Tropical Mesoscale Convective Systems Research Articles

Vertical Radar Reflectivity Structure and Cloud-to-Ground Lightning in the Stratiform Region of MCSs: Further Evidence for In Situ Charging in the Stratiform Region

Abstract This study presents further evidence in support of an in situ, noninductive charging mechanism as the process likely responsible for significant electrification of the trailing stratiform regions of mesoscale convective systems (MCSs). In contrast to previous studies of MCS electrification that have investigated observations of radar reflectivity and cloud-to-ground lightning in the horizontal (e.g., Orville et al.; Rutledge et al.), here the relationship between the location and occurrence of cloud-to-ground lightning in the stratiform regions of midlatitude and tropical MCSs and the vertical profile of radar reflectivity are examined. The vertical profile of radar reflectivity at elevations above the 0°C level is used as a proxy for the amount of mass present in the mixed-phase region of the stratiform clouds, which in turn is related to the generation of charge through a noninductive charging mechanism. To further explore the relationship between radar reflectivity, mixed-phase microphysics, a...

Gregarious Tropical Convection

A beat source with a vertical profile like that of observed tropical mesoscale convective systems (MCSs) is shown to cause, through inviscid gravity wave dynamics, upward displacement at low levels in a mesoscale region surrounding the heating. Typical values are ∼10%–30% area contraction at the surface everywhere within 270 km of the heating 6 h after it starts. As a result, conditions near an existing MCS (but beyond the area of MCS outflow) become more favorable for the development of additional convection. This theory predicts that cloud clusters should be gregarious. Infrared satellite imagery confirms that almost half of the cold cloudiness observed in a month over the oceanic warm pool region was contributed by just 14 objectively defined multiday “supercluters”.

The radiative budgets of a tropical mesoscale convective system during the EMEX‐STEP‐AMEX experiment: 1. Observations

This paper is the first of a series of two that aim to describe the spatial and temporal variation of the radiative heating associated with tropical mesoscale convective systems (MCSs). This paper describes the analysis of data collected in and around a tropical cloud cluster system studied as part of the Equatorial Mesoscale Experiment (EMEX). The data analysis indicates that the cluster originated off the northern coast of Australia along the midlevel monsoon trough and lasted approximately 12 hours. The system moved with a velocity of about 12 m/s toward the northeast and the low‐level surface northwesterly flow at the vicinity of the premonsoon trough area seems continuously to feed the EMEX 9 cloud cluster with energetic warm, moist equatorial air. Data obtained from aircraft penetrations show features similar to tropical MCSs reported elsewhere (such as an area of strong to moderate convection surrounded by a broad region of stratiform precipitation, radar echo bright band in the stratiform region, “onion” type sounding behind the convective region). The vertical structures of the EMEX 9 cloud cluster also contain two types of imbedded convection: an upright vertical structure and a pronounced rearward slope (approximately 17°), having a vertical extent of 14.5 km and above and a horizontal scale of about 40 km. The cloud base and cloud top altitude in the stratiform region are estimated to be of the order of 4.8 km and 15 to 16 km, respectively. The composite aircraft shortwave radiation data from the stratiform region show a significant attenuation of shortwave flux through the cloud (the estimated transmission is 14% at cloud base). The upward and downward solar flux profiles are almost parallel to each other in the atmosphere inside and below the cloud base, suggesting very little solar heating in these regions. The upward and downward infrared radiation fluxes measured in the tropical MCS also show little infrared heating above and below the cloud cluster.

The radiative budgets of a tropical mesoscale convective system during the EMEX‐STEP‐AMEX experiment: 2. Model results

This paper describes calculations of the spatial and temporal variation of the radiation budget of a tropical mesoscale convective system (MCS). A combination of cloud model simulations, radiation model simulations, and analyses of observations obtained during the Equatorial Mesoscale Experiment (EMEX), the Stratosphere‐Troposphere Exchange Program (STEP), and the Australian Monsoon Experiment (AMEX) are used to obtain these heating rates. The two‐dimensional version of the Colorado State University regional atmospheric modeling system is used to simulate a tropical MCS that occurred during EMEX mission 9 on February 2, 1987. The simulation is shown to broadly agree with the observations reported in a related paper. The spatial radiative heating distributions derived from a two‐stream radiative transfer model corresponding to the mature stage of the simulated cloud system indicate that significant horizontal inhomogeneities exist. According to the model results the effects of the MCS are to (1) increase in the infrared emission to the surface and to decrease in the net infrared energy loss from the atmosphere relative to the clear sky emission and (2) change the transmission of solar flux to the surface, the shortwave albedo of the atmosphere, and the solar absorption in the atmosphere. The results show how the MCS significantly reduces the solar flux to the surface relative to the clear sky values and that the largest reduction occurs under the convective portions of the mature MCS. (3) The MCS creates a total (solar plus infrared) radiative warming in the atmosphere relative to the surrounding clear sky. The value of this total heating is governed by both infrared and solar absorption. Vertical profiles of this heating show the dominance of infrared cooling near cloud top and infrared heating inside and near cloud base. The shortwave heating rate can also be as large as the infrared cooling near the cloud top region of the tropical MCS, especially at a local noon. (4) The temporal changes in radiation profiles also demonstrate how the MCS modulates the radiation budget of the atmosphere. Specifically, the total radiation energy loss of the entire two‐dimensional domain of the model atmosphere decreases and eventually becomes positive as the cloud system decays, becomes a stratiform in nature, and fills the domain. This change in the column divergence of flux translates into a total column radiative heating rate of approximately 1.7 K/d (relative to the clear sky radiative cooling rate). The solar component of this domain heating tends to be concentrated in the upper troposphere, whereas the infrared component of the heating is spread over the lower and middle troposphere. These results also show how tropical mesoscale cloud system provides an effective radiative heat source for the tropical atmosphere.

Radiative Forcing of Simulated Tropical Cloud Clusters

Abstract A number of field experiments and subsequent studies in the 1970s and 1980s have led to the belief that radiative processes play a more significant role in the evolution of tropical mesoscale convective systems (MCSS) than was once thought. In this study, an interactive radiative transfer scheme is incorporated into a two-dimensional version of the Pennsylvania State University-NCAR Mesoscale Model to simulate the evolution of these systems within a large-scale environment under a diurnally varying radiative influence. The radiative effects are examined in terms of the net rainfall, diurnal phasing, and the vertical distribution of diabatic heating within the systems. In addition, three current radiative forcing hypotheses are addressed. Simulations of individual MCSs with radiation produced more rainfall than those without it. While runs with forced background meant all peaked after the same elapsed time regardless of diurnal initialization time, the peak rainfall rates that occurred at night we...

Some characteristics of cloud‐to‐ground lightning in tropical northern Australia

Peak currents and polarities for the first return strokes of approximately 5000 cloud‐to‐ground lightning flashes have been studied. The flashes were associated with eight mesoscale convective systems (MCSs) that occurred in tropical northern Australia in 1989 during the second field phase of the Down Under Doppler and Electricity Experiment (Dundee). Statistical analysis of the lightning data produced an average peak current (independent of polarity) of 39 kA. When considered with the average cloud‐top heights of the tropical MCSs examined herein, the 39‐kA peak current average appears to be in general agreement with Orville's (1990) hypothesis of latitudinal variation in peak current. In six of the Dundee MCSs consisting of both convective and stratiform precipitation components, we found that positive peak current maxima tended to occur in the trailing stratiform region while positive peak current minima were generally situated in convective precipitation. We also observed this pattern in a middle‐latitude MCS that occurred on June 10–11, 1985. The magnitude of the positive peak current maxima in the tropical MCSs and the middle‐latitude MCS increased coincidentally in time with the growth of the stratiform regions, and reached peak values when the stratiform regions were most intense. Given the apparent dependence of positive peak current magnitude on the life‐cycle of the stratiform region and the spatial separation between positive peak current maxima and the convective line, we offer further support for a noninductive charging mechanism being responsible for the electrical charging and subsequent lightning observed in the trailing stratiform regions of MCSs.

The Down Upper Doppler and Electricity Experiment (DUNDEE): Overview and Preliminary Results

Abstract DUNDEE (Down Under Doppler and Electricity Experiment) is described. DUNDEE was carded out in the vicinity of Darwin, Northern Territory, Australia, during the wet seasons of November 1988 through February 1989, and November 1989 through February 1990. The general goal of DUNDEE was to investigate the dynamical and electrical properties of tropical mesoscale convective systems and isolated deep convective storms. Darwin, situated at the southern tip of the “maritime continent,” experiences both monsoon and “break” period conditions during the wet season. We discuss the observational network deployed for DUNDEE and present preliminary scientific results. One particularly interesting observation is a large contrast in the frequency of total lightning between break period convection (high lightning rates) and convection in the monsoon trough (low lightning rates). A relationship between CAPE (convective available potentional energy) and total flash rate is presented and discussed to explain this obs...

Middle Latitude and Tropical Mesoscale Convective Systems

Observational, numerical modelling, and theoretical studies of middle latitude and tropical Mesoscale Convective Systems are reviewed.

中規模積雲対流系における下部対流圏の昇温および乾燥化:積雲対流のパラメタリゼーシヨンへの示唆

Observations from the International Winter Monsoon Experiment (Winter MONEX) have been used to determine temperature changes in the lower troposphere beneath the stratiform components of tropical mesoscale convective systems. It is found that while the apparent heat source Q1 in the lower troposphere beneath the stratiform cloud systems is negative (due to hydrometeor melting and evaporation), the time tendency of dry static energy (*s/*t) is positive. This low-level warming is a result of mesoscale downdrafts within the mesoscale convective systems. The warming and drying at low levels in such systems may contribute importantly to temperature and moisture changes in large-scale tropical disturbances. The effects, if important, will need to be considered in the development of schemes for the parameterization of cumulus convection.

Partitioning Tropical Heat and Moisture Budgets into Cumulus and Mesoscale Components: Implications for Cumulus Parameterization

Abstract Results from recent heat and moisture budget studies of tropical mesoscale convective systems have been used to partition the total heating in tropical disturbances into cumulus and mesoscale components The mesoscale component refer to that part of tropical cloud systems which contains mesoscale anvil circulations, viz., a mesoscale updraft in an upper-tropospheric cloud shield extending from near the 0°C level to near the 0°C level to near the tropopause and a mesoscale downdraft in the lower troposphere. The cumulus component, which is determined as a residual, consists of contributions from cumulus elements of all sizes, ranging from deep cumulonimbus to shallow cumulus; however, deep cumulus effects predominate in the tropical disturbances studied here. The method of partitioning requires an estimate of the fraction of f of the total rainfall in tropical disturbances produced by mesoscale anvil systems. The total apparent heat source Q1, and moisture sink Q2 of Yanai and others and the mesosc...

A Long-Lived Mesoscale Convective Complex. Part II: Evolution and Structure of the Mature Complex

Abstract An eight-day episode in August 1977 is described, wherein 14 mesoscale convective complexes (MCCs) developed in the central United States, including one to the immediate Ice of the Rocky Mountains on each day of the episode. In Part I of this article, the daytime genesis of one of these systems was traced from its pre-convective roots in the mountains of central Colorado to its incipient MCC stage on the plains of eastern Colorado. In this paper, its continued nocturnal development into a large MCC over Kansas is followed. Satellite imagery shows that this system remained coherent for at least three days as it passed off the east coast and across the western Atlantic Ocean. Analysis is focused on the mature stage of this and a second MCC in the episode in order to compare their major dynamic features to those of similar midlatitude systems reported in the literature, and also to previous studies of tropical mesoscale convective systems. Many of the important characteristics of midlatitude MCCs fo...