Near tropopause water vapor content and exchange processes

Water vapor mixing ratios in the upper troposphere and lower stratosphere measured by the Aura Microwave Limb Sounder version 2.2 (Aura-MLS) instrument have been compared with Global Forecast System (GFS) analyses at five levels within the 300 - 100 hPa layer and North American Mesoscale (NAM) model analyses at six levels within the 300 - 50 hPa layer over the two years of 2005 and 2006 at four analysis times. Probability density functions of the vapor mixing ratios suggest that both analyses are often moister than Aura-MLS values, but NAM model analyses agree somewhat better with Aura-MLS measurements than GFS model analyses over the same North American domain at the five common levels. Examining five subsets of the global GFS domain, the GFS model analysis is moister than Aura-MLS estimates everywhere except at 150 and 100 hPa in all regions outside of the tropics. NAM model analysis water vapor mixing ratios exceeded the Aura-MLS values at all levels from 250 to 150 hPa in all four seasons of both years and some seasons at 100 and 50 hPa. Moist biases in winter and spring of both years were similar at all levels, but these moist biases in summer and fall were smaller in 2005 than in 2006 at all levels. These differences may be due to the change in the NAM from using the Eta to using the Weather Research and Forecasting (WRF) model in June 2006. NAM analysis data and the Weather Research and Forecasting (WRF) Advanced Research WRF (ARW) model version 2.2 are used to investigate the mechanisms involved in the transport of water vapor in the upper troposphere and lower stratosphere (UTLS) affected by deep convective system activity. In an examination of two convective system events occurring over the United States, it is found that hourly water vapor changes in the UTLS were mostly affected by advection and microphysical processes, with mixing playing less of a role during the period of convective system activity. Hourly moistening rates averaged over the vicinity of deep convection (VODC) in the UTLS increased during the time that convective system activity developed, and reached maximum values at the same time that the strongest convection and heaviest precipitation occurred at the surface. In the upper troposphere levels, the hourly positive water vapor tendencies were mainly due to both vertical and horizontal advection, though, the rate of water vapor tendencies due to vertical advection was greater. Water vapor tendencies due to microphysical processes were noticeable in this layer, where they tended to oppose the moistening due to advection. Near the tropopause and lower stratosphere levels, water vapor tendencies due to horizontal advection alone resulted in an increase in water vapor somewhat before/after the demise of the convective system, instead of at the time of intense convective system activity