Validation of Global Climate Model moisture trends for the Coupled Model Intercomparison Project (CMIP) using GPS Precipitable Water Vapor (PWV) observations in the U.S. Great Plains from 2000 to 2010

The IPCC Fourth Assessment concluded that increases in global Precipitable Water Vapor (PWV) are expected with rising global surface temperatures. The Global Climate Models (GCMs) used in this assessment provide a way to predict the increase in the atmosphere's total water vapor content and the resulting water vapor feedback, however a validation of the GCM's accuracy at forecasting PWV is needed. We use observations over land in order to verify key assumptions used in the climate models relating model moisture content to surface air temperature. Ground based GPS measurements of the total column water vapor provide the time continuity and absolute accuracy needed to test the model's moisture trend predictions and temporal variability.

The Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) program archive contains a large and growing database of GPS PWV measurements for the US Great Plains from networks including those of SuomiNet, GPS-Met, and the International GNSS Service (IGS). This archive also contains research quality atmospheric profile data from the ARM central facility site near Lamont, Oklahoma. Monthly mean GCM output products are available through the Earth Systems Grid (ESG) database, including NCAR's Community Climate System Model (CCSM). The CCSM data includes relative humidity, temperature, and pressure profiles, which can be vertically integrated to compute the total water vapor column.

The purpose of this study is to show model comparisons to observations of monthly mean PWV and relative humidity for over a decade (2000-2010) in the U.S. Great Plains. Results are presented to highlight trends and variability in surface temperature, relative humidity, and PWV. The dependence of moisture content on surface temperature is evaluated for both models and observations. Special attention is paid to the amplitude of the seasonal cycle of water vapor in climate models for a set of regions that span the latitude range from the Gulf of Mexico to the Northern Great Plains.