A climatological perspective of deep convection penetrating the TTL during the Indian summer monsoon from the AVHRR and MODIS instruments 1Remote Sensing Division, Swedish Meteorological and Hydrological Institute, Norrköping, Sweden
2Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge, UK
Received: 07 Nov 2009 – Published in Atmos. Chem. Phys. Discuss.: 04 Feb 2010Abstract. The impact of very deep convection on the water budget and thermal structure
of the tropical tropopause layer is still not well quantified, not least
because of limitations imposed by the available observation techniques.
Here, we present detailed analysis of the climatology of the cloud top
brightness temperatures as indicators of deep convection during the Indian
summer monsoon, and the variations therein due to active and break periods.
We make use of the recently newly processed data from the Advanced Very High
Resolution Radiometer (AVHRR) at a nominal spatial resolution of 4 km. Using
temperature thresholds from the Atmospheric Infrared Sounder (AIRS), the
AVHRR brightness temperatures are converted to climatological mean
(2003–2008) maps of cloud amounts at 200, 150 and 100 hPa. Further, we relate
the brightness temperatures to the level of zero radiative heating, which
may allow a coarse identification of convective detrainment that will
subsequently ascend into the stratosphere. The AVHRR data for the period
1982–2006 are used to document the differences in deep convection between
active and break conditions of the monsoon. The analysis of AVHRR data is
complemented with cloud top pressure and optical depth statistics (for the
period 2003–2008) from the Moderate Resolution Imaging Spectroradiometer
(MODIS) onboard Aqua satellite. Generally, the two sensors provide a very
similar description of deep convective clouds.
Revised: 18 Apr 2010 – Accepted: 06 May 2010 – Published: 18 May 2010
Our analysis shows that most of the deep convection occurs over the Bay of
Bengal and central northeast India. Very deep convection over the Tibetan
plateau is comparatively weak, and may play only a secondary role in
troposphere-to-stratosphere transport. The deep convection over the Indian
monsoon region is most frequent in July/August, but the very highest
convection (coldest tops, penetrating well into the TTL) occurs in May/June.
Large variability in convection reaching the TTL is due to monsoon
break/active periods. During the monsoon break period, deep convection
reaching the TTL is almost entirely absent in the western part of the study
area (i.e. 60 E–75 E), while the distribution over the Bay of Bengal and the
Tibetan Plateau is less affected. Although the active conditions occur less
frequently than the break conditions, they may have a larger bearing on the
composition of the TTL within the monsoonal anticyclone, and tracer
transport into the stratosphere because of deep convection occurring over
anthropogenically more polluted regions.
Citation: Devasthale, A. and Fueglistaler, S.: A climatological perspective of deep convection penetrating the TTL during the Indian summer monsoon from the AVHRR and MODIS instruments, Atmos. Chem. Phys., 10, 4573-4582, doi:10.5194/acp-10-4573-2010, 2010.