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Science questions that motivate us − How well do Cloud-Precipitation Hybrid Regimes identify tropical convective systems? − Can we examine the life cycle of convective systems using the hybrid regimes? − What are the effects of MJO on the evolution of convective systems?
In recent years, increasing attention has been given to cloud resolving models (CRMs or cloud ensemble models-CEMs) for their ability to simulate the radiative-convective system, which plays a significant role in determining the regional heat and moisture budgets in the Tropics. The growing popularity of CRM usage can be credited to its inclusion of crucial and physically relatively realistic features such as explicit cloud-scale dynamics, sophisticated microphysical processes, and explicit cloud-radiation interaction. On the other hand, impacts of the environmental conditions (for example, the large-scale wind fields, heat and moisture advections as well as sea surface temperature) on the convective system can also be plausibly investigated using the CRMs with imposed explicit forcing. In this paper, by basically using a Goddard Cumulus Ensemble (GCE) model, three different studies on tropical convective systems are briefly presented. Each of these studies serves a different goal as well as uses a different approach. In the first study, which uses more of an idealized approach, the respective impacts of the large-scale horizontal wind shear and surface fluxes on the modeled tropical quasi-equilibrium states of temperature and water vapor are examined. In this 2-D study, the imposed large-scale horizontal wind shear is ideally either nudged (wind shear maintained strong) or mixed (wind shear weakened), while the minimum surface wind speed used for computing surface fluxes varies among various numerical experiments. For the second study, a handful of real tropical episodes (TRMM Kwajalein Experiment - KWAJEX, 1999; TRMM South China Sea Monsoon Experiment - SCSMEX, 1998) have been simulated such that several major atmospheric characteristics such as the rainfall amount and its associated stratiform contribution, the Qlheat and Q2/moisture budgets are investigated. In this study, the observed large-scale heat and moisture advections are continuously applied to the 2-D model. The modeled cloud generated from such an approach is termed continuously forced convection or continuous large-scale forced convection. A third study, which focuses on the respective impact of atmospheric components on upper Ocean heat and salt budgets, will be presented in the end. Unlike the two previous 2-D studies, this study employs the 3-D GCE-simulated diabatic source terms (using TOGA COARE observations) - radiation (longwave and shortwave), surface fluxes (sensible and latent heat, and wind stress), and precipitation as input for the Ocean mixed-layer (OML) model.
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The radiation budget of a tropical mesoscale convective system (MCS) is investigated by comparing in situ aircraft measurements obtained in a tropical MCS during the Equatorial Mesoscale Experiment (EMEX), and coordinated aircraft radiation measurements, with radiation profiles calculated using cloud properties obtained from a cloud model simulation of a tropical MCS. Preliminary results indicate that the stratiform region of the tropical System B simulation represents the gross properties of the observed stratiform system between 4.5 to 15 km. The flux profiles predicted by the model are consistent with observed fluxes.
The anvil clouds of tropical mesoscale convective systems (MCSs) in West Africa, the Maritime Continent and the Bay of Bengal have been examined with TRMM and CloudSat satellite data and ARM ground-based radar observations. The anvils spreading out from the precipitating cores of MCSs are subdivided into thick, medium and thin portions. The thick portions of anvils show distinct differences from one climatological regime to another. In their upper portions, the thick anvils of West Africa MCSs have a broad, flat histogram of reflectivity, and a maximum of reflectivity in their lower portions. The reflectivity histogram of the Bay of Bengal thick anvils has a sharply peaked distribution of reflectivity at all altitudes with modal values that increase monotonically downward. The reflectivity histogram of the Maritime Continent thick anvils is intermediate between that of the West Africa and Bay of Bengal anvils, consistent with the fact this region comprises a mix of land and ocean influences. It is suggested that the difference between the statistics of the continental and oceanic anvils is related to some combination of two factors: (1) the West African anvils tend to be closely tied to the convective regions of MCSs while the oceanic anvils are more likely to be extending outward from large stratiform precipitation areas of MCSs, and (2) the West African MCSs result from greater buoyancy, so that the convective cells are more likely to produce graupel particles and detrain them into anvils
A global study of the vertical structures of the clouds of tropical mesoscale convective systems (MCSs) has been carried out with data from the CloudSat Cloud Profiling Radar. Tropical MCSs are found to be dominated by cloud-top heights greater than 10 km. Secondary cloud layers sometimes occur in MCSs, but outside their primary raining cores. The secondary layers have tops at 6 8 and 1 3 km. High-topped clouds extend outward from raining cores of MCSs to form anvil clouds. Closest to the raining cores, the anvils tend to have broader distributions of reflectivity at all levels, with the modal values at higher reflectivity in their lower levels. Portions of anvil clouds far away from the raining core are thin and have narrow frequency distributions of reflectivity at all levels with overall weaker values. This difference likely reflects ice particle fallout and therefore cloud age. Reflectivity histograms of MCS anvil clouds vary little across the tropics, except that (i) in continental MCS anvils, broader distributions of reflectivity occur at the uppermost levels in the portions closest to active raining areas; (ii) the frequency of occurrence of stronger reflectivity in the upper part of anvils decreases faster with increasing distance in continental MCSs; and (iii) narrower-peaked ridges are prominent in reflectivity histograms of thick anvil clouds close to the raining areas of connected MCSs (superclusters). These global results are consistent with observations at ground sites and aircraft data. They present a comprehensive test dataset for models aiming to simulate process-based upper-level cloud structure around the tropics.
A global study of the vertical structures of the clouds of tropical mesoscale convective systems (MCSs) has been carried out with data from the CloudSat Cloud Profiling Radar. Tropical MCSs are found to be dominated by cloud-top heights greater than 10 km. Secondary cloud layers sometimes occur in MCSs, but outside their primary raining cores. The secondary layers have tops at 6--8 and 1--3 km. High-topped clouds extend outward from raining cores of MCSs to form anvil clouds. Closest to the raining cores, the anvils tend to have broader distributions of reflectivity at all levels, with the modal values at higher reflectivity in their lower levels. Portions of anvil clouds far away from the raining core are thin and have narrow frequency distributions of reflectivity at all levels with overall weaker values. This difference likely reflects ice particle fallout and therefore cloud age. Reflectivity histograms of MCS anvil clouds vary little across the tropics, except that (i) in continental MCS anvils, broader distributions of reflectivity occur at the uppermost levels in the portions closest to active raining areas; (ii) the frequency of occurrence of stronger reflectivity in the upper part of anvils decreases faster with increasing distance in continental MCSs; and (iii) narrower-peaked ridges are prominent in reflectivity histograms of thick anvil clouds close to the raining areas of connected MCSs (superclusters). These global results are consistent with observations at ground sites and aircraft data. They present a comprehensive test dataset for models aiming to simulate process-based upper-level cloud structure around the tropics.
The spatial radiation heating budget associated with tropical mesoscale convective systems (MCSs) is studied and the change of this heating/budget throughout the life cycle of such a cloud system is investigated. The movements of an EMEX 9 cloud cluster are described. The vertical structure of the cluster contains two types of imbedded convection: an upright vertical structure and a pronounced rearward slope with vertical extent of 14.5 km or more and a horizontal scale of about 40 km. The cloud base and cloud top altitude in the stratiform region are of the order of 4.8 km and 15-16 km, respectively. The upward and downward solar flux profiles suggest very little solar heating in these regions. A tropical MCS that occurred during the EMEX Mission 9 is simulated, and the simulation is shown to broadly agree with the observations. The simulation results, which are reported in detail, show how tropical mesoscale cloud systems provide an effective radiative heat source for the tropical atmosphere.
This study uses measurements of radiation and cloud properties taken between January and August 1998 by three Tropical Rainfall Measuring Mission (TRMM) instruments, the Clouds and the Earth's Radiant Energy System (CERES) scanner, the TRMM Microwave Imager (TMI), and the Visible and InfraRed Scanner (VIRS), to evaluate the variations of tropical deep convective systems (DCS) with sea surface temperature (SST) and precipitation. This study finds that DCS precipitation efficiency increases with SST at a rate of approx. 2%/K. Despite increasing rainfall efficiency, the cloud areal coverage rises with SST at a rate of about 7%/K in the warm tropical seas. There, the boundary layer moisture supply for deep convection and the moisture transported to the upper troposphere for cirrus-anvil cloud formation increase by approx. 6.3%/K and approx. 4.0%/K, respectively. The changes in cloud formation efficiency, along with the increased transport of moisture available for cloud formation, likely contribute to the large rate of increasing DCS areal coverage. Although no direct observations are available, the increase of cloud formation efficiency with rising SST is deduced indirectly from measurements of changes in the ratio of DCS ice water path and boundary layer water vapor amount with SST. Besides the cloud areal coverage, DCS cluster effective sizes also increase with precipitation. Furthermore, other cloud properties, such as cloud total water and ice water paths, increase with SST. These changes in DCS properties will produce a negative radiative feedback for the earth's climate system due to strong reflection of shortwave radiation by the DCS. These results significantly differ from some previous hypothesized dehydration scenarios for warmer climates, and have great potential in testing current cloud-system resolving models and convective parameterizations of general circulation models.
Cloud-integrated attenuated backscatter from observations with the Lidar In-Space Technology Experiment (LITE) was studied over a range of cirrus clouds capping some extensive mesoscale convective systems (MCSS) in the Tropical West Pacific. The integrated backscatter when the cloud is completely attenuating, and when corrected for multiple scattering, is a measure of the cloud particle backscatter phase function. Four different cases of MCS were studied. The first was very large, very intense, and fully attenuating, with cloud tops extending to 17 km and a maximum lidar pulse penetration of about 3 km. It also exhibited the highest integrated attenuated isotropic backscatter, with values in the 532-nm channel of up to 2.5 near the center of the system, falling to 0.6 near the edges. The second MCS had cloud tops that extended to 14.8 km. Although MCS2 was almost fully attenuating, the pulse penetration into the cloud was up to 7 km and the MCS2 had a more diffuse appearance than MCS1. The integrated backscatter values were much lower in this system but with some systematic variations between 0.44 and 0.75. The third MCS was Typhoon Melissa. Values of integrated backscatter in tt-ds case varied from 1.64 near the eye of the typhoon to between 0.44 and 1.0 in the areas of typhoon outflow and in the 532-nm channel. Mean pulse penetration through the cloud top was 2-3 km, the lowest penetration of any of the systems. The fourth MCS consisted of a region of outflow from Typhoon Melissa. The cloud was semitransparent for more than half of the image time. During that time, maximum cloud depth was about 7 km. The integrated backscatter varied from about 0.38 to 0.63 in the 532-nm channel when the cloud was fully attenuating. In some isolated cirrus between the main systems, a plot of integrated backscatter against one minus the two-way transmittance gave a linear dependence with a maximum value of 0.35 when the clouds were fully attenuating. The effective backscatter-to-extinction ratios, when allowing for different multiple-scattering factors from space, were often within the range of those observed with ground-based lidar. Exceptions occurred near the centers of the most intense convection, where values were measured that were considerably higher than those in cirrus observed from the surface. In this case, the values were more compatible with theoretical values for perfectly formed hexagonal columns or plates. The large range in theoretically calculated back- scatter-to-extinction ratio and integrated multiple-scattering factor precluded a closer interpretation in terms of cloud microphysics.
Organization metrics were originally developed to measure how densely convective clouds are arranged at mesoscales. In this work, we apply organization metrics to describe tropical synoptic scale convective activity. Such activity is identified by cloud-precipitation (hybrid) regimes defined at 1-degree and 1-hourly resolution. Existing metrics were found to perform inadequately for such convective regime aggregates because the large domain size and co-existence of sparse aggregate occurrences with noisy isolated convection often violate assumptions inherent in these metrics. In order to capture these characteristics, in this study the existing “convective organization potential” (COP) metric was modified so as to focus on local organization and provide increased weight to aggregate size. The resulting “area-based COP” (ABCOP) follows the principle that the more numerous the objects, the higher the chance of organization. It is thus optimized to capture large-scale convective events occurring during phenomena such as ENSO and MJO, while also performs as well as existing metrics for small domain sizes.
The phenomenon of high ice water content (HIWC) occurs in mesoscale convective systems (MCSs) when a large number of small ice particles with typical sizes of a few hundred micrometers, concentrations of the order of 10 2 –10 3 L −1 , and IWC exceeding 1 g m −3 are present at high altitudes. HIWC regions in MCSs may extend vertically up to 10 km above the melting layer and horizontally up to hundreds of kilometers, filling large volumes of the convective systems. HIWC has great geophysical significance due to its effect on precipitation formation, the hydrological cycle, and the radiative properties of MCSs. It is also recognized as a hazard for commercial aviation operations since it can result in engine power loss and in the malfunctioning of aircraft data probes. This study summarizes observational and numerical simulation efforts leading to the development of a conceptual model for the production of HIWC in tropical MCSs based on the data collected during the HAIC–HIWC campaign. It is hypothesized that secondary ice production (SIP) in the vicinity of the melting layer plays a key role in the formation and sustainability of HIWC. In situ observations suggest that the major SIP mechanism in the vicinity of the melting layer is related to the fragmentation of freezing drops (FFDs). Both in situ data and numerical simulations suggest that the recirculation of drops through the melting layer led to the amplification of SIP. The proposed conceptual model and simulation results motivate further efforts to extend reproducible laboratory measurements.
No abstract available
A new approach is presented to quantify upper-level moisture transport from geostationary satellite data. Daily time sequences of GOES-7 water vapor imagery were used to produce estimates of winds and water vapor mixing ratio in the upper-troposphere sensed by the 6.7-microns water vapor channel. The winds and mixing ratio values were gridded and then combined to produce a parameter called the Water Vapor Transport Index (WVTI) which represents the magnitude of the two dimensional transport of water vapor in the upper troposphere. Daily grids of WVTI, meridional moisture transport, mixing ratio, pressure and other associated parameters were averaged to produce monthly fields for June, July and August of 1987 and 1988 over the Americas and surrounding oceanic regions. The WVTI was used to compare upper-tropospheric moisture transport between the summers of 1987 and 1988, contrasting the latter part of the 1986/87 El Nino event and the La Nina period of 1988. A similar product derived from the NCEP reanalysis was used to help validate the index and understand interannual variability of moisture transport from the modeling perspective. Both datasets showed large upper-level water vapor transport associated with synoptic features over the Americas and with outflow from tropical convective systems. Minimal transport occurred over tropical and subtropical high pressure regions where winds were light. Index values from NCEP were 2-3 times larger than that determined from the GOES satellite This difference resulted from an over estimate of the zonal winds and upper-tropospheric moisture in the reanalysis model. A comparison of the satellite-derived monthly values between the summers of 1987 and 1988 provided some insight into the impact of the ENSO event on upper-level moisture and its transport during the period. During July 1987, a large portion of the tropics in the eastern Pacific Ocean and Caribbean Sea was dominated by strong vapor transport in excess of 5 g/kg/ms with relatively small amounts in the other months. In contrast, JJA 1988 showed an opposite trend with July 1988 being less dominated by tropical water vapor transport. The meridional transport of upper-level water vapor indicated significant poleward transport from the tropics to the higher latitudes. This transport favored the Southern Hemisphere with large transport occurring south of the ITCZ which extended across the eastern Pacific and northern South America. Zonally-averaged monthly transport values were shown to provide a simple way to quantify the monthly and interannual changes in water vapor transport. Zonally-averaged WVTI values peaked in the Southern Hemisphere subtropics during both Austral winters. In the tropics a single more pronounced peak located over the equator and south latitudes occurred in 1988 as opposed to a dual peak in 1987. The second peak around 20 deg.N latitude is consistent with findings of others where upper-tropospheric winds were noted to be stronger in this region during warm ENSO events. Zonally-averaged meridional transport was southward for all summer months and stronger in 1988. The asymmetric nature of the zonally-averaged meridional transport (more southerly water vapor transport) was enhanced during JJA 1998 thus indicating a stronger upper-level branch of the Hadley circulation during this notable strong La Nina period.
A new approach is presented to quantify upper-level moisture transport from geostationary satellite data. Daily time sequences of Geostationary Operational Environmental Satellite GOES-7 water vapor imagery were used to produce estimates of winds and water vapor mixing ratio in the cloud-free region of the upper troposphere sensed by the 6.7- microns water vapor channel. The winds and mixing ratio values were gridded and then combined to produce a parameter called the water vapor transport index (WVTI), which represents the magnitude of the two-dimensional transport of water vapor in the upper troposphere. Daily grids of WVTI, meridional moisture transport, mixing ratio, pressure, and other associated parameters were averaged to produce monthly fields for June, July, and August (JJA) of 1987 and 1988 over the Americas and surrounding oceanic regions, The WVTI was used to compare upper-tropospheric moisture transport between the summers of 1987 and 1988, contrasting the latter part of the 1986/87 El Nino event and the La Nina period of 1988. A similar product derived from the National Centers for Environmental Prediction (NCEP)-National Center for Atmospheric Research (NCAR) 40-Year Reanalysis Project was used to help to validate the index. Although the goal of this research was to describe the formulation and utility of the WVTI, considerable insight was obtained into the interannual variability of upper-level water vapor transport. Both datasets showed large upper-level water vapor transport associated with synoptic features over the Americas and with outflow from tropical convective systems. Minimal transport occurred over tropical and subtropical high pressure regions where winds were light. Index values from NCEP-NCAR were 2-3 times larger than that determined from GOES. This difference resulted from large zonal wind differences and an apparent overestimate of upper-tropospheric moisture in the reanalysis model. A comparison of the satellite-derived monthly values between the summers of 1987 and 1988 provided some insight into the impact of the ENSO event on upper-level moisture and its transport during the period. During July 1987, a large portion of the Tropics in the eastern Pacific Ocean and Caribbean Sea was dominated by strong vapor transport in excess of 4.0 g/kg m/s, with relatively small amounts in the other months. JJA 1988 transport values reached similar magnitude and showed similar patterns for all three months. The meridional transport of upper-level water vapor indicated large poleward transport from the Tropics to the higher latitudes. This transport favored the Southern Hemisphere, with large transport occurring south of the ITCZ, which extended across the eastern Pacific and northern South America. Zonally averaged monthly transport values were shown to provide a simple way to quantify the monthly and interannual changes in water vapor transport. Zonally averaged WVTI values peaked in the Southern Hemisphere subtropics during both austral winters. In the Tropics, a single, more- pronounced peak located over the equator and south latitudes occurred in 1988 as opposed to a dual peak in 1987. The second peak around 20'N latitude is consistent with findings of others in which upper-tropospheric winds were noted to be stronger in this region during warm ENSO events. Zonally averaged meridional transport was southward for all summer months and was stronger in 1988. The asymmetric nature of the zonally averaged meridional transport (more southerly water vapor transport) was enhanced during JJA 1988, thus indicating a stronger upper- level branch of the Hadley circulation during this notably strong La Nina period.
We report on balloon sonde measurements of water vapor and ozone using the cryogenic frost point hygrometer and electrochemical concentration cell ozonesondes made at Alajuela, Costa Rica (10.0 N, 84.2 W) during two NASA airborne campaigns: the Tropical Convective Systems and Processes (TCSP) mission in July 2005 and the Tropical Composition, Clouds, and Climate Coupling Experiment (TC4), July - August 2007. In both campaigns we found an upper troposphere that was frequently supersaturated but no evidence that deep convection had reached the tropopause. The balloon sondes were complemented by campaigns of 4 times daily high-resolution radiosondes from mid-June through mid-August in both years. The radiosonde data reveal vertically propagating equatorial waves that caused a large increase in the variability of temperature in the tropical tropopause layer (TTL). These waves episodically produced cold point tropopauses (CPTs) above 18 km, yet in neither campaign was saturation observed above approx 380 K or 17 km. The averages of the water vapor minima below this level were 5.2 ppmv in TCSP and 4.8 ppmv in TC4, and the individual profile minima all lay at or above approx 360 K. The average minima in this 360 C380 K layer provide a better estimate of the effective stratospheric entry value than the average mixing ratio at the CPT. We refer to this upper portion of the TTL as the tropopause saturation layer and consider it to be the locus of the final dehydration of nascent stratospheric air. As such, it is the local equivalent to the tape head of the water vapor tape recorder.
The 1999 Kwajalein Atoll field experiment (KWAJEX), one of several major TRMM (Tropical Rainfall Measuring Mission) field experiments, has successfully obtained a wealth of information and observation data on tropical convective systems over the western Central Pacific region. In this paper, clouds and convective systems that developed during three active periods (Aug 7-12, Aug 17-21, and Aug 29-Sep 13) around Kwajalein Atoll site are simulated using both 2D and 3D Goddard Cumulus Ensemble (GCE) models. Based on numerical results, the clouds and cloud systems are generally unorganized and short lived. These features are validated by radar observations that support the model results. Both the 2D and 3D simulated rainfall amounts and their stratiform contribution as well as the heat, water vapor, and moist static energy budgets are examined for the three convective episodes. Rainfall amounts are quantitatively similar between the two simulations, but the stratiform contribution is considerably larger in the 2D simulation. Regardless of dimension, fo all three cases, the large-scale forcing and net condensation are the two major physical processes that account for the evolution of the budgets with surface latent heat flux and net radiation solar and long-wave radiation)being secondary processes. Quantitative budget differences between 2D and 3D as well as between various episodes will be detailed.Morover, simulated radar signatures and Q1/Q2 fields from the three simulations are compared to each other and with radar and sounding observations.