Formation Of Mesoscale Convective Systems Research Articles

Classification of large-scale environments that drive the formation of mesoscale convective systems over southern West Africa

Abstract. Mesoscale convective systems (MCSs) are frequently observed over southern West Africa (SWA) throughout most of the year. These MCS events are the dominant rain-bearing systems, contributing over 50 % of annual rainfall over SWA. However, it has not yet been identified what variations in typical large-scale environments of the seasonal cycle of the West African monsoon may favour MCS occurrence in this region. Here, nine distinct synoptic states are identified and are further associated with being a synoptic-circulation type of either a dry, transition, or monsoon season using self-organizing maps (SOMs) with inputs from reanalysis data. We identified a pronounced annual cycle of MCS numbers with frequency peaks in April and October that can be associated with the start of rainfall during the major rainy season and the maximum rainfall for the minor rainy season across SWA, respectively. Comparing daily MCS frequencies, MCSs are most likely to develop during transition conditions featuring a northward-displaced moisture anomaly (2.8 MCSs per day), which can be linked to strengthened low-level westerlies. Considering that these transition conditions occur predominantly during the pre- and post-monsoon period, these patterns may in some cases be representative of monsoon onset conditions or a delayed monsoon retreat. On the other hand, under monsoon conditions, we observe weakened low-level south-westerlies during MCS days, which reduce moisture content over the Sahel but introduce more moisture over the coast. Finally, we find a majority of MCS-day synoptic states exhibiting positive zonal wind shear anomalies. Seasons with the strongest zonal wind shear anomalies are associated with the strongest low-level temperature anomalies to the north of SWA, highlighting that a warmer Sahel can promote MCS-favourable conditions in SWA. Overall, the SOM-identified synoptic states converge towards high-moisture and high-shear conditions on MCS days in SWA, where the frequency at which these conditions occur depends on the synoptic state.

Characteristics of convective precipitation over tropical Africa in storm‐resolving global simulations

AbstractWe analyse how the representation of deep convection affects the characteristics of convective precipitation over tropical Africa in global storm‐resolving simulations with the European Centre for Medium‐Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS). The simulations consist in 48 hr forecasts initiated daily at 0000 UTC for a 40‐day period in August and September 2016, at 9 and 4 km resolution. We find that results agree best with the satellite retrieval Global Precipitation Measurement (GPM) Integrated Multi‐satellitE Retrievals for GPM (IMERG) when a term for the integrated and scaled total advective moisture tendency is added to the convective instability closure. This allows for a better coupling of the convective parametrization to the mesoscale dynamics and improves precipitation intensity (increased), precipitating area fraction (decreased), the size of individual deep convective systems (decreased) and their propagation (westward), even though the duration of rain events remains underestimated. With explicit deep convection, the duration of rain events is more realistic, but precipitation intensity is significantly overestimated, and extreme rain events (>400 mmday) are three times more likely than in GPM IMERG, causing biases in the organization of deep convection. In particular, individual convective systems are too small, hindering the formation of mesoscale convective systems. These findings are also valid for two other regions: the Maritime Continent and the contiguous United States. Our results suggest that deep convection is not completely resolved at a resolution of 9 or even 4 km, and that some scale‐aware parametrization is still needed for global numerical weather prediction.

Diurnal self-aggregation

Convective self-aggregation is a modelling paradigm for convective rain cell organisation over a constant-temperature tropical sea surface. This set-up can give rise to cloud clusters developing over timescales of weeks. In reality, sea-surface temperatures do oscillate diurnally, affecting the atmospheric state and influencing rain rates significantly. Over land, surface temperatures vary more strongly. Here, we carry out a suite of cloud-resolving numerical experiments, and find that qualitatively different dynamics emerge from modest surface temperature oscillations: while the spatial distribution of rainfall is homogeneous during the first day, already on the second day, the rain field is firmly structured. In later days, this clustering becomes stronger and alternates from day to day. We show that these features are robust to changes in resolution, domain size and mean surface temperature, but can be removed by a reduction of the amplitude of diurnal surface temperature oscillation, suggesting a transition from a random to a clustered state. Maximal clustering occurs at a scale of {l}_{max }approx 180 {rm{km}}, which we relate to the emergence of mesoscale convective systems. At {l}_{max }, rainfall is strongly enhanced and far exceeds the rainfall expected at random. Simple conceptual modelling helps interpret the transition to clustering, which is driven by the formation of mesoscale convective systems, and brings about day-to-day moisture oscillations. Our results may help clarify how continental extremes build up, and how cloud clustering over the tropical ocean could emerge as an instance of spontaneous symmetry breaking at timescales much faster than in conventional radiative–convective equilibrium self-aggregation.

The formation, character and changing nature of mesoscale convective systems

Mesoscale convective systems (MCSs) describe organized groupings of thunderstorms in the tropics and mid-latitudes that span thousands of square kilometres. While recognized for over a century, the advent of satellite and radar observations, as well as atmospheric-model simulations, has brought about their increased understanding. In this Review, we synthesize current knowledge on MCS formation, climatological characteristics, hazardous weather, predictive capacity and projected changes with anthropogenic warming. Driven by typical deep moist convective processes (moisture, lift and instability) and vertical wind shear, MCS formation occurs preferentially in locations where these ingredients are present and can be maintained by large-scale ascent and the cold pools that they produce. MCSs also generate hazardous weather, including extreme rainfall, flooding, derechos and, sometimes, tornadoes and hail, all of which have substantial economic and societal impacts. Given that MCSs also produce a large fraction of warm-season rainfall, there is critical need for both short-term forecasts and long-term projections, presently challenged by inadequate model resolution. Yet, with continually improving modelling capabilities, as well as greater theoretical basis, it is suggested that MCSs might increase in frequency and intensity under a warming climate. Further modelling progress, in turn, offers improved understanding of MCS characteristics, from their life cycle through to impacts. Mesoscale convective systems are an important source of precipitation in many tropical and mid-latitude regions, but can also produce hazardous weather, such as extreme rain, derechos and tornadoes. This Review discusses the formation of mesoscale convective systems, their hazardous weather, predictive capabilities and projected changes with anthropogenic warming.

A Machine Learning Assisted Development of a Model for the Populations of Convective and Stratiform Clouds

AbstractTraditional parameterizations of the interaction between convection and the environment have relied on an assumption that the slowly varying large‐scale environment is in statistical equilibrium with a large number of small and short‐lived convective clouds. They fail to capture nonequilibrium transitions such as the diurnal cycle and the formation of mesoscale convective systems as well as observed precipitation statistics and extremes. Informed by analysis of radar observations, cloud‐permitting model simulation, theory, and machine learning, this work presents a new stochastic cloud population dynamics model for characterizing the interactions between convective and stratiform clouds, with the goal of informing the representation of these interactions in global climate models. Fifteen wet seasons of precipitating cloud observations by a C‐band radar at Darwin, Australia are fed into a machine learning algorithm to obtain transition functions that close a set of coupled equations relating large‐scale forcing, mass flux, the convective cell size distribution, and the stratiform area. Under realistic large‐scale forcing, the derived transition functions show that, on the one hand, interactions with stratiform clouds act to dampen the variability in the size and number of convective cells and therefore in the convective mass flux. On the other, for a given convective area fraction, a larger number of smaller cells is more favorable for the growth of stratiform area than a smaller number of larger cells. The combination of these two factors gives rise to solutions with a few convective cells embedded in a large stratiform area, reminiscent of mesoscale convective systems.

Initiation and Organizational Modes of an Extreme-Rain-Producing Mesoscale Convective System along a Mei-Yu Front in East China

Abstract The initiation and organization of a quasi-linear extreme-rain-producing mesoscale convective system (MCS) along a mei-yu front in east China during the midnight-to-morning hours of 8 July 2007 are studied using high-resolution surface observations and radar reflectivity, and a 24-h convection-permitting simulation with the nested grid spacing of 1.11 km. Both the observations and the simulation reveal that the quasi-linear MCS forms through continuous convective initiation and organization into west–east-oriented rainbands with life spans of about 4–10 h, and their subsequent southeastward propagation. Results show that the early convective initiation at the western end of the MCS results from moist southwesterly monsoonal flows ascending cold domes left behind by convective activity that develops during the previous afternoon-to-evening hours, suggesting a possible linkage between the early morning and late afternoon peaks of the mei-yu rainfall. Two scales of convective organization are found during the MCS's development: one is the east- to northeastward “echo training” of convective cells along individual rainbands, and the other is the southeastward “band training” of the rainbands along the quasi-linear MCS. The two organizational modes are similar within the context of “training” of convective elements, but they differ in their spatial scales and movement directions. It is concluded that the repeated convective backbuilding and the subsequent echo training along the same path account for the extreme rainfall production in the present case, whereas the band training is responsible for the longevity of the rainbands and the formation of the quasi-linear MCS.

Cloud detection and analysis on the Tibetan Plateau using Meteosat and CloudSat

AbstractPrevious studies of clouds over the Tibetan Plateau (TiP) were subject to limitations. Surface observations are scarce, and satellite retrievals are not well adapted to the peculiar conditions of the TiP. For the most comprehensive existing cloud data set, provided by the International Satellite Cloud Climatology Project (ISCCP), issues were reported for the TiP. It also lacks sufficient spatiotemporal resolution for this topographically complex region. With the Indian Ocean Data Coverage service, European Organisation for the Exploitation of Meteorological Satellites provides a Meteosat data set between 1998 and 2008. The resolution of around 6 km at the study area is sufficient even for complex terrain. Based on this data set and on products of the active sensor onboard CloudSat, we develop a novel gross‐cloud retrieval for the TiP using logistic regression models. The approach maintains the original Meteosat resolution. Validation against independent CloudSat data reveals good performance. The approach also outperforms the ISCCP pixel level (DX) data set. The resulting data set is the first for the TiP that provides cloud information with sufficient resolution for both day and night. Patterns of cloud frequencies during winter, premonsoon, and monsoon seasons are analyzed. Strong diurnal forcing is found for the plateau. Peaks of cloud frequencies above the slopes occur during afternoon, while they are delayed in the valleys, where high cloud frequencies persist throughout the nights. Above the lower parts of the southern foothills of the Himalayas cloud frequencies were for the first time found to increase until the early morning. Katabatic flows are suspected to be responsible for this pattern by initiating the formation of mesoscale convective systems.

Numerical investigation of a localized extremely heavy rainfall event in complex topographic area during midsummer

Numerical simulations with version 3.2 of the Weather Research and Forecasting (WRF) Model are performed to study a localized extremely heavy rainfall event during midsummer in central China. The event occurred in a complex topographical area on 3 July 2008. The extremely heavy rainfall was produced by a quasi-stationary back-building mesoscale convective system (MCS), which was initiated and developed in the exit region of a low-level jet (LLJ). The main mesoscale dynamical process responsible for the MCS formation was the low-level convergence directly generated by the LLJ. The gravity waves excited by the unbalanced dynamics due to the LLJ's abrupt intensification might be another dynamical factor for the MCS initiation. The LLJ was of obvious diurnal variation, which was nearly in an opposite phase with the variations of the planetary boundary layer (PBL) height and of the surface heat and moisture fluxes. The diurnal variation of the LLJ was mainly dominated by the solar-radiation-driven evolution of the PBL height and the surface heat fluxes below the LLJ. The dynamical uplifts forced by the Wufeng mountainous area and the Wushan Mountain were favorable for the formation and development of MCS. The topographic Froude number was less than one at night due to the increase of atmospheric stability in the lower level. The LLJ was blocked mainly by the Dabashan Mountain, and partly by the Wufeng mountainous area and the Wushan Mountain, leading to the MCS in the exit region of the LLJ to remain quasi-stationary and produce the localized extremely heavy rainfall.

Observation and numerical simulations with radar and surface data assimilation for heavy rainfall over central Korea

This study investigated the impact of multiple-Doppler radar data and surface data assimilation on forecasts of heavy rainfall over the central Korean Peninsula; the Weather Research and Forecasting (WRF) model and its three-dimensional variational data assimilation system (3DVAR) were used for this purpose. During data assimilation, the WRF 3DVAR cycling mode with incremental analysis updates (IAU) was used. A maximum rainfall of 335.0 mm occurred during a 12-h period from 2100 UTC 11 July 2006 to 0900 UTC 12 July 2006. Doppler radar data showed that the heavy rainfall was due to the back-building formation of mesoscale convective systems (MCSs). New convective cells were continuously formed in the upstream region, which was characterized by a strong southwesterly low-level jet (LLJ). The LLJ also facilitated strong convergence due to horizontal wind shear, which resulted in maintenance of the storms. The assimilation of both multiple-Doppler radar and surface data improved the accuracy of precipitation forecasts and had a more positive impact on quantitative forecasting (QPF) than the assimilation of either radar data or surface data only. The back-building characteristic was successfully forecasted when the multiple-Doppler radar data and surface data were assimilated. In data assimilation experiments, the radar data helped forecast the development of convective storms responsible for heavy rainfall, and the surface data contributed to the occurrence of intensified low-level winds. The surface data played a significant role in enhancing the thermal gradient and modulating the planetary boundary layer of the model, which resulted in favorable conditions for convection.

Formation of mesoscale convective systems over the eastern Tibetan Plateau affected by plateau‐scale heating contrasts

This study revealed the formation processes of mesoscale convective systems (MCSs) over the eastern Tibetan Plateau (TP), associated with the establishment of subplateau‐scale convergences affected by the longitudinal surface wetness gradient. Composites of reanalysis data and satellite infrared images showed that large MCSs occurred in the afternoon under the condition of the eastward expansion of upper tropospheric anticyclone with the enhancement of near‐surface low pressure in the western plateau. The low‐level convergence was systematically formed through the eastward propagation of a thermally induced cyclonic circulation formed in the north central plateau before the day of the MCS genesis. A numerical model successively simulated the MCS occurrence processes with the diurnal evolution of convergences, namely, (1) the formation of horizontal wind shear between southwesterlies and northeasterlies over the strong heated land‐surface causing a thermal low in the northwest, (2) the eastward propagation of the vorticity due to intensification of upper westerlies in the night, and (3) the MCS genesis by low‐level convergences behind the migrated vortex with a convective instability condition over the eastern wet land surface. Numerical sensitivity experiments confirmed that eliminating either the western surface sensible heat flux or the eastern surface latent heat flux prevented the development of MCS. Namely, the enhanced southeast‐northwest gradation of the plateau‐scale soil moisture distribution could effectively form the MCSs in the eastern plateau during the monsoon season.

The influence of merging and individual storm splitting on mesoscale convective system formation

The aim of present study is to simulate the formation of mesoscale convective system (MCS) through splitting and merging of two single convective storms that is characteristic for mountainous regions by help of cloud-resolving mesoscale model. Features of simulated storms are as follows: they are aligned along leading flow at different stages of life cycle under sharp-sheared condition. The storm in upwind direction (denoted as primary) is initialized earlier as it is a common feature in reality. Formation of MCS passes by several phases as follows: - Primary storm moves faster along leading flow direction than secondary one that is initialized later. Both storms split into right- and left-moving storms in such a manner that primary storm splits at first; - Boundaries of cold air outflows associated by single storms collide. At their intersection air is forced lifted upward, possibly forming a new convection; - As a result the merging process produces the MCS in which the single storms are distributed along curved line. Above results encourage modelers to use more frequently the cloud-resolving models in simulations of great variety of MCS.

A Modeling Diagnosis of the Development of Mesoscale Convective Systems over the South China Sea during the Summer Monsoon Onset in 1998

In this study, PSU/NCAR MM5 is used to simulate the evolution of the 1998 East Asian summer monsoon onset over the South China Sea (SCS). Simulation outputs are further utilized to study the development of mesoscale convective systems (MCSs) and their interactions with the synoptic/large-scale environment in the SCS region. During the monsoon onset, over southern China and the SCS, high equivalent potential temperature, convective instability, and the southwesterly confluence in the lower troposphere provided favorable large-scale thermodynamic and dynamic conditions for the MCS development. Model simulated results reveal that, in a warm-moist and unstable atmosphere, the downdraft outflows as a result of the MCS development could become a possible mesoscale triggering mechanism for the formation of a new MCS. Sensitivity tests depict the effect of latent heat release playing a significant role in the establishment of monsoon circulation. The terrain effect is significant for local cloud formation over land. For well organized MCSs occurring over the SCS, the influence of the mesoscale wind field is more important. The latent heat release in MCSs heats the atmosphere, reduces the surface pressure, and therefore enhances the low-level wind convergence and cyclonic circulation. The mesoscale adjustment of wind field and mass field, on one hand, enhances the MCS development or initiates the formation of a new MCS, and the organization of MCSs, on the other hand, integrates the mesoscale disturbances into a synoptic structure and forms the local monsoon trough. Without the latent heat release, monsoon circulation cannot establish itself even if the model provides favorable large-scale boundary forcing.