Effect of an extratropical mesoscale convective system on water vapor transport in the upper troposphere/lower stratosphere: A modeling study

Abstract

North American Model (NAM) analysis data and the Weather Research and Forecasting (WRF) Advanced Research WRF (ARW) model version 2.2 are used to investigate the effect of a mesoscale convective system (MCS) in extratropical regions on the transport of water vapor in the upper troposphere and lower stratosphere (UTLS). In addition, physical mechanisms contributing most to the water vapor distribution in the UTLS and the amount of water vapor transported during the most active period of the convective system are examined. In an MCS occurring over the Midwest, the primary focus of the present study, simulated by WRF on 13–14 July 2006, hourly water vapor amount averaged near the system in the UTLS increased substantially 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, large positive hourly water vapor tendencies were mainly due to vertical advection with highest rates at the time of highest rain rates. Water vapor tendencies due to microphysical processes tended to oppose the moistening due to advection. Near the tropopause and in the lower stratosphere, however, positive hourly water vapor tendencies were primarily due to microphysics and mixing within the MCS. Horizontal advection also transported some moisture in regions downstream from the MCS at most times, with the largest impacts later in the MCS lifetime. Around the tropopause, microphysical processes related to the presence of convectively injected ice appeared to be the largest contributor to moistening for this case. The results were not found to be sensitive to model microphysical schemes.