Water Vapor

• Water vapor plays a decisive role in the transfer of radiation through the atmosphere and therefore largely determines the atmospheric response to an increase of infrared active gases such as carbon dioxide and freons through the water vapor feedback mechanism.

• Water vapor is important to the transport and release of latent heat. The distribution of latent heat release is a topic that has received considerable attention over the past decade especially with the burgeoning interest in the variability of the atmosphere on both inter- and intra-seasonal time scales.

• The amount of water vapor present in the atmosphere influences the rate of evaporation from the surface, a process of crucial importance to our understanding the surface energy budget.

• The transport and divergence of water vapor are essential ingredients in determining the distribution of solid and liquid water in the atmosphere and therefore crucial to the significant and perplexing problem of cloud feedback in climate change.

• The atmospheric distribution of water vapor is critical in determining the energy balance at the ocean surface and indirectly, in the fresh water flux into the ocean. In that context, the atmospheric water vapor is of a vital importance in determining the interaction of the ocean and the atmosphere on a wide spectrum of space and time scales.

2. Research Goals

The goal of the research is to elucidate the role of water vapor in climate and climate change especially with regard to (i) water vapor feedback, (ii) the impact of vertical redistribution on this feedback and (iii) the broad aspects of the transport of this vapor. To achieve these three goals, research is discussed in terms of two broad and connected areas. The first is concerned with research on ways of obtaining global water vapor data, particularly using existing and new satellite observations. The second phase of this research (and one definitely connected to the first) is concerned with application of the data to evaluate the hydrological cycle as it is predicted in climate models and observed in the real world. This research involves analyses of two types of climate model simulations, AMIP II where the SSTs are specified and another involving simulations of coupled ocean atmospheric GCMs. It is important to emphasize that this research is not merely about collecting water vapor data and comparison of these data to their model counterparts. Rather the research focuses on examining the sensitivity of important processes (such as radiative transfer, cloud processes among others) to water vapor and to examine how well these are treated in climate models.


3. Current Activities

1. Satellite retrievals: analyses and assessment of present satellite capabilities (Engelen and Stephens, 1998b, Englen and Stephens, 1998b).
2. Validation of satellite data: use of field study data, such as collected during the ARM IOP (see http://www.arm.gov and http://cmiss.ssec.wisc.edu/aeriwww/www/) and other current activities (e.g., LASE)
3. Analyses of existing data: A number of activities have been undertaken. These include analysis of existing data to study the climate system (e.g., Lietzke and Vonder Haar, 1998) including analysis of NVAP and reanalysis data to other components of the climate system. We are currently analyzing NVAP, ECMWF, DAO and NCEP reanalysis data sets.
In related activities, the work of CLERA (Clear Sky Longwave Radiation from ECMWF Reanalysis) is an extension of earlier work by Slingo and Webb (1992). The idea is to incorporate analyses data into a model of clear sky OLR and through comparison with measured OLR identify problems in the reanalysis measurements.

A new effort under development is to use the satellite data, together with daily wind data to study the properties of water vapor mixing. This work is being carried out in the University of Chicago under the direction of Professor R. Pierrehumbert. Satellite water vapor data sets are being developed for this activity.

We have developed a collaboration with Dr. J. Waters of JPL and are beginning to analyze MLS upper troposphere water vapor in association with TOVS and SSMT2. These data will not only be compared to other satellite data but will also be used to study the relationship between upper tropospheric water vapor variability and the variability of the atmospheric circulation.

4. Comparison with models:
We have implemented a special AMIP-2 project under this research that allows the modeling groups to simulate TOVS radiance directly. Refer to the above web address?

Experiments from four coupled climate models in which CO₂ concentrations increases at rates of 0.5-1.1% per year for periods of 75-200 years have also been used to study the radiative fluxes at the surface and the water vapor budget of the atmosphere. Results from a fifth simulation are currently being included in this analysis. (Garratt et al., 1998).

5. Signatures of water vapor feedback: The feedback cycle between radiation, clouds, water vapor and SST in the tropical atmosphere is being studied. We use a variety of sources of data providing new insight about tropical convection to demonstrate (SST).

2. Participation in Field Program

No direct participation, but indirectly through the US Department of Energy ARM water vapor IOP as mentioned above.




3. References