Ice injected into the tropopause by deep convection – Part 1: In the austral convective tropics

Abstract

Abstract. The contribution of deep convection to the amount of water vapour and ice in the tropical tropopause layer (TTL) from the tropical upper troposphere (UT; around 146 hPa) to the tropopause level (TL; around 100 hPa) is investigated. Ice water content (IWC) and water vapour (WV) measured in the UT and the TL by the Microwave Limb Sounder (MLS; Version 4.2) are compared to the precipitation (Prec) measured by the Tropical Rainfall Measurement Mission (TRMM; Version 007). The two datasets, gridded within 2∘ × 2∘ horizontal bins, have been analysed during the austral convective season, December, January, and February (DJF), from 2004 to 2017. MLS observations are performed at 01:30 and 13:30 local solar time, whilst the Prec dataset is constructed with a time resolution of 1 h. The new contribution of this study is to provide a much more detailed picture of the diurnal variation of ice than is provided by the very limited (two per day) MLS observations. Firstly, we show that IWC represents 70 % and 50 % of the total water in the tropical UT and TL, respectively, and that Prec is spatially highly correlated with IWC in the UT (Pearson's linear coefficient R=0.7). We propose a method that uses Prec as a proxy for deep convection bringing ice up to the UT and TL during the growing stage of convection, in order to estimate the amount of ice injected into the UT and the TL, respectively. We validate the method using ice measurements from the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) during the period DJF 2009–2010. Next, the diurnal cycle of injection of IWC into the UT and the TL by deep convection is calculated by the difference between the maximum and the minimum in the estimated diurnal cycle of IWC in these layers and over selected convective zones. Six tropical highly convective zones have been chosen: South America, South Africa, Pacific Ocean, Indian Ocean, and the Maritime Continent region, split into land (MariCont-L) and ocean (MariCont-O). IWC injection is found to be 2.73 and 0.41 mg m−3 over tropical land in the UT and TL, respectively, and 0.60 and 0.13 mg m−3 over tropical ocean in the UT and TL, respectively. The MariCont-L region has the greatest ice injection in both the UT and TL (3.34 and 0.42–0.56 mg m−3, respectively). The MariCont-O region has less ice injection than MariCont-L (0.91 mg m−3 in the UT and 0.16–0.34 mg m−3 in TL) but has the highest diurnal minimum value of IWC in the TL (0.34–0.37 mg m−3) among all oceanic zones.

The diurnal cycle of the clouds extending above the tropical tropopause observed by spaceborne lidar

“…Detecting occurrences of clouds extending above the tropopause by remote sensing requires documenting the vertical cloud profile with a fine resolution and a high sensitivity to optically thin clouds, which few instruments can reach. Lidar measurements are able to document such occurrences (e.g., Nee et al, 1998;Dupont et al, 2010;Gouveia et al, 2017), but for a long time they were limited to local case studies. Dessler (2009) was the first to use the cloud detections by the CALIPSO lidar (Cloud-Aerosol Lidar Infrared Pathfinder Satellite Observations) to investigate how clouds extend above the tropopause on a global scale.…”

“…The diurnal evolution of the high-altitude cirrus clouds has been documented over some specific sites using groundbased lidars (Sassen et al, 2003;Dupont et al, 2010;Gouveia et al, 2017). Gouveia et al (2017) documented the evolution of the integrated cloud fraction (no vertical distribution) over Amazonia; Sassen et al (2003) documented the diurnal evolution of the composition of cirrus clouds over Salt Lake City;and Dupont et al (2010) did the same over four observatories in France and in the United States.…”

“…The diurnal evolution of the high-altitude cirrus clouds has been documented over some specific sites using groundbased lidars (Sassen et al, 2003;Dupont et al, 2010;Gouveia et al, 2017). Gouveia et al (2017) documented the evolution of the integrated cloud fraction (no vertical distribution) over Amazonia; Sassen et al (2003) documented the diurnal evolution of the composition of cirrus clouds over Salt Lake City;and Dupont et al (2010) did the same over four observatories in France and in the United States. However, using ground-based lidar to document optically thin clouds extending above the tropopause is difficult for two reasons: (1) as the studies based on CALIPSO observations show, these clouds occur primarily in regions where operational ground-based sites are absent or very few (Pacific Ocean, Equatorial Africa and South America), and (2) these clouds are mainly associated with deep convection, which implies the presence of optically thick cloud systems in the troposphere beneath that will make the successful probing of optically thin clouds near the tropopause impossible in most cases due to the attenuation of lidar signals.…”

Ice injected into the tropopause by deep convection – Part 2: Over the Maritime Continent

“…This section summarizes the method developed by Dion et al (2019) to estimate ∆IWC, the amount of ice injected into the UT and the TL. Dion et al (2019) have presented a model relating Prec (as proxy of deep convection) from TRMM to IW C M LS over tropical convective areas during austral convective season DJF.…”

“…This section summarizes the method developed by Dion et al (2019) to estimate ∆IWC, the amount of ice injected into the UT and the TL. Dion et al (2019) have presented a model relating Prec (as proxy of deep convection) from TRMM to IW C M LS over tropical convective areas during austral convective season DJF. The IWC M LS value measured by MLS during the growing phase of the convection (at x = 01:30 LT or 13:30 LT) is compared to the Prec value at the same time x in order to define the correlation coefficient (C) between Prec and IWC M LS , as follows:…”