Estimation of Convective Downdraft Mass Flux and its Contribution to Boundary-layer Moist Static Energy Buget over the Indo-Pacific Warm Pool Using Shipborne Observational Data

As vigorous deep cumulus convection over the Indo-Pacific warm pool domain plays a crucial role in the development of various large-scale disturbances such as tropical cyclones, equatorial waves, and intraseasonal oscillations, what determine intensities of cumulus convection on spatiotemporal scales of these disturbances is one of the fundamental scientific questions in the field of tropical meteorology. To address this question, this study focuses on interaction between cumulus convection and atmospheric boundary layer (BL) and on budget processes of BL moist static energy (MSE), which is one of the essential thermodynamic parameters determining intensity of cumulus convection. In the BL quasi-equilibrium framework, increase of BL MSE due to surface turbulent heat flux is balanced with its decrease by several processes. Although convective downdraft (CD) caused by precipitation is one of the major processes, there is only a few studies that quantitatively examine the contribution of CD. Therefore, this study attempts to estimate CD mass flux across the BL top and its contribution to the BL MSE budget through analysis of in situ shipborne observation data. In particular, we analyze data obtained by research vessel Mirai during her station observation periods (nearly 250 days in total) in eight research cruises over the Indo-Pacific warm pool domain. Central Indian Ocean, off the west coast of Sumatra Island, Indonesia, and tropical Western North Pacific are particular regions of the station observation activities. Some of the cruises were conducted as a part of international field campaign projects such as Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year 2011 / Dynamics of the Madden-Julian Oscillation (CINDY2011/DYNAMO) and Years of the Maritime Continent (YMC). During the station observation periods, 3-hourly radiosonde sounding and continuous surface meteorology observation were performed. Using data of these observations, we estimate CD mass flux across the BL top on synoptic-to-intraseasonal timescales through consideration of BL dry static energy and moisture budgets with neglecting local tendency and horizontal advection. The estimated CD mass flux ranges from 0.5 to 5 g s^-1 m^-2, and is generally correlated with measures of convective activity around the vessel observed by shipborne C-band weather radar and rain gauge. In particular, the estimated CD mass flux is 10-30 times larger than precipitation mass flux. The CD mass flux is also correlated well with surface sensible heat flux, presumably reflecting enhancement of surface flux by convective cold outflow. The CD process decreases BL MSE by about 40 W m^-2 on average, which is nearly 40% of the BL MSE increase due to the surface turbulent heat flux. Furthermore, we discuss how the estimated CD mass flux is affected by taking account of the horizontal advection, through estimation of the BL horizontal advection using in situ observations by wave gliders and a moored buoy deployed at about 50 km from the vessel in one of the research cruises.