Evaluating the Bias of South China Sea Summer Monsoon Precipitation Associated with Fast Physical Processes Using a Climate Model Hindcast Approach

The present study aims to identify the precipitation bias associated with the interactions among fast physical processes in the Community Atmospheric Model v.5 (CAM5) during the abrupt onset of the South China Sea (SCS) summer monsoon, a key precursor of the overall East Asia summer monsoon (EASM). The multi-year hindcast approach is utilized to obtain the well-constrained synoptic-scale horizontal circulation each year during the onset period from the year 1998 to 2012. In the pre-onset period, the ocean precipitation over the SCS is insufficiently suppressed in CAM5 hindcasts, thus weaker land-ocean precipitation contrasts. This is associated with the weaker and shallower convection simulated over the surrounding land, producing weaker local circulation within the SCS basin. In the post-onset period, rainfall of the organized convection over the Philippine coastal ocean is underestimated in the hindcasts, with overestimated upper-level heating. These biases are further elaborated as the underrepresentation of convection diurnal cycle and coastal convection systems, as well as the issue of precipitation sensitivity to environmental moisture during the SCS onset period. The biases identified in hindcasts are consistent with the general bias of the EASM in the climate simulation of CAM5. The current results highlight the appropriate representation of land-ocean-convection interactions over coastal areas can potentially improve the simulation of seasonal transition over the monsoon regions.

Reference: Chen, W.-T., C.-M. Wu, and H.-Y. Ma (2019), Evaluating the bias of South China Sea summer monsoon precipitation associated with fast physical processes using climate model hindcast approach, J. Climate, https://doi.org/10.1175/JCLI-D-18-0660.1.

(Figure Caption: Pre-onset meridional distribution of Q1-QR diabatic heating profiles (shading: K day^-1) and anomalous zonal-vertical circulation over SCS (vectors) (a) from ERA-Int and (b) CAPT-day3. The model Q1-QR is the sum of the temperature tendency of all physical processes excluding radiation. The vectors are u wind anomaly and -100*omega anomaly, in the unit of m s^-1*Pa s^-1. Only conditionally sampling the days when open ocean areas exhibit low-level subsidence or weak updraft. Meridionally averaged over 12.5^o-18.5^oN, and the anomaly fields are relative to the areal average over 106^o-123^oE, 12.5^o-18.5^oN.)