Diurnal Cycles of Convection Modulated by the Boreal Summer Intraseasonal Oscillation in the South China Sea Region
Abstract
One of the dominant modes of the tropical intraseasonal oscillation in boreal summer over the Indian Ocean and South China Sea (SCS) is the boreal summer intraseasonal oscillation (BSISO). Weather forecasting and climate models have difficulty in simulating the BSISO due to incomplete understanding of the underlying multiscale physical processes, including the cross-scale interactions between BSISO and diurnal cycle. Active MJO (or BSISO) pulses have been shown to suppress the diurnal cycle of precipitation, especially over ocean. However, over land there is still a strong diurnal cycle present. It is still uncertain how the diurnal cycles of convection vary as a function of BSISO phase (or prevailing low-level winds), how they differ between land and ocean, how vertical precipitation structure changes diurnally, and if there are diurnal phase shifts among various convective parameters.
This study has investigated these outstanding problems by using over 20 years of three-dimensional precipitation radar observations from TRMM and GPM, as well as surface winds retrieved from scatterometers (CCMP product). Our results show that precipitation over land shows a strong diurnal cycle (maximizing at afternoon and early evening) across all BSISO phases. However, diurnal cycle of intense convection (indicated by 30dBZ reflectivity extending above the freezing level) and lightning show a different diurnal cycle pattern. Intense convection and lightning over land show a remarkable diurnal cycle during inactive periods, but they are reduced significantly during active BSISO conditions (perhaps owing to cloud shading, weaker surface heating, and reduced instability). Offshore convection peaks at midnight and through early morning during inactive BSISO phases, and the nocturnal offshore propagation is the most obvious over larger landmass (e.g., Indochina and Borneo). Offshore convection near Luzon peaks in the early evening, much earlier than that over Indochina and Borneo. This may be because offshore flows over Luzon develop earlier due to the descending precipitation cold pool from earlier convection over the high terrain.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.A43O3333X
- Keywords:
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- 3337 Global climate models;
- ATMOSPHERIC PROCESSESDE: 3339 Ocean/atmosphere interactions;
- ATMOSPHERIC PROCESSESDE: 3373 Tropical dynamics;
- ATMOSPHERIC PROCESSESDE: 4231 Equatorial oceanography;
- OCEANOGRAPHY: GENERAL