GEOS-4 Reanalysis: Assessment and sensitivity study

Shuhua Li and Steven Pawson

NASA Global Modeling and Assimilation Office


The performance of NASA GEOS-4 data assimilation system is examined by assessing the reanalysis products for the period 1991 to 1995. In addition to wind, temperature and humidity fields, we have examined features of potential vorticity, residual mean circulation, and the derived mass fluxes across the tropopause (comparing with earlier work for GEOS-1 and UKMO products), as well as the synoptic features in polar structure and water vapor transport.

A series of sensitivity studies has been conducted using the GEOS-4 system for selected periods. In this talk we will focus on the sensitivity of analysis/forecast fields to the representation of gravity wave drag (GWD). The results from a number of experiments reveal a strong sensitivity of temperature near the stratopause to the inclusion of non-zero phase speed gravity waves. This is particularly the case in winter hemisphere high latitudes. Without the non-zero phase speed GWD, there is a large "observation minus forecast'' residual that is substantially smaller when these waves are included, indicating the need for these waves in the GCM.