Clouds and Climate

This document is divided into 4 sections:


This is a single-option, single-aircraft experiment that uses the airborne Doppler radar and microphysics instrumentation to accumulate a data base of cloud precipitation properties over a wide range of environments. This study emphasizes the exploitation of both airborne in-situ and remote sensing (radar), together with satellite observations of clouds. It will provide a data base for studies of clouds and precipitation mechanisms, their effect on climate, and provide ground truth for satellite techniques.

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Program Significance

It has become widely recognized that the physics of clouds and precipitation must be considered in any realistic study of climate change. Clouds and water vapor play a pivotal role in the Earth's heat and radiation budgets. They control the amount of solar energy absorbed by the climate system as well as the infrared radiation emitted to space, and they strongly influence the redistribution of heat throughout the climate system, particularly in the tropics. Tropical clouds and cloud systems, because they lie in the zone of maximum solar input into the atmospheric system, have an important, and probably direct climatic effect. Together with the release of latent heat, the radiative heating of layered clouds in the upper tropical troposphere is a significant source of energy for driving the global circulation. A wide spectrum of tropical cloud types and sizes are important from a climate viewpoint. In some instances, the very small scale microphysical characteristics of the clouds, and interacti ons with the cloud dynamics, are important on the climate scale. Small precipitating tropical cumuli, even though their fraction of active convective updrafts may be rather small at any given instant, have an aggregate fraction of total cloud cover, including decaying clouds that is in the range of 20-30%. Hence, they have a direct effect on the radiative transfer in the tropics. In addition, they have an effect on the turbulent mixing in the upper ocean through changes in radiative heating of the sea surface, and through precipitation into the sea surface. The behavior of these small clouds is linked to the ocean, and the ocean to the behavior of these clouds. As sea surface temperature influences the atmosphere on various time and space scales, clouds and upper ocean dynamics are inextricably linked. This study is complimentary to our continuing work on studies of the dynamics and microphysics of hurricane convection. The oceanic cumulus provides a simple, easily observed convective entity that has more similarities to hurricane convective clouds than differences. One advantage is that the precise stage of an oceanic cumulus in its life cycle is usually definable. Thus answering questions about this simpler entity will complement the hurricane observation program, and greatly aid in the interpre tation of more complex data sets from large international field programs. We can exploit our extensive observational capability in the natural convective laboratory at our doorstep (Florida Bay, Bahamas, and the Caribbean Sea) for a relatively meager investment of resources. The result will be an increased understanding of principles that are applicable to convection in general. The detailed microphysical measurements will also be useful to studies of the characteristics of precipitation in the tropics. The precipitation characteristics derived from this proposed experiment will provide a data base for statistical rainfall studies underway in support of the Florida Bay Restoration Act, the Climate and Global Change Initiative, TOGA COARE, and TRMM. These studies call for rainfall estimates to be made in different precipitation regimes around the world. This data set will pr ovide data on isolated topical convective clouds.

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The experiment will document the kinematics and microphysics of a representative sample of convection, with the initial emphasis being on small precipitating convective cells. We are particularly interested in these clouds' life cycle evolving from first condensation to a precipitating stage (glaciated or not). The specific scientific objectives of this experiment include:

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Mission Description

The experiment calls for a basic one-aircraft cloud structure and evolution sampling module (Fig. 28). This simple module could be executed during dedicated flights over Florida Bay or the Keys, or on targets of opportunity during deployments. Sampling during dedicated flights will emphasize combinations of remote sensing and cloud penetrations, while remote sensing will be used during deployments.

The basic cloud sampling module utilizes one aircraft, equipped with the airborne Doppler radar and microphysics instrumentation, to investigate maritime convective clouds. Desired candidates for study should be convective clouds that can be followed th rough nearly their entire life cycle. The flight patterns of the basic cloud sampling module are shown in Fig. 28, and are relatively straightforward. The aircraft will make rapid repeated penetrations of the cloud, to sample the microphysical and electric field development at a constant distance below the cloud top. The attempt will be to document the microphysics and electric field development near cloud top from first condensation through a mature cloud stage. At each pass through the cloud, vertical incidence Doppler data will be collected to document the evolution of the vertical velocity field as the cloud matures. These patterns, or penetrations, will be oriented based upon the environmental wind shear vector. The aircraft will release a GPS-son de or perform an aircraft sounding in the environment of each cloud sampled (in the clear, upwind of the cloud). The aircraft will also attempt to sample the boundary layer air flow, rainfall characteristics, the warm cloud microphysics, and photo document the cloud behavior.

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