Hydration or dehydration: competing effects of upper tropospheric cloud radiation on the TTL water vapor 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California
28 Aug 2012
2Joint Institute for Regional Earth System Science and Engineering, University of California, Los Angeles, California, USA
Received: 14 Dec 2011 – Published in Atmos. Chem. Phys. Discuss.: 08 Feb 2012 Abstract. A tropical channel version of the Weather Research and Forecasting (WRF)
model is used to investigate the radiative impacts of upper tropospheric
clouds on water vapor in the tropical tropopause layer (TTL). The WRF
simulations of cloud radiative effects and water vapor in the upper
troposphere and lower stratosphere show reasonable agreement with
observations, including approximate reproduction of the water vapor "tape
recorder" signal. By turning on and off the upper tropospheric cloud
radiative effect (UTCRE) above 200 hPa, we find that UTCRE induces a warming
of 0.76 K and a moistening of 9% in the upper troposphere at 215 hPa.
However, UTCRE cools and dehydrates the TTL, with a cooling of 0.82 K and a
dehydration of 16% at 100 hPa. The enhanced vertical ascent due to UTCRE
contributes substantially to mass transport and the dehydration in the TTL.
The hydration due to the enhanced vertical transport is counteracted by the
dehydration from adiabatic cooling associated with the enhanced vertical
motion. UTCRE also substantially changes the horizontal winds in the TTL,
resulting in shifts of the strongest dehydration away from the lowest
temperature anomalies in the TTL. UTCRE increases in-situ cloud formation in
the TTL. A seasonal variation is shown in the simulated UTCRE, with stronger
impact in the moist phase from June to November than in the dry phase from
December to May.
Revised: 03 Jul 2012 – Accepted: 21 Aug 2012 – Published: 28 Aug 2012
Citation: Wu, L., Su, H., Jiang, J. H., and Read, W. G.: Hydration or dehydration: competing effects of upper tropospheric cloud radiation on the TTL water vapor, Atmos. Chem. Phys., 12, 7727-7735, doi:10.5194/acp-12-7727-2012, 2012.