NOGAPS-ALPHA assimilation of middle atmospheric water vapor from Aura MLS measurements 2005–2009

We present a description of middle atmospheric water vapor during the period from 2005-2009 based on the NOGAPS-ALPHA data assimilation and forecast system. Water vapor has a relatively long photochemical lifetime throughout most of the middle atmosphere, and so it is an excellent diagnostic for transport processes. Water is also an important greenhouse gas, and it is an important source of hydrogen compounds that affect stratospheric ozone photochemistry. Most of the information on middle atmospheric water vapor comes from satellite measurements, which offer good spatial coverage but lack the temporal coverage needed to obtain a synoptic “snap-shot” that would be useful for diagnosing transport processes. NOGAPS-ALPHA combines a global spectral forecast model with a 3DVAR data assimilation system to produce global synoptic analyses of water vapor mixing ratios from the surface to ~90 km every six hours. In addition to conventional operational meteorological observations (e.g., surface observations, radiosondes, satellite temperature and humidity profiles), NOGAPS-ALPHA assimilates middle atmospheric temperature, ozone, and water vapor profiles from the Aura Microwave Limb Sounder (MLS) and temperature profiles from the TIMED Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) instrument. First, we will present comparisons of NOGAPS-ALPHA water vapor profiles with independent measurements ranging from the upper troposphere/lower stratosphere to the upper mesosphere in order to validate the analyzed humidity fields. We then describe the interannual variability in middle atmospheric water vapor over the 2005-2009 period related to changes in transport. We highlight in particular the effect of the disturbed Northern Hemisphere polar winter stratospheres during January 2006, 2008, and 2009 on water vapor transport in the regions of the tropical stratopause and the extratropical summer upper mesosphere.