Radiative impacts of clouds in the tropical tropopause layer

Abstract

[1] We quantify the seasonal and spatial variations of cloud radiative impacts in the tropical tropopause layer (TTL) by using cloud retrievals from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), International Satellite Cloud Climatology Project (ISCCP) and CloudSat. Over the convective regions including Western Pacific, Africa, South America, and South Asia, we find pronounced solar heating and infrared cooling in the lower part of the TTL (<$16 km). The solar heating weakens above 16 km and nearly diminishes at 18 km, whereas the infrared cooling extends vertically throughout the TTL. The net cloud radiative forcing, which is the summation of cloud solar and infrared radiative forcing, has heating below $16 km and turns to mostly cooling above 17 km. The net cloud radiative heating over the convective regions is mainly contributed from solar radiation, whereas the weak net cloud radiative heating surrounding these regions is due to infrared heating. To further examine the impacts of different cloud types in the TTL, we classified TTL clouds in terms of cloud optical depths (t) as thin cirrus (t < 0.3), thick cirrus (0.3 t < 3), and opaque clouds (t ! 3). In the solar part, thin and thick cirrus play a relatively small role and the impact of cloud-free air above clouds is negligible. The solar heating is dominantly contributed from the solar absorption near the top of opaque clouds. In the infrared part, the thick cirrus heating is mainly confined over the convective regions in the lower part of TTL while the thin cirrus infrared heating is more prevalent both vertically and horizontally in the TTL, which is the dominant infrared heating source. The infrared cooling in cloud-free air above clouds is dominant above 17 km, whereas the infrared cooling near the top of opaque clouds is dominant below. Despite the infrared heating effects of thin and thick cirrus clouds, the infrared cooling from the opaque cloud top and cloud-free air above clouds outweighs the heating effects so that the ensemble mean cloud infrared radiative forcing is mostly cooling except outside the convective regions.

Variability in upwelling across the tropical tropopause and correlations with tracers in the lower stratosphere

“…Finally, we note that our calculations do not include the effects of clouds on the radiative balance. However, according to Yang et al (2010), the net effect of clouds on the zonal mean heating rates in the tropics is relatively small (≤ −0.05 K day −1 ) at and above the tropical tropopause.…”

Dehydration in the tropical tropopause layer estimated from the water vapor match

“…Because the default soundings in previous SOWER campaigns did not include any sensors for particles, we could not estimate the total water content in which the air mass has ice particles. In addition, for the campaign region and period (i.e., over the western Pacific in the boreal winter), cirrus clouds are common in the TTL; Shibata et al, 2007;Fujiwara et al, 2009;Massie et al, 2010;Yang et al, 2010;Inai et al, 2012;Shibata et al, 2012). If we could observe ice particles and measure their concentration and size distribution, we could estimate their growth rate under the assumption that a critical relative humidity leads to the initiation of ice nucleation, using the same technique as that employed to estimate the efficiency of dehydration (Sect.…”

Hydration or dehydration: competing effects of upper tropospheric cloud radiation on the TTL water vapor

“…TTL cirrus overlapping thick clouds could radiatively cool the TTL and result in a weakening of vertical ascent. The opaque clouds (τ ≥ 3) can warm the TTL by absorbing solar radiation (Yang et al, 2010). Observational data show that cirrus leads to a net positive cloud radiative heating in the TTL (e.g., Su et al, 2009;Yang et al, 2010).…”

“…The opaque clouds (τ ≥ 3) can warm the TTL by absorbing solar radiation (Yang et al, 2010). Observational data show that cirrus leads to a net positive cloud radiative heating in the TTL (e.g., Su et al, 2009;Yang et al, 2010). With a two-dimension (2-D) model, Rosenfield et al (1998) showed that the radiative heating of subvisible thin cirrus would result in a warming of 1-2 K and a vertical velocity increase of 0.02-0.04 mm s −1 in the TTL.…”