Observing atmospheric moisture from geostationary orbit: how many channels is enough?

Most of today's operational numerical weather prediction (NWP) models demonstrate measurable skill at predicting the onset and amount of precipitation from extratropical systems. This is somewhat surprising when you consider that atmospheric water vapor is the least measured parameter of all the dependent variables that collectively define a set of initial conditions. In many of today's operational models the moist dependent variable is often not conserved due to limitations in the moist parameterizations, missing components of the moisture budget, or due to the use of diffusion operators required for numerical stability. Forecasts of the moist dependent variable are rarely validated because of the lack of observations. Precipitation amounts, however, are routinely observed which allow modelers to subjectively tune the moist physical parameterizations that force precipitation. These same models often remain relatively insensitive to details in the initial moisture fields. As model resolution increases, so will the need for frequent high-resolution measurements of atmospheric moisture. At present there is an increasing emphasis being placed on the numerical prediction of water vapor and moisture-sensitive diagnostics such as visibility, clouds, aircraft icing potential, and convective potential. Models running at resolutions of 4 to 10 kilometers will require detailed moisture information to successfully predict these quantities. Instruments in geostationary orbit will play a key role in providing moisture observations at spatial and temporal resolutions required by future NWP models.