NASA - National Aeronautics and Space Administration

Deep convection detraining in the uppermost tropical troposphere is capable of transporting water vapor and ice into the tropical tropopause layer (TTL), but the impact of deep convection on the global and regional TTL water vapor budget remains uncertain. In particular, the role of convectively detrained ice crystals that remain suspended after active convection has subsided is not well understood. These ice crystals represent aging cirrus anvils detached from the convective core. We use a cloud microphysical model that tracks individual ice crystals throughout their lifetimes to quantify the impact of detrained ice on the humidity of the TTL during boreal winter. Convective influence of air parcels near the wintertime cold point tropical tropopause is determined by tracing thousands of backward trajectories through satellite‐derived, global, 3‐hourly convective cloud‐top altitude fields. Detrained ice, most of which is found over the tropical western Pacific, experiences cooling on the order of 1 K day−1 downstream of convection. Downstream cooling increases relative humidity and explains the observed supersaturated TTL over this region. Vapor in excess of saturation condenses onto the detrained ice, which ultimately brings the relative humidity down to saturation. Thus, convectively detrained ice crystals in aging anvils predominantly dehydrate the TTL, but the effect is small (0.01 ppmv). Moistening by active convection (0.30 ppmv), including the rapid sublimation of convectively lofted ice crystals near the tops of core anvils, overwhelms the dehydration by aging anvil ice crystals detrained from the core. The net effect is moistening by convective core anvils during boreal winter.

Plain Language Summary: Tall towering clouds in the tropics are capable of transporting water vapor and ice crystals upward to an otherwise dry region of the atmosphere. Some ice crystals remain suspended in the upper atmosphere long after the cloud tower has collapsed, but their impact on the humidity of the upper atmosphere is unknown. We use a computer model to simulate the life cycle of these ice crystals to show that they generally dry the tropical atmosphere at 14–19 km during boreal winter by a small amount. This is because most of the ice crystals from tall towering clouds originate over the tropical western Pacific where they experience cooling, causing them to grow by extracting moisture from the surrounding atmosphere. Moistening of the environment by the cloud tower itself is much larger than the drying effect of suspended ice crystals in boreal winter. Even a small change in the humidity of the upper atmosphere above ~17 km significantly affects Earth's climate. Therefore, it is important to understand the processes that affect the humidity of tropical air at these altitudes, including temperature changes downstream of tall towering clouds that determine the fate of ice crystals ejected from these clouds.