Analysis Of Triggering Conditions Of Convection On Summer Precipitation Diurnal Cycle Simulation By BCC_CSM

Up to now, there are still many questions in simulating the precipitation diurnal variation by climate models, the cumulus convection is very important to the simulation of precipitation diurnal variation, so improve th e cumulus parameterization scheme has become the hot and difficult problem of simulating the precipitation diurnal variation. The research has showed that the climate system model of National Climate Center, BCC_CSM,simulated too much night precipitation in the East Asia mainland, the cumulus parameterization scheme of this model was proposed by Wu(2012). In this paper, we start with the triggering of convection, analysis the reason of simulating too much night precipitation, and discuss the effect of triggering conditions which related to deep convection such as: convective available potential energy(CAPE), the instability of boundary layer and large scale lifting movement, to the simulations of summer precipitation diurnal variation, according to this, we propose a new triggering condition of convection; and evaluate the simulations of summer precipitation diurnal variation by BCC_CSM three different resolutions models(T266L26, T106L46 and T42L26). The main conclusions are as follows: 1.By comparing the convective precipitation intensity from TRMM, CAPE and surface buoyancy flux which represents the instability of boundary layer from BCC_AGCM2.2, we find that the peak value of surface buoyancy flux occurs in noon, convective precipitation intensity occurs in afternoon, and CAPE occurs before midnight, the result shows that peak value of precipitation occurs in afternoon contacts with the instability of boundary layer, besides the CAPE, the occur of convection also rely on the boundary layer process. While, CAPE is the only criteria of the triggering condition in original scheme, which is probably the main reason of simulating too much night precipitation. 2. By using the BCC_AGCM2.2(T106L26), 4 different triggering condition experiments are developed, the results show that, on the basis of CAPE>0, if we consider the instability of boundary layer, it can greatly improve the simulation of precipitation peak time in mainland, from midnight to noon, but it also inhibit some convections in the night, so that the simulation of night precipitation areas are smaller than the observations. On consider that the large scale lifting movement is benefit for occur of convection, we continue considering the large scale lifting movement, it can further improve the simulation of precipitation peak time in East Asia mainland, especially for the night precipitation are such as Sichuan Basin, North China plain and the south area of Tibetan Plateau. According to the analysis, we propose a new triggering condition: CAPE>0, and bound ary layer is unstable or there is large scale lifting movement exist in the bottom of the convective air parcel. 3. Evaluating the simulations of summer precipitation diurnal variation by BCC_CSM three different resolutions models(T266L26, T106L46 and T42L26). The results show that, the results of high resolution model are better than the low resolution model. For the climate state, compare to the TRMM and merged precipitation data, T266 and T106 can not only simulate the percent of convective and stratiform precipitation, which nearly occupy 50% of the total precipitation, but also the position and intensity of the main precipitation centers. For the frequency of precipitation diurnal variation, T266 can simulate the afternoon peak time, the night precipitation area(Sichuan Basin, North China plain and the south area of Tibetan Plateau) is also more accurate, in the Tibetan Plateau, Northeast China and East China, the phase of frequency of precipitation diurnal variation is more close to the observations. For the diurnal amplitude and the duration, T266 and T106 can simulate main centers of diurnal amplitude such as Tibetan Plateau and Southeast China, while the diurnal amplitude result of T42 is not obvious, duration is also too long. But for the the intensity of precipitation diurnal variation, T266 simulate the peak time is midnight, T106 and T42 simulate too much night precipitation in the East China, the result of T266 is not significantly better than the T106 and T42. Meanwhile, the precipitation peak time by T266 is still 2-4 hours earlier than the observations, so we need research deeply in the future.