Quantifying the Radiative Impact of Clouds on Tropopause Layer Cooling in Tropical Cyclones

Abstract

A ubiquitous cold signal near the tropopause, here called “tropopause layer cooling” (TLC), has been documented in deep convective regions such as tropical cyclones (TCs). Temperature retrievals from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) reveal cooling of order 0.1–1 K day−1 on spatial scales of order 1000 km above TCs. Data from the Cloud Profiling Radar (onboard CloudSat) and from the Cloud–Aerosol Lidar with Orthogonal Polarization [onboard the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO)] are used to analyze cloud distributions associated with TCs. Evidence is found that convective clouds within TCs reach the upper part of the tropical tropopause layer (TTL) more frequently than do convective clouds outside TCs, raising the possibility that convective clouds within TCs and associated cirrus clouds modulate TLC. The contribution of clouds to radiative heating rates is then quantified using the CloudSat and CALIPSO datasets: in the lower TTL (below the tropopause), clouds produce longwave cooling of order 0.1–1 K day−1 inside the TC main convective region, and longwave warming of order 0.01–0.1 K day−1 outside; in the upper TTL (near and above the tropopause), clouds produce longwave cooling of the same order as TLC inside the TC main convective region, and one order of magnitude smaller outside. Considering that clouds also produce shortwave warming, cloud radiative effects are suggested to explain only modest amounts of TLC while other processes must provide the remaining cooling.

Impacts of tropical cyclones on the thermodynamic conditions in the tropical tropopause layer observed by A-train satellites

“…8 (a)). It is also consistent with the finding of Rivoire et al (2020), who noted a cooling tendency above 100 hPa that is partially attributable to the radiative effect based on an analysis of the COSMIC data.…”

“…On the other hand, Rivoire et al (2020) pointed out that the cloud radiative cooling only partly explains the cooling 270 tendency above cyclones. Other mechanisms at play may include the adiabatic expansion (Holloway and Neelin, 2007) of the convective overshoots (Robinson and Sherwood, 2006) and the outward branch of the secondary circulation (Rivoire et al, 2020;Schubert and McNoldy, 2010). It is also interesting to find that the temperature anomaly is stronger above non-overshooting clouds (TTL-OTHERs), in comparison with DCC-OTs and NTTLs (Fig.…”

“…For instance, it is well known that thin cirrus absorbs longwave emission from the surface and heats the level it resides (e.g., see Fig. 5 (c) of Rivoire et al (2020)), although such an effect is not identified anywhere in Fig. 11.…”

Impacts of tropical cyclones on the thermodynamic conditions in the tropical tropopause layer observed by A-Train satellites

“…8a). It is also consistent with the finding of Rivoire et al (2020), who noted, based on an analysis of Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) data, a cooling tendency above 100 hPa that is partially attributable to radiative effects.…”

“…On the other hand, Rivoire et al (2020) pointed out that cloud radiative cooling only partly explains the cooling tendency above tropical cyclones. Other mechanisms at play may include the adiabatic expansion (Holloway and Neelin, 2007) in convective overshoots (Robinson and Sherwood, 2006) and the outward branches of secondary circulation (Rivoire et al, 2020;Schubert and McNoldy, 2010). It is also interesting to find that the temperature anomaly is stronger above non-overshooting clouds (TTL-OTHER) than over DCC-OT or NTTL (Fig.…”