第七届青年地学论坛

Tropical-depression (TD)-type waves are synoptic-scale disturbances embedded with deep convection over the western North Pacific. These waves contribute to considerable rainfall variability and tropical cyclone formations in this region. Studies on these disturbances began over six decades ago, however, some properties of these disturbances remain vague, e.g., the coupling mechanism between the deep convection and the waves.

This study aims to examine the rainfall progression in TD-type disturbances and associated tropospheric moisture controlling convective rainfall. First, the present study investigates the rainfall and moisture features of TD-type waves using TRMM-derived rainfall products and the ERA-Interim reanalysis data during the period June-October 1998-2013. The rainfall features a north-south asymmetrical pattern with respect to a TD-type disturbance, that enhanced convective and stratiform rainfall occur in the southern portion. Along with the northwestward propagation, deep convective and stratiform rainfall occurs in-phase with the TD-type disturbance without significant preceding shallow convective rainfall. Following the deepest convection, shallow convective rainfall increases in the anomalous southerlies. Such a rainfall progression differs from the paradigm from shallow, to deep convection, then to stratiform rainfall, which is suggested in other convectively coupled equatorial waves. The rainfall progression and the atmospheric moisture anomaly are phase-locked to the TD-type disturbances that the relative displacements change little when the disturbances propagate northwestward. The latent heat release in deep convection, which is obtained from TRMM-3G25 dataset, superposes with a broad warm anomaly in the mid-upper troposphere, suggesting wave growth through the generation of available potential energy from diabatic heating.

Then, the large-scale controls on the convective rainfall progression have been investigated through budgets of moist static energy (MSE) and moisture. A buildup of column-integrated MSE occurs in advance of deep convection, and a export of MSE occurs following deep convection, consistent with the MSE recharge-discharge paradigm. The MSE recharge-discharge is controlled by horizontal processes, whereby horizontal moisture advection causes net MSE import prior to deep convection. Such moistening by horizontal advection creates a moist mid-troposphere, which helps destabilize the atmospheric column, leading to the development of deep convective rainfall. Following the heaviest rainfall, negative horizontal moisture advection dries the troposphere, inhibiting convection. Such moistening and drying processes explain why deep convection can develop without preceding shallow convection. The advection of moisture anomalies by the mean horizontal flow controls the tropospheric moistening and drying processes. As the TD-type waves propagate northwestward in coincidence with the northwestward environmental flow, the moisture, or convective rainfall, is phase-locked to the waves. The critical role of the MSE import by horizontal advection in modulating the rainfall progression is supported by the anomalous gross moist stability (AGMS), where the lowest AGMS corresponds to the quickest increase in the precipitation rate prior to the rainfall maximum.

References:
Feng, T.*, X.-Q. Yang, J.-Y. Yu, and R. Huang, 2020: Convective coupling in tropical-depression-type waves Part I: Precipitation characteristics and moisture structure.  Journal of the Atmospheric Sciences, in minor revision.
Feng, T*., J.-Y. Yu, X.-Q. Yang, and R. Huang, 2020: Convective coupling in tropical-depression-type waves Part II: Moisture and moist static energy budgets. Journal of the Atmospheric Sciences, in minor revision.

 

tropical wave,convective coupling,moisture