A multisatellite analysis of deep convection and its moist environment over the Indian Ocean during the winter monsoon

Abstract

[1] The aim of this paper is to characterize the deep convective systems over the Indian Ocean during Indian Ocean Experiment (INDOEX) and their relationship to cloudiness and to the Upper Tropospheric Humidity (UTH) of their environment together with the relevant longwave radiation fields. Multisatellite analyses are performed (Meteosat, Scanner for Radiation Budget (ScaRaB), and Special Sensor Microwave Imager (SSM/I)) to measure these environmental parameters. The use of Meteosat water vapor (WV) channel appears very efficient not only for estimating UTH but also for separating high level cloudiness, including thin cirrus, from clear sky and low clouds. The Meteosat infrared (IR) and WV channels are also used for correlating Meteosat and ScaRaB measurements, allowing to retrieve continuously the longwave radiative flux. The longwave flux is used to compute the cloud radiative forcing as well as the clear-sky greenhouse effect. Spatial relationships between upper level cloudiness and UTH are established. A strong positive linear relationship is found suggesting a local moistening of the upper troposphere by convection. The temporal analysis reveals that during the active phase of the intraseasonal oscillation, the longwave cloud radiative forcing reaches a mean value up to 40 W m À2 over a large region in the open ocean, while the average clear-sky greenhouse effect is in excess of 180 W m À2 . These radiative parameters are strongly correlated with the upper level cloudiness and upper level moisture, respectively. The temporal variability of UTH explains up to 80% of the greenhouse effect variability. The structure of the convective cloud systems is then studied. The observed population of systems spans a wide spectrum of area from 100 to 1,000,000 km 2 . The contribution to the high level cloudiness of the systems with a strong vertical development is dominant. These systems, with at least one convective cell reaching the highest levels (below 210 K), present indices of overshooting tops and are the most horizontally extended. The largest system exhibits an average longwave radiative forcing of around 100 W m À2. Their contribution to the cloud forcing over the Indian Ocean is overwhelming. The spatial and temporal variability of the systems is finally related to the UTH and to the clear-sky greenhouse effect. Strong correlations are found indicating that these organized convective systems at mesoscale play a leading role in the Indian Ocean climate. The analysis suggests that deeper convection is associated with larger cloud desks with larger cloud radiative forcing. It is also associated with a moister upper troposphere and a larger clear-sky greenhouse effect. These two effects would provide a positive feedback on the surface conditions.

A numerical study of tropical cross-tropopause transport by convective overshoots

“…A satellite analysis of convective systems over the Indian Ocean shows that, for mesoscale organized convection (spanning an area larger than 10 4 km 2 ), the larger the deepest systems, the larger the fraction of overshoot areas (Roca et al, 2002). It is anticipated that more import of water vapour in the stratosphere should be found over organized systems as was observed during the African Monsoon Multidisciplinary Analysis (AMMA) campaign (J.-P. Pommereau 2006, personal communication).…”

“…The 3-K threshold was arbitrarily chosen to highlight the BTD signature. It is less than the empirical value of 5 K used by Roca et al (2002) to analyse deep convection over the Indian Ocean. During the two Geophysica flight legs, BTDs over this particular threshold were effectively found over the coldest clouds, where particles were observed in the lower stratosphere.…”

Atmospheric pollutant outflow from southern Asia: a review

“…Chatfield et al (2004) show that the SAW-ABC provides an episodic and significant source of ozone pollution to the Atlantic troposphere, via occasional convective lofting of the pollutants over the northern to central Indian Ocean, addition of NO x from convection (particularly over Africa), westward transport in the upper troposphere, and merging in of stratospheric O 3 and ozone precursors from convective lofting over Africa, while subsidence over the Atlantic "tends to enhance the O 3 column as high O 3 fills in from above". Although the northern Indian Ocean tends to be relatively cloud-free during the winter monsoon, down to the ITCZ, occasional deep convection in mesoscale convective systems does occur (Roca et al, 2002), leading to the possibility of this long-range contribution to the tropical Atlantic ozone columns.…”

A new METEOSAT “water vapor” archive for climate studies

“…In cloudy sky, the 6.3-mm radiance corresponds to a signal similar to the window infrared channel modulated by the water vapor above the clouds, if any. More details about the interpretation of these measurements are given by Ramond et al [1981], Szejwach [1982], Picon and Desbois [1990], Soden and Bretherton [1993], Schmetz et al [1995], Roca et al [2002a], and references therein. Hence thanks to these radiometric observations, some information about the long-term variability of the UTH distribution is available.…”