IMPROVED FORECASTING OF COLD AIR OUTBREAKS OVER SOUTHERN CHINA THROUGH OROGRAPHIC GRAVITY WAVE DRAG PARAMETERIZATION

The parameterization of gravity wave drag induced by sub-grid scale orography(GWDO), which has been used in the regional model based on the Global/Regional Assimilation and Prediction System for Tropical Mesoscale Model(GRAPES_TMM), is extended to include the effect of mountain flow blocking drag(MBD). The extended scheme is evaluated against non-GWDO parameterization, including a cold air outbreak over southern China and a monthly verification in February 2012. The experiment results show that the GWDO and MBD parameterization both improves the forecasting of the cold air outbreaks over southern China, as well as alleviations of system bias of GRAPES_TMM.(1) The extended scheme alleviates the strong southerly wind and high surface temperature simulation during the cold air outbreak, especially over northern Guangxi and Guangdong(NGG) province, where local high surface temperature simulation reduces nearly 5 degree.(2) The MBD parameterization improves southerly wind simulations over NGG, as well as surface temperature forecasts improvement over Guangxi, Guizhou province and southern Yunnan-Guizhou plateau(YUP), and low level southerly wind simulation improvement over intertidal zone over south China.(3) The formation of MBD is mainly in the mountain area(Wuyi, Daba mountain, east of YUP) and coastal area. The MBD over plateau, which is mainly formed at the west of 105°E, is stronger and thicker than that over Nanling mountain.(4) The improvement of GWDO and MBD parameterization is stable in model physics. MBD parameterization demonstrates more overall improvements in the forecasts than GWDO, and the larger of the model forecast error is, the greater improvements of MBD contribute to. Overall, the extended GWDO scheme successfully improves the simulations of meteorological elements forecasting during cold air outbreaks.

地形重力波拖曳参数化方案在华南中尺度模式(GRAPES)中的应用试验

在华南中尺度模式GRAPES中发展KA95方案地形重力波拖曳参数化方案(GWDO),并对2012年的2月8—12日影响华南冷空气过程、8月23—24日发生在广东的局地性暴雨和5—8月共4个月的KA95方案预报试验进行检验。结果表明,KA95地形重力波拖曳参数化方案的引入,改善了GRAPES模式对冬、春季低层南风偏强和地面温度偏高的现象,一定程度上纠正了在模式积分过程中出现的南风偏差,改正了在业务预报中因南风偏强而导致温度偏差过大的现象;改善了模式对局地强降水强度和落区的预报,减小了大雨和暴雨的漏报率和预报偏差,提高了GRAPES模式对降水的预报效果。

地形重力波拖曳参数化对热带气旋强度和路径预报影响的研究

本文在GRAPES_TMM(Global/Regional Assimilation and Prediction System for Tropical Mesoscale Model)——中国南海台风模式版(面向南海和东南亚)中发展和引进了KA95(Kim and Arakawa,1995)地形重力波拖曳参数化方案(GWDO),并对2012年主要的9个登陆台风进行了试验对比研究,考察了不同标准Richardson数(c Ri)的GWDO试验对台风路径和强度预报的影响。结果表明,在引入地形重力波拖曳参数化过程后,模式对台风登陆时路径和强度的预报能力均要有提高,对台风预报时长越长,GWDO的影响也更为显著。对双台风"SAOLA"和"DAMREY"试验结果表明,GWDO对台风外围距台风中心150 km的对流层中下层风速减弱较为明显,减弱了GRAPES区域模式对台风强度预报偏强的现象,对台风强度长时间预报改善更为明显。不同标准c Ri对重力波拖曳力的计算较为敏感,当c Ri取1.0时,动能迅速的在低层被频散,能量无法有效地上传;c Ri取0.25时,大部分的能量在中高层被频散。总的来说,c Ri取0.75时对台风路径和强度预报改进更为显著,其结果可为业务预报提供指导意义。

一次大暴雨过程中地形重力波拖曳作用的研究

利用WRFV3.9数值模式,对2011年6月9—10日发生在湘鄂赣交界附近山区的一次江淮气旋大暴雨天气过程进行模拟。进行了地形重力波拖曳作用的控制试验和不考虑地形重力波拖曳作用的敏感性试验。结果表明,控制试验能较好地再现江淮气旋及其产生的强降水;模式引入地形重力波拖曳后能够更好地模拟出地形附近雨带的中心位置和强度;地形重力波拖曳在本次大暴雨过程中能够减弱地形附近的经向风速,增强低层气流辐合,促进垂直上升运动,从而有利于对流的触发和维持。

GRAPES中地形重力波拖曳物理过程的引进和应用试验

在中国新一代全球中期/区域中尺度同化与预报系统(GRAPES)模式中引进了ECMWF地形重力波拖曳物理过程,填补了GRAPES全球中期数值预报系统中物理过程的空白。重新计算了地形重力波过程需要的地形静态资料数据,并与原ECMWF模式的地形静态参数进行了对比分析,验证了模式地形参数的正确性。利用GRAPES模式,进行了地形重力波拖曳物理过程影响的敏感性数值试验;结果表明:引进地形重力波拖曳过程以后,在存在大地形的区域,风场会发生变化,当纬向风遇到青藏高原时,一部分气流会产生爬坡效应而越过高原,使高原上空的西风气流减弱;另一部分气流会绕过高原,在高原的南侧产生绕流;随着模式积分时间的延长,风场变化会越来越明显,地形越复杂,风场的变化也越复杂;连续的模式积分试验结果显示,引进地形重力波过程,可以延长GRAPES模式的可用预报时效,提高了全球形势预报的准确率。通过对一次降水过程的模拟,对地形重力波过程影响降水预报的原因进行了简单分析。结果显示:引进地形重力波拖曳过程后,改变了大气流场的分布,使预报的流场更接近于大气真实状态,从而提高了降水预报的准确率。

Improved Wind and Precipitation Forecasts over South China Using a Modified Orographic Drag Parameterization Scheme

To improve the wind and precipitation forecasts over South China, a modified orographic drag parameterization（OP） scheme that considers both the gravity wave drag（GWD） and the mountain blocking drag（MBD） effects was implemented in the Global/Regional Assimilation and Prediction System Tropical Mesoscale Model（GRAPES-TMM）. Simulations were performed over one month starting from 1200 UTC19 June 2013. The initial and lateral boundary conditions were obtained from the NCEP global forecast system output. The simulation results were compared among a control（CTL） experiment without the OP scheme, a GWDO experiment with the OP scheme that considers only the GWD effect, and an MBD experiment with the modified OP scheme（including both GWD and MBD）. The simulation with the modified OP scheme successfully captured the main features of precipitation, including its distribution and intensity,and improved the wind circulation forecast in the lower troposphere. The modified OP scheme appears to improve the wind forecast by accelerating the ascending air motion and reinforcing the convergence in the rainfall area. Overall, the modified OP scheme exerts positive impacts on the forecast of large-scale atmospheric fields in South China.