the stratosphere. Water in aerosol form provides the site for surface catalysis that repartitions the dominant chemical families in the troposphere and the stratosphere and alters the chemical composition (and thus the formation characteristics) of the aerosol particles themselves. As the temperature drops in the upper troposphere, particularly in the tropics and polar regions, the rate of chemical transformation increases exponentially, in some important cases by as much as three orders of magnitude over an interval of 10°C. The formation of cirrus, both visible and subvisible, is driven by the combination of water vapor density, temperature, pressure, organic aerosols, and sulfate/ nitrate loading. The role that cirrus formation plays in the climate system is a critical quantitative question but one for which little information exists. An understanding of water vapor in its phases has emerged as the main link between chemistry, radiation, dynamics, and climate. Progress in clarifying these relationships will depend on a strategic blend of in situ and remote observations that explore the links among sea surface temperature, convective drive, horizontal water vapor redistribution, and mechanisms controlling the ratio of dry subsidence regions and moist vertically ascending zones in the climate system. A consistent attack on this problem is needed, using a combination of sonde and aircraft observations with an array of chemical tracers and isotopes, together with innovative satellite observations.

For the purpose of initially determining secular trends in water, in situ measurements (using sondes with improved accuracy and more fully instrumented aircraft) offer the advantage of high vertical resolution and provide a test of existing satellite data.

Instruments such as SAGE II,24 the Halogen Occultation Experiment, and the Microwave Limb Sounder on the Upper Atmosphere Research Satellite have demonstrated that stratospheric water vapor can be measured from satellites with adequate precision to characterize temporal trends at some levels. Because of the global coverage that space-based platforms provide, continuous measurements using these techniques are critical for tracking the potential causes and effects of global climate change.

Compared to H2O concentrations typically found in the lower and midtroposphere, such concentrations in the upper troposphere are extremely small. (H2O concentrations from the surface to the tropopause typically decrease by about three orders of magnitude or more.) As a result, the technologies used for routine weather soundings do not have the accuracy or the sensitivity to reliably monitor H2 O in the upper troposphere. Moreover, current space-based platforms (e.g., SAGE II) are only able to quantify upper tropospheric H2O when aerosol loadings are low.25 For these reasons, new approaches must be developed for measuring H2O trends in the upper troposphere. Ideally, these approaches would be amenable to remote sensing from small satellite platforms, thus affording a strategy for obtaining global coverage at a reasonable cost.

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