The impact of tropical tropopause cooling on Sahelian extreme deep convection

Previous studies have suggested that the recent increase in tropical extreme deep convection, in particular over Asia and Africa during the boreal summer, has occurred in association with cooling in the tropical lower stratosphere. The present study is focused on the Sahel region of West Africa, where an increased occurrence of extreme precipitation events has been reported over recent decades. The results indicate that the changes over West Africa since the 1980s involve a cooling trend in the tropical lower stratosphere and tropopause layer, combined with warming in the troposphere. This feature is similar to that which might result from increased greenhouse-gas levels but is distinct from the interannual variation of precipitation associated with the transport of water vapor from the Atlantic Ocean. It is suggested that the decrease in the vertical temperature gradient in the tropical tropopause region enhances extreme deep convection over the Sahel, where penetrating convection is frequent, whereas tropospheric warming suppresses the shallower convection over the Guinea Coast. Therefore, the essential feature of the recent changes over West Africa is the depth of convection rather than the total amount of surface precipitation

Maritime-Continental Contrasts in the Properties of Low-Level Clouds: A Case Study of the Summer of the 2003 Yamase, Japan, Cloud Event

Satellite data were used to investigate maritime-continental differences in the characteristics of the low-level cloud (the Yamase cloud) that covered northeast Japan during the summer of 2003. The features of the Yamase cloud were found to be almost the same as those of general stratus clouds but with a smaller effective radius (re) and a greater optical thickness (τ) over land, as compared with general stratus clouds. The values of re over land (average, 11.8 μm) were smaller than those over the ocean (13.5 μm), and the values of τ and the cloud water path over land (20 and 145 gm−2, resp.) showed larger spatial variances than those over the ocean (10 and 86 gm−2, resp.), although the cloud top altitude was nearly the same over both ocean and land (1–3 km). We suggest that this maritime-continental contrast is a result of the combined effects of topography and aerosols characteristics. The Yamase wind blowing from the ocean is forced upwards in coastal regions by the steep mountainous terrain. The updraft drives the inhomogeneity in cloud parameters, and a convective-like cloud develops without precipitation. The relationship between re and τ suggests high aerosol concentrations and unstable conditions over land

Impact of abrupt stratospheric dynamical change on Tropical Tropopause Layer

We have studied the impact of stratospheric circulation change on water vapor in the Tropical Tropopause Layer (TTL) during the stratospheric sudden warming (SSW) events (e.g., Eguchi and Kodera, 2010). Increased Brewer-Dobson circulation associated with SSW produces cooling in the tropical lower stratosphere (LS). The cooling generally produces more cirrus clouds and decreases of the water vapor mixing ratio (WV) in the TTL, except for some regions over Africa and South American continents where penetrating clouds are expected. This time, we found a new stratospheric phenomenon which produces abrupt warming in the tropical stratosphere converse to the SSW event. The dynamical aspect of this phenomenon and the impact on the convective activity will be discussed. Due to the interaction between the subtropical jet and polar night jet in the upper stratosphere, tropical stratosphere warmed about two weeks in early December 2011. Accordingly, temperature in the TTL suddenly increased (approximately 0.5 K at 100 hPa) and the tropical convection ceased. Further, the downward velocity anomaly appeared from stratosphere to lower troposphere through the TTL. The present study mainly focuses on the variation of WV, temperature and cirrus clouds in the upper troposphere (UT) and LS in the period of the stratospheric dynamical change. The data from EOS MLS (Earth Observing System, Microwave Limb Sounder) is used. Before the start of abrupt warming event, the tropical convection temporarily enhanced at the south of the Equator. Then the temperature in the TTL decreased with the Kelvin wave like vertical structure: the WV at 146 hPa increased, while the WV at 100 hPa decreased, and the dryer air extended to 83 hPa with a few days lag. Following the start of the warming event, the tropical convection was suppressed. In the UT and LS, the warm and wet tendencies were found in the temperature and WV anomalies from seasonal march, respectively, and the ice cloud suddenly disappeared with increasing temperature, suggesting adiabatic heating. The result found of the present study clearly shows that the stratospheric dynamical change controls the WV variation in the UT and LS, as well as the tropical convection

Rapid Convective Transport of Tropospheric Air into the Tropical Lower Stratosphere during the 2010 Sudden Stratospheric Warming

International audienceA possible transport mechanism from the tropical troposphere to the lower stratosphere (LS) across the tropical tropopause layer (TTL) is through convective overshooting clouds (COV) that inject air with tropospheric characteristics (high carbon monoxide (CO) and low ozone (O3) concentrations) into the LS over a few days. Evidence of such convective intrusions was observed at the end of January 2010, associated with increased convective activity over the southern African continent following the onset of a sudden stratospheric warming (SSW) in the northern hemisphere, lasting approximately two weeks. The modulation of tropical stratospheric upwelling by SSW appears to have forced stronger and deeper tropical convection, particularly in the Southern Hemisphere tropics. The tropospheric (CO-rich, O3-poor) air injected into the TTL by COV then gradually moved upward via the tropical stratospheric upwelling strengthened by SSW. Meanwhile the O3 decrease started in the middle stratosphere and descended gradually to the TTL, indicating that the effect of stratospheric upwelling reached the TTL. The present results suggest that the direct and indirect (strengthened convective clouds) effects of stratospheric upwelling modulated by SSW can have a large impact on the trace gas fields in the TTL and LS