Insights into Cloud-Top Height and Dynamics from the Seasonal Cycle of Cloud-Top Heights Observed by MISR in the West Pacific Region

Abstract

The connection between environmental stability and the height of tropical deep convective clouds is analyzed using stereo cloud height data from the Multiangle Imaging Spectroradiometer (MISR), focusing on the seasonal cycle of clouds over the western Pacific Ocean. Three peaks in cloud-top height representing low, mid-topped, and deep convective clouds are found as in previous studies. The optically thickest cloud heights are roughly 2 km higher on the summer side of the equator, where CAPE is higher, than on the winter side. Overall cloud height, however, is about the same on both sides of the equator, but ?600 m higher in December?February (DJF) than in June?August (JJA). Because of variations in stratospheric upwelling, temperatures near the tropopause exhibit a significant seasonal cycle, mainly above 13 km. Using an ensemble of simulations by the Weather Research and Forecasting (WRF) cloud-resolving model and a simple overshooting parcel calculation, the authors show that the cloud height variation can be explained by that of near-tropopause stability changes, including influence from heights above 14 km, even though the cloud height peaks only near 12 km. This suggests that mixing above cloud top?not typically accounted for in simple models of convection?is important in setting the height of the laminar (anvil) high clouds that result. The MISR data indicate a seasonal variation in peak cloud-top temperature of ?5 K, despite the recent proposal that cloud-top heights should track a fixed isotherm. That proposal must therefore be applied with caution to any climate-change scenario that may involve significant changes in stratospheric upwelling.