The impact of gravity waves and cloud nucleation threshold on stratospheric water and tropical tropospheric cloud fraction

摘要

Using the Modern Era Retrospective?￠????Analysis for Research and Applications (MERRA) and MERRA?￠????2 reanalysis winds, temperatures, and anvil cloud ice, we explore the impact of varying the cloud nucleation threshold relative humidity (RH) and high?￠????frequency gravity waves on stratospheric water vapor (H 2 O) and upper tropical tropopause cloud fraction (TCF). Our model results are compared to 2008/2009 winter TCF derived from Cloud?￠????Aerosol Lidar with Orthogonal Polarization and H 2 O observations from the Microwave Limb Sounder (MLS). The RH threshold affects both model H 2 O and TCF, while high?￠????frequency gravity waves mostly impact TCF. Adjusting the nucleation RH and the amplitude of high?￠????frequency gravity waves allows us to tune the model to observations. Reasonable observational agreement is obtained with a nucleation threshold between 130% and 150% RH consistent with airborne observations. For the MERRA reanalysis, we lower the tropopause temperature by 0.5?￠????K roughly consistent with GPS radio occultation measurements and include ~0.1?￠????K high?￠????frequency gravity wave temperature oscillations in order to match TCF and H 2 O observations. For MERRA?￠????2 we do not need to adjust the tropopause temperature nor add gravity waves, because there are sufficient high?￠????frequency temperature oscillations already present in the MERRA?￠????2 reanalysis to reproduce the observed TCF.