Thomas M. Rickenbach

Trimodal characteristics of tropical convection

It has long been known that trade wind cumulus and deep cumulonimbus represent primary components of the broad spectrum of cumulus clouds in the Tropics, which has led to the concept of a bimodal distribution of tropical clouds. However, recent analyses of shipboard radar data from Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (COARE) provide evidence of abundant populations of a third cloud type, cumulus congestus. Congestus clouds constitute over half the precipitating convective clouds in COARE and contribute over one-quarter of the total convective rainfall. Global Atmospheric Research Program Atlantic Tropical Experiment studies reveal a similar midlevel peak in the distribution of radar-echo tops. These findings lead to the conclusion that shallow cumulus, congestus,and cumulonimbus are all prominent tropical cumulus cloud types. They are associated with trimodal distributions of divergence, cloud detrainment, and fractional cloudiness in the Tropics. The peaks in the distributions of radar-echo tops for these three cloud types are in close proximity to prominent stable layers that exist over the Pacific warm pool and the tropical eastern Atlantic: near 2 km (the trade stable layer), ;5 km (near 08C), and ;15‐16 km (the tropopause). These stable layers are inferred to inhibit cloud growth and promote cloud detrainment. The 08C stable layer can produce detrainment from cumulonimbi (attendant shelf clouds) and help retard the growth of precipitation-laden and strongly entraining congestus clouds. Moreover, restriction of growth of congestus clouds to just above the 08C level limits further enhancement of cloud buoyancy through glaciation. The three cloud types are found to vary significantly during COARE on the timescale of the 30‐60-day intraseasonal oscillation. The specific roles of clouds of the congestus variety in the general circulation are not yet clear, but some (the shallower ones) contribute to moistening and preconditioning the atmosphere for deep convection; others (the deeper ones) contribute an important fraction of the total tropical rainfall, and both likely produce many midlevel clouds, thereby modulating the radiative heating of the tropical atmosphere.

Convection in TOGA COARE: Horizontal Scale, Morphology, and Rainfall Production

Abstract The occurrence frequency and rainfall production of mesoscale convective systems (MCSs) relative to smaller groups of convective clouds over the tropical oceans is not well known. Eighty days of shipboard radar data collected during the recent Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) were used to provide a detailed view of convection in the western Pacific warm pool, a region of global climatological significance. The aim of this study was to document the frequency of occurrence, rainfall production, and depth of convection observed during TOGA COARE within a simple and meaningful framework of convective horizontal organization. Organization was characterized in terms of the horizontal scale and morphology of convective systems. Precipitation events were defined based on whether they attained the length scale of an MCS, and on whether convection was organized into lines. About four-fifths of rainfall during COARE was associated with MCS-scale squa...

A Report of the Field Operations and Early Results of the South China Sea Monsoon Experiment (SCSMEX)

Abstract The South China Sea Monsoon Experiment (SCSMEX) is an international field experiment with the objective to better understand the key physical processes for the onset and evolution of the summer monsoon over Southeast Asia and southern China aiming at improving monsoon predictions. In this article, a description of the major meteorological observation platforms during the intensive observing periods of SCSMEX is presented. In addition, highlights of early results and discussions of the role of SCSMEX in providing valuable in situ data for calibration of satellite rainfall estimates from the Tropical Rainfall Measuring Mission are provided. Preliminary results indicate that there are distinctive stages in the onset of the South China Sea monsoon including possibly strong influences from extratropical systems as well as from convection over the Indian Ocean and the Bay of Bengal. There is some tantalizing evidence of complex interactions between the supercloud cluster development over the Indian Oce...

Vertical-Mode Decompositions of 2-Day Waves and the Madden–Julian Oscillation

Abstract Vertical structures of 2-day waves and the Madden–Julian oscillation (MJO) are projected onto vertical normal modes for a quiescent tropical troposphere. Three modes capture the gross tropospheric structure of 2-day waves, while only two modes are needed to represent most of the baroclinic structure of the MJO. Deep circulations that project onto the first baroclinic mode are associated with deep cumulonimbus and stratiform rainfall. Shallow circulations that project onto higher wavenumber modes are associated with precipitating shallow cumulus and congestus and stratiform rainfall. For both disturbances the horizontal divergence contributed by shallow modes is an important factor in the column-integrated moist enthalpy budget. These modes converge moist static energy for a time prior to when deep circulations export moist static energy. These results highlight the importance of properly representing the effects of shallow cumulus, congestus, and stratiform precipitation in theories of convective...

Environmental Characteristics of Convective Systems during TRMM-LBA

Abstract In this paper, data collected from 51 days of continual upper-atmospheric soundings and the Tropical Ocean Global Atmosphere (TOGA) radar at Anglo–Brazilian Amazonian Climate Observation Study (ABRACOS) Hill during the Tropical Rainfall Measuring Mission component of the Brazilian Large Scale Biosphere–Atmosphere (TRMM-LBA) experiment are used to describe the mean thermodynamic and kinematic airmass properties of wet season convection over Rondonia, Brazil. Distinct multiday easterly and westerly lower-tropospheric wind regimes occurred during the campaign with contrasting airmass characteristics. Westerly wind periods featured modest CAPE (1000 J kg−1), moist conditions (>90% RH) extending through 700 mb, and shallow (900 mb) speed shear on the order of 10−4 s−1. This combination of characteristics promoted convective systems that featured a relatively large fraction of stratiform rainfall and weak convection nearly devoid of lightning. In contrast, easterly regime convective systems were more s...

Nocturnal Cloud Systems and the Diurnal Variation of Clouds and Rainfall in Southwestern Amazonia

Abstract This paper examines the origins of a secondary nocturnal maximum in cloudiness and precipitation in southwestern Amazonia, a diurnal feature observed previously by many investigators. Analysis is based on satellite, radar, sounding, and profiler observations of precipitating systems and cloudiness from the Tropical Rainfall Measuring Mission Large-Scale Biosphere–Atmosphere (TRMM-LBA) and the coincident Wet-Season Atmospheric Mesoscale Campaign (WETAMC) field programs during the early 1999 wet season. The general finding is that following the collapse of the nearly ubiquitous and locally generated afternoon (“noon balloon”) convection, organized deep convection contributes to a postmidnight maximum in raining area and high cloudiness, and to a lesser extent rainfall. Nocturnal convective systems have the effect of weakening and delaying the onset of the following afternoons convection. Many of these nocturnal convective events are traced to large- scale squall lines, which propagate westward tho...

Cloud-Top Evolution of Tropical Oceanic Squall Lines from Radar Reflectivity and Infrared Satellite Data

Abstract Precipitation estimation over the tropical oceans is commonly performed using passive infrared (IR) measurements of cloud-top brightness temperature from geostationary satellites to infer the location of deep convection. It has been recognized in recent years that the majority of tropical precipitation is produced by mesoscale convective systems (MCSs). However, the relationship between the IR cloud-top patterns associated with MCSs and the underlying precipitation is not well understood. The assumption that the coldest cloud tops are associated with deep, active convection has been central to the characterization of cloud system motion and organization, and to many IR-based rainfall retrievals. Previous studies suggested that this view may be oversimplified when applied to propagating convective systems, such as squall lines. The goal of this study was to understand the evolution of the cold cloud associated with tropical oceanic squall line MCSs, and to discuss the implications for the retrieva...

The Relationship between Anvil Clouds and Convective Cells: A Case Study in South Florida during CRYSTAL-FACE

Abstract One of the important goals of NASA’s Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL-FACE) was to further the understanding of the evolution of tropical anvil clouds generated by deep convective systems. An important step toward understanding the radiative properties of convectively generated anvil clouds is to study their life cycle. Observations from ground-based radar, geostationary satellite radiometers, aircraft, and radiosondes during CRYSTAL-FACE provided a comprehensive look at the generation of anvil clouds by convective systems over South Florida during July 2002. This study focused on the relationship between convective rainfall and the evolution of the anvil cloud shield associated with convective systems over South Florida on 23 July 2002, during the CRYSTAL-FACE experiment. Anvil clouds emanating from convective cells grew downwind (to the southwest), reaching their maximum area at all temperature thresholds 1–2 h after the active c...

An Ensemble of Convective Systems on 11 February 1993 during TOGA COARE:Morphology, Rainfall Characteristics, and Anvil Cloud Interactions

Abstract An active day during the Coupled Ocean–Atmosphere Response Experiment (COARE) Intensive Observation Period (IOP) is examined in which nine convective systems evolved and moved eastward across the region of shipboard radar coverage in the Intensive Flux Array (IFA) within westerly wind burst conditions. The detailed genesis, morphology, and interactions between these cloud systems are documented from a radar and satellite perspective. One of these systems was a large and complex elliptical cluster, among the largest observed during the Tropical Ocean Global Atmosphere COARE. Multiple, parallel deep convective lines spaced 20–30 km apart and embedded within this system were initially oriented from north-northwest to south-southeast, oblique to the storm motion. Furthermore, the lines underwent counterclockwise realignment as the system moved eastward. The influence of strong lower-tropospheric directional and speed shear on these convective system properties is examined in the context of a dynamic,...

Regional Contrast of Mesoscale Convective System Structure prior to and during Monsoon Onset across South America

AbstractIn this study, a 10-yr (1998–2007) climatology of observations from the Tropical Rainfall Measuring Mission (TRMM) satellite is used to study regional mechanisms of monsoon onset across tropical and subtropical South America. The approach is to contrast regional differences in the structure, intensity, and rainfall of mesoscale convective systems (MCSs) prior to and after onset, in the context of thermodynamic conditions from the National Centers for Environmental Prediction (NCEP) reanalysis data. This is accomplished by analyzing the mean annual cycle time series, 10-yr frequency histograms, and 3-month-averaged values prior to and following onset in four regions of distinct rainfall variability. Observed MCS metrics and NCEP variables include lightning flash rate, convective rain fraction, height of the 30-dBZ isosurface, minimum 85-GHz polarization corrected temperature, and the fluxes of sensible and latent heat.The west-central Amazon region had a distinct maximum of MCS intensity 2 months p...