Abstract
Abstract. The presence of clouds above the tropopause over tropical convection centers has so far been documented by spaceborne instruments that are either sun-synchronous or insensitive to thin cloud layers. Here we document, for the first time through direct observation by spaceborne lidar, how the tropical cloud fraction evolves above the tropopause throughout the day. After confirming previous studies that found such clouds most frequently above convection centers, we show that stratospheric clouds and their vertical extent above the tropopause follow a diurnal rhythm linked to convective activity. The diurnal cycle of the stratospheric clouds displays two maxima: one in the early night (19:00–20:00 LT) and a later one (00:00–01:00 LT). Stratospheric clouds extend up to 0.5–1 km above the tropopause during nighttime, when they are the most frequent. The frequency and the vertical extent of stratospheric clouds is very limited during daytime, and when present they are found very close to the tropopause. Results are similar over the major convection centers (Africa, South America and the Warm Pool), with more clouds above land in DJF (December–January–February) and less above the ocean and in JJA (June–July–August).
Highlights
The presence of ice clouds near the tropical tropopause has long been documented by in situ measurements (e.g., Thomas et al, 2002; Jensen et al, 2013; Frey et al, 2014)
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
Our results show how clouds in the tropical stratosphere are strongly concentrated above deep-convection centers, are almost absent in subtropical regions, and are more frequent in DJF than JJA and over land than over the ocean
Summary
Low-stratospheric clouds impact the atmospheric system in several ways. First, their larger heating rate than in the clear sky (Corti et al, 2006) increases the upward mass flux and fosters the large-scale upward transport of water above the tropopause. 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 This explains why the ground-based lidars do not document the diurnal cycle of the stratospheric clouds with a satisfying spatial and temporal coverage.
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