Clouds and Climate
This document is divided into 4 sections:
Summary
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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Objectives
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:
- Building a data base, or census, of small precipitating cumulus; e.g., dimensions (top height, diameter, and depth) and precipitation characteristics that has potential uses in several facets of climatic analysis.
- Documenting the thermodynamic and wind environment of the clouds. Mapping the three dimensional flow field within an active convective feature, and computing the hydrometeor trajectories into the region surrounding the storm using the airborne Dopple
r
radar.
- Collecting rainfall statistics of oceanic convection for use in statistical rainfall studies.
- Testing the capability of determining the hydrometeor distributions from the reflectivity and Doppler mean velocity data at, or near, vertical incidence.
- Documenting the initial electrification and the evolution of the electric field within a sample of clouds.
- Documenting the characteristics of significant convective updrafts - water mass flux, the evolution of ice particles in the updrafts and the conversion rates to ice.
- Studying the relationship between initial and subsequent precipitation formation and the interaction between precipitation loading and the dynamics of the convective cell.
- Studying the interactions between warm cloud and ice microphysics at different stages of cloud development. Emphasis will be placed on the warm rain development versus rain from glaciation.
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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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