On June 5th 2018, DESS Ph.D. Candidate Yang Mengmiao published a paper as the first author called “Precipitation and Moisture in Four Leading CMIP5 Models: Biases across Large-Scale Circulation Regimes and Their Attribution to Dynamic and Thermodynamic Factors” on Journal of Climate, which is a prestigious journal on atmospheric science. Professor Zhang Guangjun is the corresponding author, and Professor Sun Dezheng from the department of Atmospheric and Oceanic Science in University of Colorado Boulder is the co-author. This paper reveals precipitation and moisture biases in CMIP5 models and their causes.

This paper analyzed the simulation of precipitation and moisture in four leading CMIP5 models. It is found that precipitation in these models is overestimated in most areas. However, moisture bias has large intermodal differences. The model biases in precipitation and moisture are further examined in conjunction with large-scale circulation by regime-sorting analysis. Results show that in a given regime of vertical velocity at 500 hPa, models produce too much precipitation compared to observation and reanalysis data. For moisture, their biases differ from model to model and also from level to level, but have a common bias: too dry in upward motion regimes and too moist in downward motion regimes. Furthermore, error causes are revealed through decomposing contribution biases into dynamic and thermodynamic components. For precipitation, the contribution errors in strong upward motion regimes are attributed to the overly frequent w500. In the weak upward motion regime, the biases in the dependence of precipitation on w500 and the w500 probability density function (PDF) make comparable contribution. On the other hand, the biases in the column integrate water vapor contribution are mainly due to errors in the frequency of occurrence of w500, while thermodynamic components contribute little. These findings suggest that errors in the frequency of w500 occurrence are a significant cause of biases in the precipitation and moisture simulation.

Figure 1. the joint distribution of precipitation as a function of w500 and IWV for (a) ERA-Interim-TRMM combined data, (b) CCSM4, (c)GFDL-CM3, (d) HadGEM2-ES, and (e) MPI-ESM-MR in the (left to right) Pacific ITCZ, SPCZ, Indian Ocean and Atlantic ITCZ regions.

PhD Candidate Yang Mengmiao is in her fourth year of PhD in the department of Earth System Science. Her research filed is climate model diagnosis, mainly focus on convection related physical processes and convective parameterization scheme in models.