During November 2004-December 2005, we were in the field on the Caribbean Islands of Antigua and Barbuda carrying out RICO, a comprehensive study of trade wind clouds.  Below is a summary of the goals and objectives of this experiment.

Shallow, maritime, cumulus convection is one of the most prevalent cloud types on the planet. Trade wind cumuli typically extend to no greater than 4 km altitude, the height of the tropical trade wind inversion, and are dominated by warm rain processes. They are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding the global energy balance and climate. Because of the disparity in the range of scales from the microphysical/cloud scale (microns to a kilometer), to the cloud-interaction scale (kilometers to tens of kilometers), to the ensemble cloud field scale (tens of kilometers or more), past work has tended to focus on processes occurring on only one of these three scales – often neglecting the important interactions that occur across scales.

The objective of RICO in the broadest sense is to characterize and understand the properties of trade wind cumulus at all scales, with particular emphasis on determining the importance of precipitation. At the smallest scale, the most fundamental problem – recognized for over a half century – is explaining the rapid onset of precipitation in shallow tropical clouds. At the intermediate scale, processes controlling the mesoscale structure and coverage of shallow tropical cloud systems are not well understood. At the largest scale, our inability to describe the statistical behavior of trade wind cloud fields confounds our attempts to properly represent the exchange of radiant energy, moist enthalpy, momentum and trace constituents between the atmosphere and ocean over vast expanses of the planet. These scales are inextricably linked and the nature of these linkages is an important aspect of RICO.

RICO focused on the following interrelated scientific issues:

a.   The Microphysical/Cloud Scale

Research on spectral broadening and the initiation of precipitation in trade wind cumulus: The microphysical mechanisms directly responsible for spectral broadening and the rapid onset of precipitation remain mired in controversy. A goal of RICO is to obtain the critical observations that, when analyzed in conjunction with numerical modeling experiments, will provide the key evidence required to confirm or refute hypotheses related to precipitation initiation. The RICO scientific objectives in this area focus on 1) the ultragiant nuclei hypothesis;

2) the effects of turbulence on spectral broadening and precipitation initiation; 3) the effects of entrainment on spectral broadening; 4) the effects of pre-existing clouds and cloud processing on precipitation initiation. The measurement of aerosol, particularly CCN, is essential to the evaluation of all hypotheses explaining the onset of precipitation.

Research on the microphysics of the transition to a mature rainshaft: It is unknown whether the processes responsible for the onset of precipitation are also important for the continued production of precipitation in trade wind cumulus. An objective of RICO is to understand the microphysical processes that are important as clouds undergo and complete the transition to a mature rainshaft.

b.   The cloud-interaction scale

Research on the mesoscale organization of trade wind clouds: The mechanisms by which convection organizes in the trade wind environment are poorly understood. An objective of RICO is to understand the cloud and boundary layer processes that lead to the development and organization of convective structures in the trades. In particular, it is important to understand how dynamic and thermodynamic processes within and near precipitation-generated cold pools influence the organization and evolution of tropical cumulus.

c.   The ensemble cloud field scale

Research on the water budget of trade wind cumulus: Although shallow cumulus cover vast expanses of the world’s oceans, very little is known about how much they precipitate and how precipitation affects their statistical properties. One objective of RICO will be to estimate the water budget of trade wind cumulus using modern radar and satellite remote sensing, calibrated with in situ data. These estimates will be compared to inferences from thermodynamic budgets.

Research on the large-scale trade wind cloud environment: An objective of RICO will be to augment precipitation estimates with detailed characterizations of the large-scale environment, clouds, and their microphysical and aerosol properties. These data will be used to test hypotheses regarding scaling laws (e.g., for cloudiness, cloud base mass flux, entrainment and detrainment rates, cloud-environment differences, and momentum transport) for trade wind cumulus derived from recent large-eddy simulations (LES). They will also provide a basis for the next generation of LES studies that will strive to understand the dependence of trade-cumulus on microphysical and radiative processes, both affected by the properties of the environmental aerosol. The coupling of simulation/theoretical studies with field data across a range of scales will provide a basis for representing shallow convection in large-scale climate and weather forecast models.

d.   Related research studies

RICO will involve a number of additional studies. These include research on 1) the age of cloud parcels, 2) the chemistry and origin of aerosols, 3) radar remote-sensing, 4) developing a satellite cloud climatology, and 5) studies of the effects of clouds on radiation.

BAMEX

We are finishing up work from the Snowband Dynamics Experiment (SNOWBAND). We came out of the field in January 1998. High resolution wind fields and in-cloud thermodynamic and microphysical measurements were collected with the Electra within the snowbands, providing the first data set of its kind in these storms. We are using these data and complementrary modeling studies to document and understand the dynamic and thermodynamic structures of heavy precipitation bands, determine the dynamic forcing producing the circulations associated with the bands, and determine how boundary layer fluxes of heat and moisture modify frontal structure, stability and precipitation band dynamics.

Cross section of equivalent potential temperature and Eldora radar reflectivity for the IOP-5 snowband.  This cross section was taken from northern Indiana to northern Lake Michigan across the band appearing on the satellite image to the right..

This figure shows two cross-sections of radar reflectivity in a heavy snowband sampled by Electra during a SNOWBAND flight. The bottom panel shows the radial velocity - note the front at 2.5 km.

IR image of parent cyclone. The cross section on the left is from northern Indiana to northern Lake Michigan.