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 Q ₁, and moisture sink Q ₂ of Yanai and others and the mesoscale anvil Q ₁, and Q ₂ profiles of Johnson and Young form the basis for the partitioning. For realistic estimates of ƒ, the total heating, which has a peak near 450 mb (6 km), is found to be a consequence of two distinctly different circulation features 1) the mesoscale anvil, which has a heating peak near 330 mb (8 km) and a cooling peak below new 700 mb (3 km) and 2) the cumulus, which produces a heating peak centered near 600 mb (4 km).

The partitioning of the apparent moisture sink Q ₂ produces qualitatively similar results. The mesoscale anvils give a drying peak in the upper troposphere near 350 mb (8 km) and a moistening peak (through evaporation) near 800 mb (2 km). However, the effect of the cumulus in this case (which dry the lower troposphere through removal of water vapor by net condensation) are such that the cumulus drying has a peak somewhat lower in the troposphere (near 750 mb or 2.5 km). Thus, the double-peak structure in Q ₂ often seen in tropical budget composite studies is a consequence of the combined, but vertically-separated drying effects of two distinct convective phenomena: mesoscale anvils and deep cumulus.

The results of this study have implications for cumulus parameterization schemes in general, but particularly for those that assign vertical distributions to the convective heating. It has been shown that the cumulus and mesoscale heating distributions are considerably different. Schemes that use an assigned vertical distribution of convective heating chosen to match those obtained from large-scale tropical budget studies should consider carefully the different contributions to total convective heating by the separate cumulus and mesoscale components. Possible errors may result if the proportion of cumulus versus mesoscale-produced rainfall in the region of model application is different from that in the region where the assigned distribution was derived. The results of this study suggest that cumulus parameterization schemes that permit vertical heating distributions to evolve in a realistic way during the course of model integrations are preferred, at least on a physical basis, over those that prescribe the distributions.

Abstract

Composites of rawinsonde observations of the subcloud layer over south central Florida obtained during the 1975 Florida Area Cumulus Experiment are presented. At a single location south of Lake Okeechobee three soundings per day were released, whenever possible, at approximately 1000, 1300 and 1600 LST during July and August. Soundings have been stratified according to the state of the weather in the vicinity of the release site and time of day. Weather conditions over the south Florida peninsula during the summer period normally ranged from undisturbed (no convective activity) in the morning to highly disturbed (intense convection with precipitation downdrafts) during the afternoon.

Analysis of 47 soundings that ascended into clear air shows rapid growth of the mixed layer beginning 2–3 h after sunrise until shortly before noon, the approximate time of the onset of convective precipitation downdrafts over the peninsula. From the application of simple mixed-layer theory, it is evident that significant subsidence within the cloud environment compensating the net cumulus mass flux occurs on the mesoscale (∼10–100 km). The maximum environmental sinking occurs several hours before the time of the heaviest peninsula-scale rainfall, indicating that the intensity of cumulus downdrafts relative to updrafts increases as the convective activity increases during the day. The subcloud layer structure for a number of soundings that ascended into the bases of nonprecipitating cumuli is also examined.

Abstract

A study has been carried out of the structure of the planetary boundary layer over south central Florida under locally undisturbed conditions using data from the 1975 Florida Area Cumulus Experiment (FACE). From a series of rawinsonde, observations (129 total) during July and August at approximately 1000, 1300 and 1600 LST, 24 soundings have been identified as revealing both mixed-layer and cumulus-layer structures. The soundings have been composited to provide a representation of the average structure of the two layers. The inversion that is found to exist at the top of the cumulus layer is much less pronounced than the trade wind inversion as might be expected considering the relatively rapid growth and short lifetime of this phenomenon.

A model for the growth of the mixed and cumulus layers is presented that includes effects of updraft mass flux, detrainment and evaporation. A closure assumption is proposed that relates the buoyancy flux in the cloud environment at the base of the cumulus inversion to the buoyancy flux within cumulus updrafts at that level. Application of the model to the 1975 FACE data has provided lower bound estimates to the downward buoyancy flux at the cumulus inversion.

Abstract

The large-scale response of the atmospheric boundary layer to the passage of tropical wave disturbances is investigated. Observations from GATE indicate that during periods of extensive deep convective activity rather shallow mixed layers frequently are found, primarily in association with mesoscale precipitation systems. Convective-scale precipitation downdrafts accompanying such systems contribute to the formation of large trailing “wakes” wherein there exist considerably enhanced fluxes of sensible heat from the mean surface. Observations in the convectively active trough portion of tropical waves indicate that a considerable fraction of the total area is covered by wakes. However, there remain regions between rain systems in the wave trough covering ∼15–30% of the total area, depending on how these regions are defined, which have mixed layers more characteristic of those observed in the undisturbed ridge portion of the wave than those in wakes.

A simple model is developed to study the maintenance of the mixed layer in regions between wakes. In the model, horizontal and vertical boundaries of the well-mixed layer are treated as zero-order discontinuities. The model takes into account, in a large-scale sense, the convergence of relatively cool downdraft air into the region between convective systems or wakes. Variables at the top of the mixed layer are specified using large-scale rawinsonde observations and a diagnostic model for the cloud layer above. Application of the model to a composite of a number of GATE wave disturbances has shown that during periods of abundant deep convective activity, environmental subsidence (subsidence away from cumulus and mesoscale downdraft systems) is weak and yet the mixed layer there does not grow without bound because of the inversion-strengthening effect of cool downdraft air outflow into the between-cloud region. Although the model is not fully closed, in the sense that a budget is not carried out to determine thermodynamic properties of downdraft air near the ground, it does suggest that large-scale numerical prediction models may be able to carry the mixed layer height as a predicted variable even during convectively disturbed situations.

Abstract

Observational evidence from GATE suggests that a significant fraction of precipitation in tropical disturbances over the eastern Atlantic occurs beneath anvil-type mesoscale convective systems that are frequently associated with intense cumulonimbus convection. A diagnostic model is developed that includes heat, moisture and mass transports by both the convective-scale and mesoscale components of these systems. An entraining plume model is used for individual convective-scale elements yielding a spectral distribution of cumulus sizes. Mesoscale downdrafts are modeled using a bulk cloud model. A parameterization links properties of the mesoscale systems to those of associated deep convective-scale (cumulonimbus) clouds. A method based on a subjective comparison of model results to observed cloud cover is used to determine mesoscale downdraft areas in relation to cumulonimbus areas. For this reason only qualitative results derived from application of the theory can be considered reliable.

The model is applied to GATE tropical-wave composite data (Thompson et al., 1979) for Phase III. Radiative beating rates derived from observations during this period and storage of water in clouds determined from satellite and ground-based whole-sky camera cloud coverage data are included in the study. Diagnostic model results may be interpreted as indicating the existence of deep convection and mesoscale downdraft systems well in advance of the composite wave trough, followed by suppressed low-level cloud activity with a later increase in deep cumulus convective and mesoscale downdrafts to a maximum centered at the trough axis. Multiple layers of cumulus cloud detrainment are diagnosed to occur in the vicinity of the wave trough. Convective-scale and mesoscale downdrafts are diagnosed to be important contributors to the mass, heat and water budgets of the composite wave, indicating that their effects should be included in cumulus parameterization schemes.

Abstract

Abstract not available.

Abstract

A diagnostic model has been developed to determine the contribution by convective-scale precipitation downdrafts to the total cumulus fluxes of mass, heat and moisture an the synoptic scale. Populations of cumulus clouds are assumed to consist of individual cloud elements of various sizes, each possessing an updraft and downdraft that are modeled as steady-state, entraining plumes. An integral equation for a function representing the cloud base mass flux for each type of cloud is derived based on an assumption regarding the relative intensities of the updraft and downdraft.

The model is tested on the Reed and Recker (1971) western Pacific composite easterly wave data and on a heavy-rain-producing tropical depression that occurred over northern Florida in 1969. Results from both studies indicate that cumulus downdrafts contribute significantly to the total convective mass transport in the lower troposphere. It is shown that the neglect of cumulus downdrafts and their associated rainfall evaporation leads to the diagnosis of excessively large populations of shallow cumulus clouds in highly convective situations. Analysis of the subcloud-layer moisture budget shows downdraft water vapor transport to be important in the water vapor balance for this layer. The implications of the findings of this diagnostic study on theories for the parameterization of cumulus convection are discussed.

Abstract

An observational study of short-term (∼1 week) tropical tropopause height variations has been undertaken using measurements from the December 1978 Winter Monsoon Experiment (Winter MONEX). Sounding data from three Soviet ships in the southern South China Sea and conventional rawinsonde stations in the region of the Indonesian maritime continent am used in the analysis.

During a one-week period (10-17 December), a ∼1.5 km reduction in the height of the tropopause and a warning of 5-8°C in the lower stratosphere were observed over the southern South China Sea. Outside this region a systematic lowering of the tropopause was not observed. In the region of tropopause descent there was a concurrent increase in the amount of deep convective activity. Within the time period of them events, two winter monsoon cold surges (on the l0th and 15th) and a weak tropical depression (on the 14th) affected the circulation over the South China Sea.

During the period of tropopause lowering. tropospheric pressure height and surface pressure falls (having amplitudes ∼20 m and 4 mb, respectively) were observed over the northern South China Sea. These observations suggest that the tropopause descent and lower-stratospheric warming may be a consequence of regional dynamical processes which are analogous to those occurring during the development of cyclones at midlatitudes. While dynamical processes seem to be a likely explanation for the observed variations, other process related to the increased convective activity in the region may also play a role. Several such mechanisms are discussed.

Abstract

A diagnostic study has been carried out of convective transports in tropical African wave disturbances that occurred during GATE. Data from the Reed et al. (1977) three-dimensional composite of eight wave disturbances during Phase III of GATE have been used in the analysis. In the compositing procedure the wave trough has been defined as the position of maximum relative vorticity at 700 mb, and eight categories in the cast-west direction and seven latitude bands (separated by 4° latitude) have been assigned based on this center. The wave composite combines data from both ocean and land regions. Precipitation observations for the wave are compared to precipitation rates given by the cloud layer model to determine the magnitudes of the cumulus updraft and downdraft mass fluxes.

The findings for the innermost latitude bands confirm results from earlier studies that downdraft mass fluxes are significant in regions of deep tropical convection. In the most active categories of the wave the magnitude of the downdraft mass flux at cloud base is found to be as great as two-thirds the magnitude of the updraft mass flux at cloud base. Important meridional differences in the response of deep convection to large-scale forcing (low-level convergence) have been found. At and north of the central latitude band (11.5°N) the maximum deep cloud activity occurs about 10–20 h after the maximum low-level convergence, whereas to the south the response of deep clouds to large-scale forcing is nearly instantaneous. These differences are explained in terms of the meridional variation in the mean moisture stratification. The important role of shallow trade wind cumuli in moistening the lower troposphere in advance of the trough region at the central latitude is discussed.

Abstract

Lateral detrainment from cumulus updrafts and its effect on the properties of cumulus cloud populations (as determined from large-scale observations) are examined. This detrainment, which can be related to the cumulus life cycle, has been specified for a spectrum of cloud sizes by considering initially the character of detrainment for the two extremes of convection: deep cumulonimbi and shallow cumuli. The main purpose is to determine for some reasonable assumption of individual cloud detrainment to what extent diagnosed cloud ensemble properties differ from those given by applying a steady-state plume model for convective updrafts and downdrafts.

Application of the model to tropical western Pacific data indicates, as earlier studies have shown, that in convectively disturbed situations a bimodal cloud population exists of predominantly deep and shallow cumuli. However, the contribution to the total cloud-base mass flux from deep cumuli is increased and from shallow cumuli decreased somewhat when the effects of lateral detrainment are taken into account. Convective downdrafts maintained by precipitation evaporation are found to contribute in an important way to the total convective mass flux regardless of whether side detrainment from updrafts is included or not.