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Abstract
Diabatic heating in the Maritime Continent region is controlled by a unique blend of mesoscale variability associated with steep topography and complex coastlines and intraseasonal variability associated with propagating planetary-scale disturbances. In this study, the diabatic heating from a 10-yr austral summer simulation over the Maritime Continent with a 4-km horizontal grid length is analyzed with respect to diurnal, spatial, and intraseasonal variations. Results are compared, where possible, to analogous estimates from the TRMM precipitation radar. We show that the heating budget is largely a balance between latent heating and vertical advection, with rays of heating and cooling extending upward and outward from the coast evident in the advection terms, consistent with the gravity wave representation of the tropical sea breeze. By classifying rainfall into convective and stratiform components, it is shown that simulated convective heating over Sumatra peaks in MJO phases 2 and 3, while simulated stratiform heating peaks in phase 4. Similarly, spectral latent heating estimates from the TRMM Precipitation Radar show that stratiform heating peaks in phases 3 and 4, while convective heating peaks in phases 2 and 3. It is also shown that stratiform precipitation plays a greater role in offshore precipitation during the night, albeit with embedded convective cores, than over the land during the day. These results emphasize the importance of achieving a realistic representation of convective and stratiform processes in high-resolution simulations in the tropics, both for total rainfall estimates and for realistic latent heating.
Abstract
Diabatic heating in the Maritime Continent region is controlled by a unique blend of mesoscale variability associated with steep topography and complex coastlines and intraseasonal variability associated with propagating planetary-scale disturbances. In this study, the diabatic heating from a 10-yr austral summer simulation over the Maritime Continent with a 4-km horizontal grid length is analyzed with respect to diurnal, spatial, and intraseasonal variations. Results are compared, where possible, to analogous estimates from the TRMM precipitation radar. We show that the heating budget is largely a balance between latent heating and vertical advection, with rays of heating and cooling extending upward and outward from the coast evident in the advection terms, consistent with the gravity wave representation of the tropical sea breeze. By classifying rainfall into convective and stratiform components, it is shown that simulated convective heating over Sumatra peaks in MJO phases 2 and 3, while simulated stratiform heating peaks in phase 4. Similarly, spectral latent heating estimates from the TRMM Precipitation Radar show that stratiform heating peaks in phases 3 and 4, while convective heating peaks in phases 2 and 3. It is also shown that stratiform precipitation plays a greater role in offshore precipitation during the night, albeit with embedded convective cores, than over the land during the day. These results emphasize the importance of achieving a realistic representation of convective and stratiform processes in high-resolution simulations in the tropics, both for total rainfall estimates and for realistic latent heating.
Abstract
This study analyzes data obtained by intensive observation during a pilot field campaign of the Years of the Maritime Continent Project (Pre-YMC) to investigate the diurnal cycle of precipitation in the western coastal area of Sumatra Island. The diurnal cycle during the campaign period (November–December 2015) is found to have a number of similarities with statistical behavior of the diurnal cycle as revealed by previous studies, such as afternoon precipitation over land, nighttime offshore migration of the precipitation zone, and dependency on Madden–Julian oscillation (MJO) phase. Composite analyses of radiosonde soundings from the Research Vessel (R/V) Mirai, deployed about 50 km off the coast, demonstrate that the lower free troposphere starts cooling in late afternoon (a couple of hours earlier than the cooling in the boundary layer), making the lower troposphere more unstable just before precipitation starts to increase. As the nighttime offshore precipitation tends to be more vigorous on days when the cooling in the lower free troposphere is larger, it is possible that the destabilization due to the cooling contributes to the offshore migration of the precipitation zone via enhancement of convective activity. Comparison of potential temperature and water vapor mixing ratio tendencies suggests that this cooling is substantially due to vertical advection by an ascent motion, which is possibly a component of shallow gravity waves. These results support the idea that gravity waves emanating from convective systems over land play a significant role in the offshore migration of the precipitation zone.
Abstract
This study analyzes data obtained by intensive observation during a pilot field campaign of the Years of the Maritime Continent Project (Pre-YMC) to investigate the diurnal cycle of precipitation in the western coastal area of Sumatra Island. The diurnal cycle during the campaign period (November–December 2015) is found to have a number of similarities with statistical behavior of the diurnal cycle as revealed by previous studies, such as afternoon precipitation over land, nighttime offshore migration of the precipitation zone, and dependency on Madden–Julian oscillation (MJO) phase. Composite analyses of radiosonde soundings from the Research Vessel (R/V) Mirai, deployed about 50 km off the coast, demonstrate that the lower free troposphere starts cooling in late afternoon (a couple of hours earlier than the cooling in the boundary layer), making the lower troposphere more unstable just before precipitation starts to increase. As the nighttime offshore precipitation tends to be more vigorous on days when the cooling in the lower free troposphere is larger, it is possible that the destabilization due to the cooling contributes to the offshore migration of the precipitation zone via enhancement of convective activity. Comparison of potential temperature and water vapor mixing ratio tendencies suggests that this cooling is substantially due to vertical advection by an ascent motion, which is possibly a component of shallow gravity waves. These results support the idea that gravity waves emanating from convective systems over land play a significant role in the offshore migration of the precipitation zone.
