A Numerical Study of Tropical Cyclone Formation from Two Mesoscale Convective Systems in a Large-scale Convective System

Abstract The effects of multiple mesoscale convective systems (MCSs) on the formation of Typhoon Ketsana (2003) are analyzed in this study. Numerical simulations using the Weather Research and Forecasting (WRF) model with assimilation of Quick Scatterometer (QuikSCAT) and Special Sensor Microwave Imager (SSM/I) oceanic winds and total precipitable water are performed. The WRF model simulates well the large-scale features, the convective episodes associated with the MCSs and their periods of development, and the formation time and location of Ketsana. With the successive occurrence of MCSs, midlevel average relative vorticity is strengthened through generation of mesoscale convective vortices (MCVs) mainly via the vertical stretching mechanism. Scale separation shows that the activity of the vortical hot tower (VHT)-type meso-γ-scale vortices correlated well with the development of the MCSs. These VHTs have large values of positive relative vorticity induced by intense low-level convergence, and thus play an important role in the low-level vortex enhancement with aggregation of VHTs as one of the possible mechanisms. Four sensitivity experiments are performed to analyze the possible different roles of the MCSs during the formation of Ketsana by modifying the vertical relative humidity profile in each MCS and consequently the strength of convection within. The results show that the development of an MCS depends substantially on that of the prior ones through remoistening of the midtroposphere, and thus leading to different scenarios of system intensification during the tropical cyclone (TC) formation. The earlier MCSs are responsible for the first stage vortex enhancement, and depending on the location can affect quite largely the simulated formation location. The extreme convection within the last MCS before formation largely determines the formation time.

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

Journal of Hydrometeorology ◽

10.1175/jhm-d-20-0244.1 ◽

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.

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Toward an Understanding of Tropical Cyclone Formation with a Nonhydrostatic, Mesoscale-Convection-Resolving Model§

The Open Atmospheric Science Journal ◽

10.2174/1874282301307010037 ◽

2013 ◽

Vol 7 (1) ◽

pp. 37-50

Author(s):

Masanori Yamasaki

Keyword(s):

Tropical Cyclone ◽

Numerical Experiments ◽

Vertical Shear ◽

Initial Time ◽

Convective System ◽

Vortex Center ◽

Moist Convection ◽

Mesoscale Convection ◽

Tropical Cyclone Formation ◽

Convective Systems

This paper describes results from numerical experiments which have been made toward a better understanding of tropical cyclone formation. This study uses a nonhydrostatic version of the author’s mesoscale-convection-resolving model that was developed in the 1980s to improve paramerization schemes of moist convection. In this study the horizontal grid size is taken to be 20 km in an area of 6,000 km x 3,000 km, and a non-uniform coarse grid is used in two areas to its north and south. Results from two numerical experiments are presented; one (case 1) without any environmental flow, and the other (case 2) with an easterly flow without low-level vertical shear. Three circular buoyancy perturbations are placed in the west-east direction at the initial time. Convection is initiated in the imposed latently unstable (positive CAPE) area. In both cases, a vortex with a pressure low is formed, and two band-shaped convective systems are formed to the north and the south of the vortex center. The vortex and two convective systems are oriented in the westsouthwest – eastnortheast direction, and their horizontal scales are nearly 2,000 km. In case 1, the band-shaped convective system on the southern side is stronger, and winds are stronger just to its south. In contrast, in case 2, the northern convective system is stronger, and winds are stronger just to its north. Therefore, the distributions of the equivalent potential temperature in the boundary layer and latent instability (positive buoyancy of the rising air) are also quite different between cases 1 and 2. The TC formation processes in these different cases are discussed, with an emphasis on the importance of examining the time change of latent instability field.

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Lightning and precipitation produced by severe weather systems over Belém, Brazil

Revista Brasileira de Meteorologia ◽

10.1590/0102-778620130039 ◽

2014 ◽

Vol 29 (spe) ◽

pp. 41-59 ◽

Cited By ~ 3

Author(s):

Wanda Maria do Nascimento Ribeiro ◽

José Ricardo Santos Souza ◽

Márcio Nirlando Gomes Lopes ◽

Renata Kelen Cardoso Câmara ◽

Edson José Paulino Rocha ◽

...

Keyword(s):

Large Scale ◽

Electric Activity ◽

Weather Conditions ◽

Rain Gauge ◽

Mesoscale Convective Systems ◽

Major Influence ◽

Rainfall Events ◽

Convective Systems ◽

Mesoscale Convective ◽

Cg Lightning

CG Lightning flashes events monitored by a LDN of the Amazon Protection System, which included 12 LPATS IV VAISALA sensors distributed over eastern Amazonia, were analyzed during four severe rainstorm occurrences in Belem-PA-Brazil, in the 2006-2007 period. These selected case studies referred to rainfall events, which produced more than 25 mm/hour, or more than 40 mm/ 2 hours of precipitation rate totals, registered by a tipping bucket automatic high-resolution rain gauge, located at 1º 47' 53" S and 48º 30' 16" W. Centered at this location, a 30 ,10 and 5 km radius circles were drawn by means of a geographic information system, and the data from lightning occurrences within this larger area, were set apart for analysis. During these severe storms the CG lightning events, occurred almost randomly over the surrounding defined circle, previously covered by mesoscale convective systems, for all cases studied. This work also showed that the interaction between large-scale and mesoscale weather conditions have a major influence on the intensity of the storms studied cases. In addition to the enhancement of the lightning and precipitation rates, the electric activity within the larger circles can precede the rainfall at central point of the areas

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Ensemble cloud-resolving modelling of a historic back-building mesoscale convective system over Liguria: the San Fruttuoso case of 1915

Climate of the Past ◽

10.5194/cp-13-455-2017 ◽

2017 ◽

Vol 13 (5) ◽

pp. 455-472 ◽

Cited By ~ 10

Author(s):

Antonio Parodi ◽

Luca Ferraris ◽

William Gallus ◽

Maurizio Maugeri ◽

Luca Molini ◽

...

Keyword(s):

Extreme Event ◽

Flash Flood ◽

Western Mediterranean ◽

Mesoscale Convective Systems ◽

Convective System ◽

Water Vapour Content ◽

Ligurian Sea ◽

Convective Systems ◽

Position Errors ◽

Mesoscale Convective

Abstract. Highly localized and persistent back-building mesoscale convective systems represent one of the most dangerous flash-flood-producing storms in the north-western Mediterranean area. Substantial warming of the Mediterranean Sea in recent decades raises concerns over possible increases in frequency or intensity of these types of events as increased atmospheric temperatures generally support increases in water vapour content. However, analyses of the historical record do not provide a univocal answer, but these are likely affected by a lack of detailed observations for older events. In the present study, 20th Century Reanalysis Project initial and boundary condition data in ensemble mode are used to address the feasibility of performing cloud-resolving simulations with 1 km horizontal grid spacing of a historic extreme event that occurred over Liguria: the San Fruttuoso case of 1915. The proposed approach focuses on the ensemble Weather Research and Forecasting (WRF) model runs that show strong convergence over the Ligurian Sea (17 out of 56 members) as these runs are the ones most likely to best simulate the event. It is found that these WRF runs generally do show wind and precipitation fields that are consistent with the occurrence of highly localized and persistent back-building mesoscale convective systems, although precipitation peak amounts are underestimated. Systematic small north-westward position errors with regard to the heaviest rain and strongest convergence areas imply that the reanalysis members may not be adequately representing the amount of cool air over the Po Plain outflowing into the Ligurian Sea through the Apennines gap. Regarding the role of historical data sources, this study shows that in addition to reanalysis products, unconventional data, such as historical meteorological bulletins, newspapers, and even photographs, can be very valuable sources of knowledge in the reconstruction of past extreme events.

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Tracking Mesoscale Convective Systems that are Potential Candidates for Tropical Cyclogenesis

Monthly Weather Review ◽

10.1175/mwr-d-19-0070.1 ◽

2020 ◽

Vol 148 (2) ◽

pp. 655-669 ◽

Cited By ~ 2

Author(s):

Kelly M. Núñez Ocasio ◽

Jenni L. Evans ◽

George S. Young

Keyword(s):

Propagation Speed ◽

Extreme Rainfall ◽

Life Cycles ◽

Mesoscale Convective Systems ◽

Tropical Cyclogenesis ◽

Convective System ◽

Background Flow ◽

Extreme Rainfall Events ◽

Convective Systems ◽

Mesoscale Convective

Abstract This study introduces the development of the Tracking Algorithm for Mesoscale Convective Systems (TAMS), an algorithm that allows for the identifying, tracking, classifying, and assigning of rainfall to mesoscale convective systems (MCSs). TAMS combines area-overlapping and projected-cloud-edge tracking techniques to maximize the probability of detecting the progression of a convective system through time, accounting for splits and mergers. The combination of projection on area overlapping is equivalent to setting the background flow in which MCSs are moving on. Sensitivity tests show that area-overlapping technique with no projection (thus, no background flow) underestimates the real propagation speed of MCSs over Africa. The MCS life cycles and propagation derived using TAMS are consistent with climatology. The rainfall assignment is also more reliable than with previous methods as it utilizes a combination of regridding through linear interpolation with high temporal and spatial resolution data. This makes possible the identification of extreme rainfall events associated with intense MCSs more effectively. TAMS will be utilized in future work to build an AEW–MCS dataset to study tropical cyclogenesis.

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Observed structure of mesoscale convective systems and implications for large-scale heating

Quarterly Journal of the Royal Meteorological Society ◽

10.1002/qj.49711548702 ◽

1989 ◽

Vol 115 (487) ◽

pp. 425-461 ◽

Cited By ~ 321

Author(s):

Robert A. Houze

Keyword(s):

Large Scale ◽

Mesoscale Convective Systems ◽

Convective Systems ◽

Mesoscale Convective

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Soil Moisture Influence on the Incidence of Summer Mesoscale Convective Systems in the U.S. Great Plains

Monthly Weather Review ◽

10.1175/mwr-d-21-0140.1 ◽

2021 ◽

Author(s):

Rachel Gaal ◽

James L. Kinter

Keyword(s):

Soil Moisture ◽

Great Plains ◽

Land Surface ◽

Large Scale ◽

Mesoscale Convective Systems ◽

Antecedent Soil Moisture ◽

Initiation Point ◽

Convective Systems ◽

Mesoscale Convective ◽

The U.S

AbstractMesoscale convective systems (MCS) are known to develop under ideal conditions of temperature and humidity profiles and large-scale dynamic forcing. Recent work, however, has shown that summer MCS events can occur under weak synoptic forcing or even unfavorable large-scale environments. When baroclinic forcing is weak, convection may be triggered by anomalous conditions at the land surface. This work evaluates land surface conditions for summer MCS events forming in the U.S. Great Plains using an MCS database covering the contiguous United States east of the Rocky Mountains, in boreal summers 2004-2016. After isolating MCS cases where synoptic-scale influences are not the main driver of development (i.e. only non-squall line storms), antecedent soil moisture conditions are evaluated over two domain sizes (1.25° and 5° squares) centered on the mean position of the storm initiation. A negative correlation between soil moisture and MCS initiation is identified for the smaller domain, indicating that MCS events tend to be initiated over patches of anomalously dry soils of ~100-km scale, but not significantly so. For the larger domain, soil moisture heterogeneity, with anomalously dry soils (anomalously wet soils) located northeast (southwest) of the initiation point, is associated with MCS initiation. This finding is similar to previous results in the Sahel and Europe that suggest that induced meso-β circulations from surface heterogeneity can drive convection initiation.

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On the Predictability of Mesoscale Convective Systems: Two-Dimensional Simulations

Weather and Forecasting ◽

10.1175/2008waf2007057.1 ◽

2008 ◽

Vol 23 (5) ◽

pp. 773-785 ◽

Cited By ~ 23

Author(s):

Matthew S. Wandishin ◽

David J. Stensrud ◽

Steven L. Mullen ◽

Louis J. Wicker

Keyword(s):

Wind Speed ◽

Relative Humidity ◽

Ensemble Member ◽

Mesoscale Convective Systems ◽

Convective System ◽

Forecast Errors ◽

Two Dimensional ◽

Substantial Impact ◽

Convective Systems ◽

Mesoscale Convective

Abstract Mesoscale convective systems (MCSs) are a dominant climatological feature of the central United States and are responsible for a substantial fraction of warm season rainfall. Yet very little is known about the predictability of MCSs. To help alleviate this situation, a series of ensemble simulations of an MCS are performed on a two-dimensional, storm-scale (Δx = 1 km) model. Ensemble member perturbations in wind speed, relative humidity, and instability are based on current 24-h forecast errors from the North American Model (NAM). The ensemble results thus provide an upper bound on the predictability of mesoscale convective systems within realistic estimates of environmental uncertainty, assuming successful convective initiation. The simulations are assessed by considering an ensemble member a success when it reproduces a convective system of at least 20 km in length (roughly the size of two convective cells) within 100 km on either side of the location of the MCS in the control run. By that standard, MCSs occur roughly 70% of the time for perturbation magnitudes consistent with 24-h forecast errors. Reducing the perturbations for all fields to one-half the 24-h error values increases the MCS success rate to over 90%. The same improvement in forecast accuracy would lead to a 30%–40% reduction in maximum surface wind speed uncertainty and a roughly 20% reduction in the uncertainty in maximum updraft strength, and initially slower growth in the uncertainty in the size of the MCS. However, the occurrence of MCSs drops below 50% as the midlayer mean relative humidity falls below 65%. The response of the model to reductions in forecast errors for instability, moisture, and wind speed is not consistent and cannot be easily generalized, but each can have a substantial impact on forecast uncertainty.

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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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