattern of MCSs in the IMC shows maturity time of MCSs mostly occurred in the evening to early morning. The typical occurrence of MCSs in this study is different compared to United States and China types, whereas IMC has balanced proportion amounts between linear and circular shape types, while United States and China region were dominated by linear type.

Understanding the summertime diurnal cycle of precipitation over sub-Saharan West Africa: regions with daytime rainfall peaks in the absence of significant topographic features

Convection-permitting regional model output is analyzed to better understand the diurnal cycle of rainfall during the height of the West African summer monsoon. This investigation focuses on two regions, the western Bodele of Chad and eastern Burkina Faso, that have a propensity for daytime rainfall, but why this occurs does not conform to our conventional understanding of being directly associated with the presence of significant topographic feature(s). The August diurnal cycle of rainfall for the Bodele is characterized by an afternoon peak, with mesoscale convective systems (MCSs) originating within 500 km accounting for 80% of the afternoon precipitation. These MCSs are associated with a deepening of the monsoon trough over the western Sahara related to northern storm track African easterly wave (AEW) disturbance activity, combined with anomalous ridging over eastern Chad associated with cold pool outflow from convection that originates over the Marra Mountains the previous afternoon. These circulation features enhance the moist low-level southwesterly flow and increase instability over the Bodele. Over Burkina Faso rainfall has a primary afternoon peak, and a secondary morning peak. MCSs account for 95% of the total rainfall. Morning rainfall is primarily due to MCSs forming over the Damergou Gap of Niger, while the afternoon rainfall is associated with MCSs that originate over the Damergou Gap as well as locally. While both types of MCSs are associated with an approaching southern storm track AEW disturbance, it is differences in northern storm track activity that helps explain why some MCSs originate over the Damergou Gap.

Long-Lived Mesoscale Systems in a Low–Convective Inhibition Environment. Part I: Upshear Propagation

Abstract Dynamical models of organized mesoscale convective systems have identified the important features that help maintain their overarching structure and longevity. The standard model is the trailing stratiform archetype, featuring a front-to-rear ascending circulation, a mesoscale downdraft circulation, and a cold pool/density current that affects the propagation speed and the maintenance of the system. However, this model does not represent all types of mesoscale convective systems, especially in moist environments where the evaporation-driven cold pools are weak and the convective inhibition is small. Moreover, questions remain about the role of gravity waves in creating and maintaining organized systems and affecting their propagation speed. This study presents simulations and dynamical models of self-organizing convection in a moist, low–convective inhibition environment and examines the long-lived convective regimes that emerge spontaneously. This paper, which is Part I of this study, specifically examines the structure, kinematics, and maintenance of long-lived, upshear-propagating convective systems that differ in important respects from the standard model of long-lived convective systems. Linear theory demonstrates the role of ducted gravity waves in maintaining the long-lived, upshear-propagating systems. A steady nonlinear model approximates the dynamics of upshear-propagating density currents that are key to the maintenance of the mesoscale convective system.

Characteristics of Mesoscale Convective Systems over China and Its Vicinity Using Geostationary Satellite FY2

Abstract This study investigates mesoscale convective systems (MCSs) over China and its vicinity during the boreal warm season (May–August) from 2005 to 2012 based on data from the geostationary satellite Fengyun 2 (FY2) series. The authors classified and analyzed the quasi-circular and elongated MCSs on both large and small scales, including mesoscale convective complexes (MCCs), persistent elongated convective systems (PECSs), meso-β circular convective systems (MβCCSs), meso-β elongated convective system (MβECSs), and two additional types named small meso-β circular convective systems (SMβCCSs) and small meso-β elongated convective systems (SMβECSs). Results show that nearly 80% of the 8696 MCSs identified in this study fall into the elongated categories. Overall, MCSs occur mainly at three zonal bands with average latitudes around 20°, 30°, and 50°N. The frequency of MCSs occurrences is maximized at the zonal band around 20°N and decreases with increase in latitude. During the eight warm seasons, the period of peak systems occurrences is in July, followed decreasingly by June, August, and May. Meanwhile, from May to August three kinds of monthly variations are observed, which are clear northward migration, rapid increase, and persistent high frequency of MCS occurrences. Compared to MCSs in the United States, the four types of MCSs (MCCs, PECSs, MβCCSs, and MβECSs) are relatively smaller both in size and eccentricity but exhibit nearly equal life spans. Moreover, MCSs in both countries share similar positive correlations between their duration and maximum extent. Additionally, the diurnal cycles of MCSs in both countries are similar (local time) regarding the three stages of initiation, maturation, and termination.

A strategy for representing the effects of convective momentum transport in multiscale models: Evaluation using a new superparameterized version of the Weather Research and Forecast model (SP‐WRF)

AbstractThis paper describes a general method for the treatment of convective momentum transport (CMT) in large‐scale dynamical solvers that use a cyclic, two‐dimensional (2‐D) cloud‐resolving model (CRM) as a “superparameterization” of convective‐system‐scale processes. The approach is similar in concept to traditional parameterizations of CMT, but with the distinction that both the scalar transport and diagnostic pressure gradient force are calculated using information provided by the 2‐D CRM. No assumptions are therefore made concerning the role of convection‐induced pressure gradient forces in producing up or down‐gradient CMT. The proposed method is evaluated using a new superparameterized version of the Weather Research and Forecast model (SP‐WRF) that is described herein for the first time. Results show that the net effect of the formulation is to modestly reduce the overall strength of the large‐scale circulation, via “cumulus friction.” This statement holds true for idealized simulations of two types of mesoscale convective systems, a squall line, and a tropical cyclone, in addition to real‐world global simulations of seasonal (1 June to 31 August) climate. In the case of the latter, inclusion of the formulation is found to improve the depiction of key synoptic modes of tropical wave variability, in addition to some aspects of the simulated time‐mean climate. The choice of CRM orientation is also found to importantly affect the simulated time‐mean climate, apparently due to changes in the explicit representation of wide‐spread shallow convective regions.

Constraining mass–diameter relations from hydrometeor images and cloud radar reflectivities in tropical continental and oceanic convective anvils

Abstract. In this study the density of ice hydrometeors in tropical clouds is derived from a combined analysis of particle images from 2-D-array probes and associated reflectivities measured with a Doppler cloud radar on the same research aircraft. Usually, the mass–diameter m(D) relationship is formulated as a power law with two unknown coefficients (pre-factor, exponent) that need to be constrained from complementary information on hydrometeors, where absolute ice density measurement methods do not apply. Here, at first an extended theoretical study of numerous hydrometeor shapes simulated in 3-D and arbitrarily projected on a 2-D plan allowed to constrain the exponent βof the m(D) relationship from the exponent σ of the surface–diameterS(D)relationship, which is likewise written as a power law. Since S(D) always can be determined for real data from 2-D optical array probes or other particle imagers, the evolution of the m(D) exponent can be calculated. After that, the pre-factor α of m(D) is constrained from theoretical simulations of the radar reflectivities matching the measured reflectivities along the aircraft trajectory. The study was performed as part of the Megha-Tropiques satellite project, where two types of mesoscale convective systems (MCS) were investigated: (i) above the African continent and (ii) above the Indian Ocean. For the two data sets, two parameterizations are derived to calculate the vertical variability of m(D) coefficients α and β as a function of the temperature. Originally calculated (with T-matrix) and also subsequently parameterized m(D) relationships from this study are compared to other methods (from literature) of calculating m(D) in tropical convection. The significant benefit of using variable m(D) relations instead of a single m(D) relationship is demonstrated from the impact of all these m(D) relations on Z-CWC (Condensed Water Content) and Z-CWC-T-fitted parameterizations.

Robust Observational Quantification of the Contribution of Mesoscale Convective Systems to Rainfall in the Tropics

Satellite estimation of precipitation and satellite-derived statistics of mesoscale convective systems (MCS) are analyzed conjunctively to quantify the contribution of the various types of MCS to the water budget of the tropics. This study focuses on two main mesoscale characteristics of the systems: duration and propagation. Overall, the systems lasting more than 12 h are shown to account for around 75% of the tropical rainfall, and 60% of the rainfall is due to systems traveling more than 250 km, a typical GCM grid. A number of regional features are also revealed by factoring in the convective systems’ morphological parameters in the water budget computation. These findings support the challenging effort to account for such mesoscale features when considering the theory on the future evolution of the water budget as well as the physical parameterizations of climate models. Finally, this analysis provides a simple metric for evaluating high-resolution numerical simulations of the tropical water budget. Furthermore, results are shown to be robust to the selection of the satellite rainfall products.

Mesoscale convective systems observed during AMMA and their impact on the NO<sub>x</sub> and O<sub>3</sub> budget over West Africa

Abstract. During the "African Monsoon Multidisciplinary Analysis" (AMMA) field phase in August 2006, a variety of measurements focusing on deep convection were performed over West Africa. The German research aircraft Falcon based in Ouagadougou (Burkina Faso) investigated the chemical composition in the outflow of large mesoscale convective systems (MCS). Here we analyse two different types of MCS originating north and south of the intertropical convergence zone (ITCZ, ~10° N), respectively. In addition to the airborne trace gas measurements, stroke measurements from the Lightning Location Network (LINET), set up in Northern Benin, are analysed. The main focus of the present study is (1) to analyse the trace gas composition (CO, O3, NO, NOx, NOy, and HCHO) in the convective outflow as a function of distance from the convective core, (2) to investigate how different trace gas compositions in the boundary layer (BL) and ambient air may influence the O3 concentration in the convective outflow, and (3) to estimate the rate of lightning-produced nitrogen oxides per flash in selected thunderstorms and compare it to our previous results for the tropics. The MCS outflow was probed at different altitudes (~10–12 km) and distances from the convective core (<500 km). Trace gas signatures similar to the conditions in the MCS inflow region were observed in the outflow close to the convective core, due to efficient vertical transport. In the fresh MCS outflow, low O3 mixing ratios in the range of 35–40 nmol mol−1 were observed. Further downwind, O3 mixing ratios in the outflow rapidly increased with distance, due to mixing with the ambient O3-rich air. After 2–3 h, O3 mixing ratios in the range of ~65 nmol mol−1 were observed in the aged outflow. Within the fresh MCS outflow, mean NOx (=NO+NO2) mixing ratios were in the range of ~0.3–0.4 nmol mol−1 (peaks ~1 nmol mol−1) and only slightly enhanced compared to the background. Both lightning-produced NOx (LNOx) and NOx transported upward from the BL contributed about equally to this enhancement. On the basis of Falcon measurements, the mass flux of LNOx in the investigated MCS was estimated to be ~100 g(N) s−1. The average stroke rate of the probed thunderstorms was 0.04–0.07 strokes s−1 (here only strokes with peak currents ≥10 kA contributing to LNOx were considered). The LNOx mass flux and the stroke rate were combined to estimate the LNOx production rate. For a better comparison with other published results, LNOx estimates per LINET stroke were scaled to Lightning Imaging Sensor (LIS) flashes. The LNOx production rate per LIS flash was estimated to 1.0 and 2.5 kg(N) for the MCS located south and north of the ITCZ, respectively. If we assume, that these different types of MCS are typical thunderstorms occurring globally (LIS flash rate ~44 s−1), the annual global LNOx production rate was estimated to be ~1.4 and 3.5 Tg(N) a−1.

Employment of MM5 in simulating MCSs developed in and around Bangladesh

The Mesoscale Convective Systems (MCSs) produce numerous weather hazards with their variety of forms. The formation mechanism of MCSs is thus important to know for Bangladesh and its surroundings, because this region is one of the heaviest rainfall areas in the tropical zone. The meteorologists are studying and analyzing the formation mechanism of different types of MCSs using radar and satellite observations data. Observations are limited to real time, but for planning purposes projected parameters over a certain period obtained from a mesoscale model is the requirement. Consequently, the motivation of this paper is to obtain the evolution and life cycle of MCSs developed in and around Bangladesh during pre-monsoon period through the simulation by a mesoscale model named MM5. In this work the calibration of MM5 model for different cumulus parameterization has been performed during the pre-monsoon period of this region. In the present study two domains with mesh resolutions 45 km × 45 km and 15 km × 15 km are prepared. MM5 runs using different cumulus parameterizations are carried out for sensitivity test. The precipitation simulated by the model are compared structurally and numerically with that of Tropical Rainfall Measuring Mission (TRMM) data products, available data from radar scan and observed rain-gauges rainfall in Bangladesh. Important features like lifetime, maintenance mechanism, traversed path, propagation speed and direction of MCSs developed during pre-monsoon period of 2002 in and around Bangladesh have been pointed out.

Climatological distribution and diurnal variation of mesoscale convective systems over China and its vicinity during summer

The climatological distribution of mesoscale convective systems (MCSs) over China and its vicinity during summer is statistically analyzed, based on the 10-year (1996–2006, 2004 excluded) June–August infrared TBB (Temperature of black body) dataset. Comparing the results obtained in this paper with the distribution of thunderstorms from surface meteorological stations over China and the distribution of lightning from low-orbit satellites over China and its vicinity in the previous studies, we find that the statistic characteristics of TBB less than −52°C can better represent the spatiotemporal distribution of MCSs over China and its vicinity during summer. The spreading pattern of the MCSs over this region shows three transmeridional bands of active MCSs, with obvious fluctuation of active MCSs in the band near 30°N. It can be explained by the atmospheric circulation that the three bands of active MCSs are associated with each other by the summer monsoon over East Asia. We focus on the diurnal variations of MCSs over different underlying surfaces, and the result shows that there are two types of MCSs over China and its vicinity during summer. One type of MCSs has only one active period all day long (single-peak MCSs), and the other has multiple active periods (multi-peak MCSs). Single-peak MCSs occur more often over plateaus or mountains, and multi-peak MCSs are more common over plains or basins. Depending on lifetimes and active periods, single-peak MCSs can be classified as Tibetan Plateau MCSs, general mountain MCSs, Ryukyu MCSs, and so on. The diurnal variation of multi-peak MCSs is very similar to that of MCCs (mesoscale convective complexes), and it reveals that multi-peak MCSs has longer life cycle and larger horizontal scale, becomes weaker after sunset, and develops again after midnight. Tibetan Plateau MCSs and general mountain MCSs both usually develop in the afternoon, but Tibetan Plateau MCSs have longer life cycle and more active MαCSs. Ryukyu MCSs generally develop after midnight, last longer time, and also have more active MαCS. The abundant moisture and favorable large-scale environment over Indian monsoon surge areas lead to active MCSs and MαCSs almost at any hour all day during summer. Due to local mountain-valley breeze circulation over the Sichuan Basin, MCSs are developed remarkably more often during the nighttime, and again there are also more active MαCSs. Because of local prominent sea-land breeze circulation over Guangxi and Guangdong, the MCSs over this region propagate from sea to land in the afternoon and from land to sea after midnight. The statistic characteristics of TBB less than −52°C clearly display the different climatological characteristics of MCSs owing to the thermal difference among water, land and rough terrain. Not only the large-scale atmospheric circulation but also the local atmospheric circulation caused by the thermal difference among water, land and rough terrain, to a great extent, determines the climatological distribution of MCSs over China and its vicinity during summer.

Organization of mesoscale convective systems: 2. Linear theory

The anelastic system of equations (3‐D momentum, continuity and thermodynamic energy) is used to investigate the organization of mesoscale convective systems (MCSs) using the reference layer concept presented in part 1. Latent heating is assumed to be proportional to the vertical velocity. The WKBJ method is used to solve this system of equations by perturbing the solution linearly from that at the reference level, which is the top of the reference layer. The reference layer is defined as a layer with maximum wind shear and unstable moist stratification over a minimum thickness of 200 mbar. The characteristics of the reference layer, such as the magnitude of the shear and moist stratification, determine the type of MCS' organization and associated properties in the analysis. This result agrees well with numerical simulations of linear MCSs presented in part 1 of this series of study. Three types of MCSs are identified from the linear theory: one is nonlinear and two are linear. The group speed of all three types turns out to be proportional to the mean wind at the reference level. Nonlinear MCSs occur when the vertical wind shear is weak and the stratification is unstable. Type 1 linear solutions are neutral, shear‐parallel lines; their reference layers are usually in the middle troposphere. Type 2 linear solutions are amplifying, shear‐perpendicular lines; their reference layers are usually in the lower troposphere. The theory predicts the widths and growth rates of the type 2 linear MCSs. The width of an amplifying MCS is determined as a simple function of the Richardson number, while the growth rate is a function of vertical wind shear and Richardson number. Tests of the linear theory using data from several field experiments show that it gives fairly realistic results for a variety of the observed MCSs.

Structure of Mesoscale Convective Systems during the Late Baiu Season in the Global Warming Climate Simulated by a Non-Hydrostatic Regional Model

Mesoscale convective systems (MCSs) that bring rainfall in the vicinity of Kyushu Island, Japan during the late Baiu season in the present and global warming climates are examined by a non-hydrostatic regional climate model (NHM) with the horizontal grid of 5 km.In the global warming climate, two types of MCSs appear in the vicinity of Kyushu Island. One travels from the Chinese Continent and the other from the southern part of the East China Sea to Kyushu Island. These two MCSs often merge over the sea southwest of Kyushu Island, and they rapidly develop to bring heavy precipitation to the vicinity of Kyushu Island. Among the latter, MCSs with low cloud-tops below 4 km MSL (Mean Sea Level) are found.In the comparison with the present climate, the averaged cloud and rain water mixing ratios in the vicinity of Kyushu Island become much larger, and the peak altitude of the mixing ratios are about 0.5-1.0 km higher in the global warming climate. The cloud water mixing ratio between 2-4 km MSL increases in the global warming climate, corresponding to MCSs with low cloud-tops.These results suggest one of the processes to produce heavy rainfall in the vicinity of Kyushu Island in July in the global warming climate.

On the characteristics of the rainfall events in the Sahel with a view to the analysis of climatic variability

The lack of appropriate data has long been a major obstacle to the study of the Sahelian rainfall at the event scale. In this paper, use is made of the EPSAT-Niger recording rain-gauge data to characterize the convective rain events of the central Sahel. Although some considerations lead to the identification of two main types of mesoscale convective systems, it is shown here that the most relevant stratification in terms of statistical analysis of the event rainfall distribution is between the events of the margins and those of the core of the rainy season. In fact, the average storm rain-depth appears to be non-stationary in time, with storm rain-depths slightly higher in the core of the rainy season than on the margins. Separation between the core and the margins thus allows the fitting of an exponential model to the observed storm rain-depth distributions of each period (core and margins), although a better fit would certainly be obtained if a proper modelling of the time non-stationarity was carried out. It is then shown that there is little, if any, correlation between the mean storm rain-depth of a given year and the overall abundance of the corresponding rainy season. This is a validation of previous works, which reached the same conclusion using daily rainfall data only. One major result of this work is that the statistics characterizing the rain events in the Sahel display little fluctuations, either in space or from year-to-year, as compared with those observed for the total seasonal rainfall. Each year and at each station the average storm rainfall remains close to 12 mm during the margins of the rainy season and close to 15 mm in the core. During the same period, the average seasonal rainfall over the study area ranged from 400 to 660 mm and for any given year the ratio between the maximum and the minimum point seasonal rainfall was of the order of 2. It is therefore concluded that the main source of rainfall variability in the Sahel is linked to the variability in the number of events rather than in the magnitude of these events. © 1998 Royal Meteorological Society

The Impact of Mesoscale Convective Systems on Regional Visibility and Oxidant Distributions during Persistent Elevated Pollution Episodes

It is generally accepted that atmospheric pollutants can be transported over distances described as synoptic in scale. However, the redistribution and removal of the various pollutant species may be significantly influenced, often in a highly episodic way, by phenomena which are inherently mesoscale in nature. Detailed mesoanalyses over a 31 hour period reveal the dramatic influence of several types of mesoscale convective systems on regional ozone and visibility. In the impacted air mass, afternoon ozone was significantly depleted (20–40 ppb vs 100–120 ppb in the polluted regional air mass). Visibilities, which prior to the wind shift/rain had typically been 4–13 km rose in a matter of a few hours to 27–80 km at many locations. This convective aerosol removal event covered Virginia, northern North Carolina, Delmarva, and more than 500 km into the Atlantic, as clearly evidenced by GOES satellite imagery. Assuming a relationship between visibility and surface aerosol mass concentrations, it is speculated that approximately 30 × 106 kg of sulfate were removed from the planetary boundary layer (PBL). The removal was not due solely to wet deposition. Rather, consideration of thunderstorm dynamics suggests that perhaps two-thirds of the removal from the PBL could have been due to convective displacement into the free atmosphere, perhaps to altitudes as high as the tropopause.