Propagation Of Mesoscale Convective Systems Research Articles

A Mechanism for the Southward Propagation of Mesoscale Convective Systems Over the Bay of Bengal

AbstractEquatorward propagating precipitation episodes over the Bay of Bengal have been documented in many previous observational studies. Proposed propagation mechanisms include mean surface to midtropospheric wind shear driving the convection orthogonal to the lower tropospheric winds and the gravity currents generated by outflow from convection initiated by the diurnally varying land‐ocean circulations dispersing south. In this study, we perform high‐resolution simulations using the Weather Research and Forecast model capable of resolving mesoscale convective systems during the South Asian summer monsoon season. This mesoscale system is shown to have squall line‐like structure with leading line/trailing stratiform. The rear inflow due to saturated downdraft and jump updraft indicates a gravity current‐like structure. The rear inflow jet produces horizontal momentum tendencies in the direction of propagation. The center of convection is shown to move faster than the midtropospheric winds and at the same speed as that of the rear inflow jet near the surface. These systems are also shown to be tightly coupled to the diurnal land surface heating cycle. We perform additional model simulations with varying horizontal resolution and with the inclusion of cumulus parameterization. A model with cumulus parameterization is unable to simulate the updraft‐downdraft pair and the gravity current structure of this southward propagating mesoscale system. We find that high model resolution is needed to resolve the updraft‐downdraft pair and cumulus parameterization assumptions break down at such high resolutions. Using cloud microphysics exclusively becomes essential in simulating these mesoscale systems.

Simulation of vegetation feedbacks on local and regional scale precipitation in West Africa

Abstract Planned changes to land use in West Africa have been proposed to both combat desertification and to preserve biodiversity in the region, however, there is an urgent need for tools to assess the effects of these proposed changes on local and regional scale precipitation. We use a high-resolution, convection-permitting numerical weather prediction (NWP) model to study how the initiation and propagation of mesoscale convective systems (MCS) depends on the surface vegetation cover. The simulations covered a 4-day period during the West African monsoon in August 2006. In many aspects of the simulations, there was evidence of vegetation type exerting a significant influence on the location of precipitation where the influence of orography and coastal water was minimal. In this study, vegetation was classified according to the fractional coverage of tree (>30%) and grass (>30%) plant functional types. Tree-grass boundary cover was defined where more than 3 grid cells of both tree and grass occurred in a moving 3 × 3 window, which was further enlarged using a 3 grid cell (∼12 km) buffer. We found that over the whole study region (5N to 17N and 11W to 9E) 33.8% of convective initiations occur over tree-grass boundaries that cover only 28.4% of the land surface. This is significantly more than would be expected by chance (p = 0.0483), providing support to the hypothesis that vegetation gradients provide heat and moisture gradients, of a similar magnitude to that of soil moisture. Additionally, we found that on average, more time under an MCS occurred over boundary cover and orography, followed by tree cover, during the afternoon and evening period, thus supporting the hypothesis that land cover type influences the location of larger propagating systems. Contrasting patterns were found in the quantity of precipitation between small-scale convective cells and larger scale MCS. More small-scale precipitation accumulated, on average, over grass cover during the afternoon period, indicating a tendency for small-scale convection, initiated over boundaries, to prefer the drier and warmer grass side of vegetation boundaries in the afternoon period. However, once these smaller scale convective cells merge together to form larger MCS, a tendency for the most intense precipitation to fall over tree cover was observed. When intense precipitation (>10 mm per hour) occurred simultaneously over tree, boundary and grass cover, we found the highest precipitation rate to be most frequently over tree cover (48.4%), and least frequently over boundary cover (19.9%), indicating a preference of MCS for cooler, more moist forest cover. These results show for the first time that convection-permitting NWP models do exhibit responses to vegetation similar to those observed in the real world, and therefore are useful tools to assess the impacts of proposed future land use changes.

Mechanism of the northward propagation of mesoscale convective systems observed on 15 June 2005 during PALAU2005

This paper describes the structure and evolution of northward propagating mesoscale convective systems (MCSs) observed over the tropical western Pacific on 15 June 2005. A wedge‐shaped convective cloud area, consisting of three groups of MCSs, was generated near the equator to the north of New Guinea at around 0000 LST on 15 June, and one of the groups of MCSs propagated toward Palau (7°–8°N). Dropsonde analysis revealed that the MCSs developed at the northern edge of a cold air mass that had a horizontal scale of more than 400 km. Global objective analysis data from the Japan Meteorological Agency showed that the cold air mass observed by the dropsonde analysis originated in a land breeze from New Guinea. The cold air mass was initially formed by cold advection from the south and was maintained by cooling with the MCSs. When the initial MCSs were generated along the land‐breeze front, an easterly wave, which was accompanied by low‐level southerly wind and a low‐pressure area, was located near Palau. The cold air mass associated with the land breeze between 132° and 137°E was locally extended toward the center of the low‐pressure area. The results suggest that the easterly wave located around Palau helped locally promote northward cold advection from New Guinea, inducing the long‐distance northward propagation of MCSs generated along the land‐breeze front.

Idealized Mesoscale Convective System Structure and Propagation Using Convective Parameterization

Abstract The development and propagation of mesoscale convective systems (MCSs) was examined within the Weather Research and Forecasting (WRF) model using the Kain–Fritsch (KF) cumulus parameterization scheme and a modified version of this scheme. Mechanisms that led to propagation in the parameterized MCS are evaluated and compared between the versions of the KF scheme. Sensitivity to the convective time step is identified and explored for its role in scheme behavior. The sensitivity of parameterized convection propagation to microphysical feedback and to the shape and magnitude of the convective heating profile is also explored. Each version of the KF scheme has a favored calling frequency that alters the scheme’s initiation frequency despite using the same convective trigger function. The authors propose that this behavior results in part from interaction with computational damping in WRF. A propagating convective system develops in simulations with both versions, but the typical flow structures are distorted (elevated ascending rear inflow as opposed to a descending rear inflow jet as is typically observed). The shape and magnitude of the heating profile is found to alter the propagation speed appreciably, even more so than the microphysical feedback. Microphysical feedback has a secondary role in producing realistic flow features via the resolvable-scale model microphysics. Deficiencies associated with the schemes are discussed and improvements are proposed.

Observations of synoptic-scale land surface variability and its coupling with the atmosphere

Cloud-free brightness temperatures from satellite are used to interpret changes in the state of the land surface energy balance from day to day across the tropical semi-arid region of North Africa. The method provides an assessment of the spatial and temporal patterns in the surface characteristics. The variability in brightness temperature is closely linked to precipitation, as inferred from available rain-gauge data and satellite cold-cloud imagery. Temperatures drop sharply after rainfall, and increase gradually in subsequent days consistent with surface drying. The analysis shows that surface temperatures (and therefore fluxes) are well-organized at the synoptic scale, and can be linked to the westward propagation of mesoscale convective systems and African easterly waves (AEWs). The modulation of rainfall and cloud cover by weather systems is a key element in producing variability in surface fluxes of heat and moisture. To examine the relationships between surface variability and the atmosphere, a composite of anomalously warm (and dry) surface conditions with spatial coherence over a given longitude band is constructed from satellite data covering a single wet season. Operational analyses are used to examine atmospheric anomalies associated with these composited ‘hotspots’. Surface moisture variations on the scale of several hundred km and larger can generate anomalous heat-lows during the day and, according to the operational analyses, generate cyclonic vorticity overnight. These vortices appear to be responsible for the observed modulation of cold cloud in the vicinity of the hotspot, and may influence the characteristics of AEWs in the Northern Sahel. Furthermore, theoretical ideas suggest that surface hotspots are likely to trigger a westward-propagating Rossby-wave response; this process is demonstrated here in a simple numerical model. Copyright © 2005 Royal Meteorological Society

Effects of Orography on the Generation and Propagation of Mesoscale Convective Systems in a Two-Dimensional Conditionally Unstable Flow

Abstract Effects of orography, cold-air outflow, and gravity waves on the generation and propagation of convective systems in a conditionally unstable airstream over a mesoscale mountain are studied using a two-dimensional cloud model. Based on the propagation of convective systems, three regimes are identified: (I) an upstream propagating convective system, (II) a quasi-stationary convective system, and (III) quasi-stationary and downstream propagating systems. In regime I [low moist Froude number (Fw)], the convective cells are generated by upstream deceleration associated with orographic forcing, by gravity waves associated with convective cells over the upslope area at earlier stages, and by the upstream propagating density current at later stages when the density current is fully developed. In this flow regime, quasi-continuous and heavy rainfall is produced over the upslope and plain areas as individual convective cells develop farther upstream at the head of the density current and then propagate d...

Phase Lag between Deep Cumulus Convection and Low-Level Convergence in Tropical Synoptic-Scale Systems

Abstract This study deals with the origin of the phase lag between deep cumulus convection and low-level convergence in tropical synoptic-scale systems, known since 1974. Several possible causes, including 1) propagation of the heating field, 2) β, 3) vertical shear of the basic flow, and 4) vertical tilt of the heat source, are examined. The last one is found to be the reason for the phase lag. The vertical tilt of the heat source occurs as a result of evolution and propagation of mesoscale convective systems within the synoptic system. During this evolution the change of vertical heating profile results in the tilt of heating field. Previous efforts of incorporating such phase lag in wave-CISK studies are commented on.

Numerical simulation of tropical cyclones by an axisymmetric nonhydrostatic model

As an approach to study the mesoscale processes within a typhoon, an axisymmetric nonhydrostatic numerical model is developed without the use of convective parameterization. Many simulated characteristics are consistent with radar and aircraft observations, such as the maximum of vertical and tangential wind, the inflow concentrated near the surface, the outward slope of the eyewall updraft, etc. The model duplicates not only the outward propagation of mesoscale convective systems, but also the inward movement of convective rings, the rate of which coincides with the observation. Besides, the model gives good simulations of the life cycle of convective rings, and indicates that the convective rings far from the eyewall play important roles in the fluctuation of typhoon intensity. Numerical results also exhibit the existence of coupling between outer and inner core structure. Analyses of the simulations show that convective momentum transport generates local maximum absolute angular momentum in the middle and upper troposphere. The momentum anomaly results in symmetric instability, which provides the environment to form convective rings. While the momentum anomaly moves outward with the outflow in the middle and upper troposphere, it initiates a series of convective rings with aid of other direct factors, which explains the outward propagation of convective systems. The simulations exhibit the life cycle of a typical convective ring in terms of three stages, or the developing, mature and dissipating stage. Analysis shows that the symmetric instability and the convective instability promote each other, and their cooperation makes the life of convective rings longer.