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Daniel Rosenfeld and William L. Woodley

Abstract

Spaceborne inferences of cloud microstructure and precipitation-forming processes with height have been used to investigate the effect of ingested aerosols on clouds and to integrate the findings with past cloud physics research. The inferences were made with a method that analyzes data from National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (NOAA AVHRR) and Tropical Rainfall Measuring Mission Visible and Infrared Scanner (TRMM VIRS) sensors to determine the effective radius of cloud particles with height. In addition, the TRMM Precipitation Radar (PR) made it possible to measure the rainfall simultaneously with the microphysical retrievals, which were validated by aircraft cloud physics measurements under a wide range of conditions. For example, the satellite inferences suggest that vigorous convective clouds over many portions of the globe remain supercooled to near −38°C, the point of homogeneous nucleation. These inferences were then validated in Texas and Argentina by in situ measurements using a cloud physics jet aircraft.

This unique satellite vantage point has documented enormous variability of cloud conditions in space and time and the strong susceptibility of cloud microstructure and precipitation to the ingested aerosols. This is in agreement with past cloud physics research. In particular, it has been documented that smoke and air pollution can suppress both water and ice precipitation-forming processes over large areas. Measurements in Thailand of convective clouds suggest that the suppression of coalescence can decrease areal rainfall by as much as a factor of 2. It would appear, therefore, that pollution has the potential to alter the global climate by suppressing rainfall and decreasing the net latent heating to the atmosphere and/or forcing its redistribution. In addition, it appears that intense lightning activity, as documented by the TRMM Lightning Imaging Sensor (LIS), is usually associated with microphysically highly “continental” clouds having large concentrations of ingested aerosols, great cloud-base concentrations of tiny droplets, and high cloud water contents. Conversely, strongly “maritime” clouds, having intense coalescence, early fallout of the hydrometeors, and glaciation at warm temperatures, show little lightning activity. By extension these results suggest that pollution can enhance lightning activity.

The satellite inferences suggest that the effect of pollution on clouds is greater and on a much larger scale than any that have been documented for deliberate cloud seeding. They also provide insights for cloud seeding programs. Having documented the great variability in space and time of cloud structure, it is likely that the results of many cloud seeding efforts have been mixed and inconclusive, because both suitable and unsuitable clouds have been seeded and grouped together for evaluation. This can be addressed in the future by partitioning the cases based on the microphysical structure of the cloud field at seeding and then looking for seeding effects within each partition.

This study is built on the scientific foundation laid by many past investigators and its results can be viewed as a synthesis of the new satellite methodology with their findings. Especially noteworthy in this regard is Dr. Joanne Simpson, who has spent much of her career studying and modeling cumulus clouds and specifying their crucial role in driving the hurricane and the global atmospheric circulation. She also was a pioneer in early cloud seeding research in which she emphasized cloud dynamics rather than just microphysics in her seeding hypotheses and in her development and use of numerical models. It is appropriate, therefore, that this paper is offered to acknowledge Dr. Joanne Simpson and her many colleagues who paved the way for this research effort.

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