SOLA Vol. 12A(2016) No. Special_Edition
: The First Asian Conference on Meteorology

A 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.

A microphysical cloud model is employed to study the ice nucleation processes generating the extremely thin (∼10 m) cirrus cloud observed by the Airborne Tropical Tropopause Experiment in the Tropical Tropopause Layer. This cirrus is unique in its high concentration (10⁴ L⁻¹) of small (1.0-3.2 μm diameter) ice particles, suggesting that dynamical processes such as sedimentation and mixing have not yet occurred. This is advantageous for the investigation of cloud physical processes in numerical simulations. The maximum ice concentration (N_ice^max) is calculated from a series of parameter sweep experiments by changing the cooling rate, aerosol particle size, initial water vapor mixing ratio, and accommodation coefficient, assuming homogeneous ice nucleation in the presence of monodisperse aerosol particles in an air parcel subject to isentropic uplift. The cooling rate necessary for the formation of this cirrus is investigated using the cross sections of N_ice^max in parameter space. The required cooling rate, estimated to be a few Kelvin per hour under the observed atmospheric conditions, may exceed typical values associated with synoptic disturbances, but falls within typical values for mesoscale motion, although some uncertainty remains, especially in the value of the accommodation coefficient.

Seasonal and diurnal rainfall characteristics, including the rainfall amount (RA), rainfall frequency (RF), and rainfall intensity (RI), were investigated over the tropical Asian monsoon region using Tropical Rainfall Measuring Mission Precipitation Radar (TRMM-PR) data. The results showed that the number of hours of high RF varied seasonally over land, although the diurnal peaks were mostly unchanged. Over Indochina, precipitation ended in the evening in April and May when the RA was low, whereas precipitation lasted until early morning from July to September when the RA was high. The seasonal changes in RF likely contributed to the changes in RA over land. Additionally, RI had two seasonal peaks occurring at the beginning and end of the summer monsoon over land regions; thus, RI was strongest during the two transition seasons. In contrast, both RF and RI had a single seasonal peak over the ocean.