NOGAPS-ALPHA stratospheric water vapor simulations during the 2005 and 2006 Northern Hemisphere winters

NOGAPS-ALPHA is a high-altitude version of the Navy Operational Global Atmospheric Prediction System (NOGAPS) global spectral forecast model. This study compares NOGAPS-ALPHA simulations of stratospheric water vapor with Aura Microwave Limb Sounder (MLS) measurements and with operational analyses of upper level specific humidity fields during January and February of 2005 and 2006. For this analysis, NOGAPS-ALPHA utilizes a new parameterization for production of stratospheric water vapor based on methane photolysis rates from the NRL CHEM2D model. This new parameterization, known as CHEM2D-H2O, specifies the water vapor production as a function of latitude and pressure throughout the year. It has been developed for use in high-altitude numerical weather prediction models where full photochemical calculations are too computationally expensive. The simulations reported here are produced with the 60-level (L60) version of NOGAPS-ALPHA, which extends up to ~85 km altitude (0.005 hPa), at T79, T239, and T479 spectral truncations. Overall we find good agreement between the NOGAPS-ALPHA and MLS water vapor profiles throughout the stratosphere in both winters. The NOGAPS-ALPHA simulations highlight the large differences in transport and photochemistry in the Northern polar stratosphere and lower mesosphere between the relatively stable January-February 2005 period and the highly disturbed January-February 2006 period. These results demonstrate how strong dynamical coupling between the troposphere, stratosphere, and mesosphere can contribute to the observed interannual variability in the Northern Polar stratosphere.