Impact of Convectively Detrained Ice Crystals on the Humidity of the Tropical Tropopause Layer in Boreal Winter

• Published in: Journal of geophysical research. Atmospheres 2020-07, Vol.125 (14), p.n/a
• Main Authors: Ueyama, R., Jensen, E. J., Pfister, L., Krämer, M., Afchine, A., Schoeberl, M.
• Format: Article
• Language: English
• Subjects: Ageing; Aging; Air parcels; Air temperature; Anvils; Atmosphere; Cirrus clouds; Cloud computing; Cloud microphysics; Clouds; Computer simulation; Convection; Convection cooling; Convective clouds; Cooling; Crystals; Dehydration; detrained ice crystals; Drying; Geophysics; Humidity; Ice; Ice crystals; Life cycle; Life cycles; Relative humidity; Saturation; Sublimation; Temperature changes; Towers; Transportation; Tropical atmosphere; Tropical climate; Tropical environments; Tropical tropopause; tropical tropopause layer; Tropopause; Troposphere; Upper atmosphere; Water vapor; Water vapor budget; Water vapour; Wetting; Winter
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2020JD032894

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 to