SENSITIVITY OF PRECIPITATION TO SEA SURFACE TEMPERATURE AND ITS DIURNAL VARIATION: A PARTITIONING ANALYSIS BASED ON SURFACE RAINFALL BUDGET

The sensitivity of precipitation to sea surface temperature(SST) and its diurnal variation is investigated through a rainfall partitioning analysis of two-dimensional cloud-resolving model experiments based on surface rainfall budget.For all experiments,the model is set up using zero vertical velocity and a constant zonal wind and is integrated over 40 days to reach quasi-equilibrium states.The 10-day equilibrium grid-scale simulation data and a time-invariant SST of 29°C are used in the control experiment.In the sensitivity experiments,time-invariant SSTs are 27°C and 31°C with an average value of 29°C when the minimum and maximum values of diurnal SST differences are 1°C and 2°C,respectively.The results show that the largest contribution to total rainfall is from the rainfall with water vapor convergence and local atmospheric drying and hydrometeor gain/divergence(~30%) in all experiments.When SST increases from 27°C to 29°C,the contribution from water vapor convergence decreases.The increase of SST reduces the contribution of the rainfall with water vapor convergence primarily through the decreased contribution of the rainfall with local atmospheric drying and hydrometeor gain/divergence and the rainfall with local atmospheric moistening and hydrometeor loss/convergence.The inclusion of diurnal variation of SST with the diurnal difference of 1°C decreases the rainfall contribution from water vapor convergence primarily through the decreased contribution of the rainfall with local atmospheric moistening and hydrometeor loss/convergence.The contribution of the rainfall from water vapor convergence is barely changed as the diurnal difference of SST increases from 1°C to 2°C.     

EVALUATION OF CONVECTIVE-STRATIFORM RAINFALL SEPARATION SCHEMES BY PRECIPITATION AND CLOUD STATISTICS

In this study,two convective-stratiform rainfall partitioning schemes are evaluated using precipitation and cloud statistics for different rainfall types categorized by applying surface rainfall equation on grid-scale data from a two-dimensional cloud-resolving model simulation.One scheme is based on surface rainfall intensity whereas the other is based on cloud content information.The model is largely forced by the large-scale vertical velocity derived from the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment(TOGA COARE).The results reveal that over 40% of convective rainfall is associated with water vapor divergence,which primarily comes from the rainfall type with local atmospheric drying and water hydrometeor loss/convergence,caused by precipitation and evaporation of rain.More than 40% of stratiform rainfall is related to water vapor convergence,which largely comes from the rainfall type with local atmospheric moistening and hydrometeor loss/convergence attributable to water clouds through precipitation and the evaporation of rain and ice clouds through the conversion from ice hydrometeor to water hydrometeor.This implies that the separation methods based on surface rainfall and cloud content may not clearly separate convective and stratiform rainfall.     

Cloud Microphysical Budget Associated with Torrential Rainfall During the Landfall of Severe Tropical Storm Bilis (2006)

Effects of vertical wind shear, radiation, and ice clouds on cloud microphysical budget associated with torrential rainfall during landfall of severe tropical storm Bilis (2006) are investigated by using a series of analysis of two-day grid-scale sensitivity experiment data. When upper-tropospheric upward motions and lower-tropospheric downward motions occur on 15 July 2006, the removal of vertical wind shear and ice clouds increases rainfall contributions from the rainfall type (CM) associated with positive net condensation and hydrometeor loss/convergence, whereas the exclusion of cloud radiative effects and cloud-radiation interaction reduces rainfall contribution from CM. The elimination of vertical wind shear and cloud-radiation interaction increases rainfall contribution from the rainfall type (Cm) associated with positive net condensation and hydrometeor gain/divergence, but the removal of cloud radiative effects and ice clouds decreases rainfall contribution from Cm. The enhancements in rainfall contribution from the rainfall type (cM) associated with negative net condensation and hydrometeor loss/convergence are caused by the exclusion of cloud radiative effects, cloud-radiation interaction and ice clouds, whereas the reduction in rainfall contribution from cM results from the removal of vertical wind shear. When upward motions appear throughout the troposphere on 16 July, the exclusion of all these effects increases rainfall contribution from CM, but generally decreases rainfall contributions from Cm and cM.     

Effects of time-dependent large-scale forcing, solar zenith angle, and sea surface temperature on time-mean rainfall: a partitioning analysis based on surface rainfall budget

Effects of time-dependent large-scale forcing, solar zenith angle, and sea surface temperature on time-mean rainfall during the Tropical Ocean Global Atmosphere Coupled Ocean?CAtmosphere Response Experiment (TOGA COARE) are examined through a partitioning analysis of a series of sensitivity cloud-resolving model experiment data based on surface rainfall budget. The model is forced by time-dependent large-scale forcing (LSF), solar zenith angle (SZA), and sea surface temperature (SST) in the control experiment and is forced only by either time-dependent LSF, SZA, or SST while others are replaced with their time averages in the sensitivity experiments. The rainfall associated with water vapor divergence and local atmospheric drying and hydrometeor loss/convergence has the largest contribution to total rainfall among eight rainfall types. The largest rainfall contribution is increased in the simulations where either time-dependent LSF, SZA, or SST is replaced with its average, whereas it is decreased in the simulation where COARE-derived large-scale vertical velocity is replaced with zero vertical velocity. The contribution of the rainfall associated with water vapor convergence to total rainfall is decreased in the simulations with time-mean LSF, SZA, and SST, whereas it is increased in the simulation without large-scale vertical velocity.     

Water Vapor, Cloud, and Surface Rainfall Budgets Associated with the Landfall of Typhoon Krosa （2007）： A Two-Dimensional Cloud-Resolving Modeling Study

Water vapor, cloud, and surface rainfall budgets associated with the landfall of Typhoon Krosa on 6--8 October 2007 are analyzed based on a two-dimensional cloud-resolving model simulation. The model is integrated with imposed zonally-uniform vertical velocity, zonal wind, horizontal temperature, and vapor advection from NCEP/Global Data Assimilation System (GDAS) data. The simulation data that are validated with observations are examined to study physical causes associated with surface rainfall processes during the landfall. The time- and domain-mean analysis shows that when Krosa approached the eastern coast of China on 6 October, the water vapor convergence over land caused a local atmospheric moistening and a net condensation that further produced surface rainfall and an increase of cloud hydrometeor concentration. Meanwhile, latent heating was balanced by advective cooling and a local atmospheric warming. One day later, the enhancement of net condensation led to an increase of surface rainfall and a local atmospheric drying, while the water vapor convergence weakened as a result of the landfall-induced deprivation of water vapor flux. At the same time, the latent heating is mainly compensated the advective cooling. Further weakening of vapor convergence on 8 October enhanced the local atmospheric drying while the net condensation and associated surface rainfall was maintained. The latent heating is balanced by advective cooling and a local atmospheric cooling.     

EFFECTS OF SEA SURFACE TEMPERATURE AND ITS DIURNAL VARIATION ON DIURNAL VARIATION OF RAINFALL: A PARTITIONING ANALYSIS BASED ON SURFACE RAINFALL BUDGET

The effects of sea surface temperature(SST) and its diurnal variation on diurnal variation of rainfall are examined in this study by analyzing a series of equilibrium cloud-resolving model experiments which are imposed with zero large-scale vertical velocity.The grid rainfall simulation data are categorized into eight rainfall types based on rainfall processes including water vapor convergence/divergence,local atmospheric drying/moistening,and hydrometeor loss/convergence or gain/divergence.The rainfall contributions of the rainfall types with water vapor convergence are insensitive to the increase in SST from 27°C to 29°C during the nighttime,whereas they are decreased during the daytime.The rainfall contributions of the rainfall types with water vapor convergence are decreased as the SST increases from 29°C to 31°C but the decreases are larger during the nighttime than during the daytime.The rainfall contributions of the rainfall types with water vapor convergence are decreased by the inclusion of diurnal variation of SST with diurnal difference of 1°C during the nighttime,but the decreases are significantly slowed down as the diurnal difference of SST increases from 1°C to 2°C.The rainfall contributions of the rainfall types with water vapor convergence are insensitive to the inclusion of diurnal variation of SST during the daytime.     

A Modeling Study of Surface Rainfall Processes Associated with a Torrential Rainfall Event over Hubei, China, during July 2007

The surface rainfall processes associated with the torrential rainfall event over Hubei,China,during July 2007 were investigated using a two-dimensional cloud-resolving model.The model integrated the large-scale vertical velocity and zonal wind data from National Centers for Environmental Prediction(NCEP)/Global Data Assimilation System(GDAS) for 5 days.The time and model domain mean surface rain rate was used to identify the onset,mature,and decay periods of rainfall.During the onset period,the descending motion data imposed in the lower troposphere led to a large contribution of stratiform rainfall to the model domain mean surface rainfall.The local atmospheric drying and transport of rain from convective regions mainly contributes to the stratiform rainfall.During the mature periods,the ascending motion data integrated into the model was so strong that water vapor convergence was the dominant process for both convective and stratiform rainfall.Both convective and stratiform rainfalls made important contributions to the model domain mean surface rainfall.During the decay period,descending motion data input into the model prevailed,making stratiform rainfall dominant.Stratiform rainfall was mainly caused by the water vapor convergence over raining stratiform regions.     

A cloud-resolving modeling study of short-term surface rainfall processes

The short-term tropical surface rainfall processes in rainfall regions (raining stratiform and convective regions) and rainfall-free regions (non-raining stratiform and clear-sky regions) are investigated based on the hourly data from a two-dimensional cloud-resolving model simulation. The model is integrated over a 21-day period with imposed zonally uniform vertical velocity, zonal wind, horizontal temperature and vapor advection, and sea surface temperature from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE). The analysis of the model domain-mean surface rainfall budget reveals that surface rainfall is mainly associated with water vapor convergence and local atmospheric drying. The mean surface rainfall lags the mean water vapor convergence by 3?h. The convective?Cstratiform rainfall separation analysis shows that convective rainfall is associated with water vapor convergence, whereas stratiform rainfall is related to the local atmospheric drying and hydrometeor loss/convergence. The transport of water vapor from rainfall-free regions to rainfall regions creates the main water vapor source for rainfall while it balances local atmospheric drying in rainfall-free regions. Surface evaporation plays a minor role in short-term surface rainfall processes.     

2010年6月中旬浙闽赣地区暴雨特征和冰云热力效应的数值模拟分析

以2010年6月19日发生在浙闽赣地区的一次强降水过程为例,利用中尺度WRF模式进行模拟,用模拟资料对该地区降水收支特征和冰云热力作用进行分析。依据局地水汽/热量变化项、水汽/热量辐合辐散项和云凝物辐合辐散项这3个因子可将降水分为8类,其中局地水汽变干和大气变暖、水汽辐合和热量辐散以及云凝物辐合时,降水强度(雨强)最强,而局地水汽变湿和大气变冷、水汽辐合和热量辐散以及云凝物辐合时,降水覆盖率最大。冰云热力效应包括辐射和潜热两部分。基准试验与敏感性试验对比分析表明冰云辐射减弱降水,而冰云潜热增强降水。热量收支对比分析发现冰云辐射造成辐射冷却的减弱在对流层中低层随高度增加,减弱大气不稳定和降水;而冰云潜热造成潜热增强在对流层中高层随高度减小,增强大气不稳定和降水。     

Torrential rainfall responses to ice microphysical processes during pre-summer heavy rainfall over southern China

In this study,the effects of key ice microphysical processes on the pre-summer heavy rainfall over southern China during 3-8 June 2008 were investigated.A series of two-dimensional sensitivity cloud-resolving model simulations were forced with zonally uniform vertical velocity,zonal wind,horizontal temperature,and water vapor advection data from the National Centers for Environmental Prediction(NCEP)/Global Data Assimilation System(GDAS).The effects of key ice microphysical processes on the responses of rainfall to large-scale forcing were analyzed by comparing two sensitivity experiments with a control experiment.In one sensitivity experiment,ice crystal radius,associated with depositional growth of snow from cloud ice,was reduced from 100 μm in the control experiment to 50 μm,and in the other sensitivity experiment the efficiency of the growth of graupel from the accretion of snow was reduced to 50% from 100% in the control experiment.The results show that the domain-mean rainfall responses to these ice microphysical processes are stronger during the decay phase than during the onset and mature phases.During the decay phase,the increased mean rain rate resulting from the decrease in ice crystal radius is associated with the enhanced mean local atmospheric drying,the increased mean local hydrometeor loss,and the suppressed mean water vapor divergence.The increased mean rain rate caused by the reduction in accretion efficiency is related to the reduced mean water vapor divergence and the enhanced mean local hydrometeor loss.     

Water Cycle and Microphysical Processes Associated with a Mesoscale Convective Vortex System in the Dabie Mountain Area

The water vapor budget and the cloud microphysical processes associated with a heavy rainfall system in the Dabie Mountain area in June 2008 were analyzed using mesoscale reanalysis data(grid resolution 0.03 × 0.03,22 vertical layers,1-h intervals),generated by amalgamating the local analysis and prediction system(LAPS).The contribution of each term in the water vapor budget formula to precipitation was evaluated.The characteristics of water vapor budget and water substances in various phase states were evaluated and their differences in heavy and weak rainfall areas were compared.The precipitation calculated from the total water vapor budget accounted for 77% of actual precipitation;surface evaporation is another important source of water vapor.Water vapor within the domain of interest mainly came from the lower level along the southern boundary and the lower-middle level along the western boundary.This altitude difference for water vapor flux was caused by different weather systems.The decrease of local water vapor in the middle-lower layer in the troposphere during the system development stage also contributed to precipitation.The strength and the layer thickness of water vapor convergence and the content of various water substances in the heavy rainfall areas were obviously larger than in the weak rainfall areas.The peak values of lower-level water vapor convergence,local water vapor income,and the concentration of cloud ice all preceded the heaviest surface rainfall by a few hours.     

0679香山局地大暴雨的中小尺度天气分析

利用Doppler雷达、地面自动站、Profile垂直风廓线及GPS水汽分布等多种新型探测资料, 对2006年7月9日夜间发生在北京西郊香山附近的局地大暴雨天气的影响系统和γ-中尺度强降雨落区形成的动力机理进行了精细分析。对雷达等本地多种探测资料的精细研究表明:地形辐合回波带是造成这次过程的主要影响系统。地形辐合回波带上中气旋回波块的滚动更迭是大暴雨落区形成的直接原因; 近地面辐合对大暴雨落区强降雨的发生具有重要作用。大暴雨落区形成阶段近地面3种辐合同时存在:平原东南风与山区偏北风风向切变辐合、平原东南风在山脉阻挡作用下的抬升辐合、大暴雨落区中心的γ-中尺度气旋性辐合。研究还表明:山前近地面地形辐合扰动, 向上传播, 引发边界层扰动的动力过程是香山大暴雨落区形成的主要动力源, 而来自东南方向近地面层的暖湿平流为大暴雨提供了有效的水汽和能量。     

一次区域暴雨过程综合诊断分析

利用NCEP 1°×1°的6h再分析资料对2008年7月22日河南省南阳市的区域性暴雨天气进行了综合诊断分析,结果表明：此次暴雨过程是中低层的西南涡在高空急流的引导下,沿着河套高压与副高之间的辐合带移出造成的。降雨的水汽供应主要来自对流层中低层,且水汽强辐合出现在强降雨前。随着对流活动的发展,水汽通量和水汽辐合都向高层发展,湿层明显增厚。在整个降雨过程中,700hPa垂直螺旋度正值中心的位置和强度与西南涡的移动和强弱变化有很好的对应关系,垂直螺旋度正值长轴区与切变线辐合区相吻合,在某种程度上能反映出西南涡的移动和强度的演变;垂直螺旋度强弱的变化与暴雨强度变化基本一致。高层辐散、低层辐合的大气垂直结构能增强大气的抽吸作用,促进垂直上升运动的发展,反之,抑制垂直上升运动,降雨减弱。上、下层负、正垂直螺旋度耦合的结构对暴雨的发生和维持非常有利。在雷达速度PPI上,逆风区的出现预示着局地强降雨的产生。     

西行台风背景下云南一次短时强降水过程的成因分析

利用云南省自动气象站雨量资料、卫星和闪电定位仪资料、雷达回波资料、NCEP 1 °×1 °再分析资料和地面、高空常规观测资料对云南一次典型的台风前侧短时强降水过程的成因进行分析,结果表明:在2017年13号台风“天鸽”西行影响云南的天气背景之下,短时强降水出现在台风前侧700 hPa风速辐合区和边界层辐合线附近。台风前侧偏东低空急流向云南境内输送水汽和能量,边界层辐合线触发垂直上升运动,700 hPa风速辐合区利于垂直上升运动的维持和加强,促使水汽的辐合与不稳定能量的释放,引发短时强降水。在中等强度深层垂直风切变的作用下,云南东部中尺度对流系统(MCS)频繁产生并向西传播发展,MCS相互作用组织成飑线系统,在东北气流的引导下,飑线从云南中部移至云南西南部,MCS和飑线是大范围短时强降水的直接影响系统。      

2005年7月4-11日淮河流域强降水过程的水汽收支分析

通过对水汽通量向量的势函数和流函数分解以及水汽方程的诊断计算,分析了2005年7月4-11日淮河流域强降水过程水汽收支情况。结果表明,此次强降水过程主要水汽来源于菲律宾经南海向北转向流入我国中东部地区的较强盛的水汽输送带。这条水汽输送带由两部分组成：起重要作用的是强中心位于太平洋上,沿热带赤道以南地区一致向西到达菲律宾的水汽流,另一部分是沿赤道南侧的东风气流,在非洲东岸索马里附近越赤道向东北方向经孟加拉湾北部到达菲律宾的水汽流;水汽汇合区内,淮河流域附近存在东南与西北水汽汇集辐合,这可能是造成淮河流域持续出现强降水重要原因之一;水汽强辐合区域与强降水发生的区域相对应,降水发生区域的总体水汽收支和平均面降雨量的变化趋势相吻合,随着水汽收入的增加,降水量开始加大;中低层的水汽垂直输送与降水过程的平均面降雨量的变化具有较好的对应关系。     

一次由列车效应引发的暴雨过程分析

利用自动站资料、探空资料、NCEP再分析资料以及雷达资料,对2016年6月26日浙西地区一次暴雨过程的列车效应特征、物理量特征以及中尺度特征进行诊断分析。结果表明:暴雨产生于中低层暖切及地面低压倒槽的背景场中。强降水时段内回波在江西地区生成并沿同一路径东移影响浙西地区,产生列车效应,同时雷达速度场的低层辐合区有利于列车效应中东移回波增强并产生强烈降水。中低层湿度增加、垂直上升速度增大以及暖式切变线的加强有利于水汽辐合抬升的加强,使得列车效应增强,有利于降水的增大。26日11时以后衢州中北部地区列车效应的产生与维持与中低层正涡度和水汽通量辐合在这个区域的持续作用密不可分。Barnes滤波显示中尺度辐合线的维持和移动与列车效应的维持和生消有较好地对应关系,同时地面加密风场中稳定的低压环流有利于列车效应回波源地的维持,实际业务中应加强对地面加密风场和低层中尺度辐合线的分析。     

一次梅雨暴雨过程的数值模拟

运用中尺度暴雨MRM模式,采用常规报文资料作为初始场,对2003年7月8-10日的一次江淮地区暴雨过程进行数值模拟。结果表明：该模式对降水场模拟结果同实况基本相似,模式对暴雨的位置、强度、中心都有较好的模拟,嬲评分较高;西南气流对水汽的输送作用及江淮地区上空水汽通量的高值区,为暴雨的形成与维持提供了重要的水汽条件,水汽辐合区与暴雨落区相对应;中低层辐合、高层辐散的散度垂直分布形势,对暴雨的发生提供了十分有利的动力条件;强降雨出现在低层正涡度中心和负散度中心附近。     

An Analysis of Thermally-Related Surface Rainfall Budgets Associated with Convective and Stratiform Rainfall

Both water vapor and heat processes play key roles in producing surface rainfall.While the water vapor effects of sea surface temperature and cloud radiative and microphysical processes on surface rainfall have been investigated in previous studies,the thermal effects on rainfall are analyzed in this study using a series of two-dimensional equilibrium cloud-resolving model experiments forced by zonally-uniform,constant,large-scale zonal wind and zero large-scale vertical velocity.The analysis of thermally-related surface rainfall budget reveals that the model domain mean surface rain rate is primarily associated with the mean infrared cooling rate.Convective rainfall and transport of hydrometeor concentration from convective regions to raining stratiform regions corresponds to the heat divergence over convective regions,whereas stratiform rainfall corresponds to the transport of hydrometeor concentration from convective regions and heat divergence over raining stratiform regions.The heat divergence over convective regions is mainly balanced by the heat convergence over rainfall-free regions,which is,in turn,offset by the radiative cooling over rainfall-free regions.The sensitivity experiments of rainfall to the effects of sea surface temperature and cloud radiative and microphysical processes show that the sea surface temperature and cloud processes affect convective rainfall through the changes in infrared cooling rate over rainfall-free regions and transport rate of heat from convective regions to rainfall-free regions.     

四川盆地一次强降水过程水汽特征

利用WRF模式、地基GPS资料以及常规气象观测资料,结合模式输出资料的高空间分辨率(10km)和GPS大气可降水量(GPS-PWV)资料的高时间分辨率(30min)的优点,对2008年7月20—22日四川盆地一次暴雨过程的水汽变化特征及各物理量与大气可降水量的关系进行综合分析。结果表明:此次降雨过程是由高原涡和西南涡共同作用引起,WRF模式能够较好地模拟出降雨落区和强度。GPS-PWV反映的大气可降水量增减趋势与WRF模拟的较为一致。水汽密度垂直分布反映了大气可降水量分布,水汽密度随高度增加而递减,降雨初期,水汽密度随高度减小迅速,降雨强盛时期,水汽密度随高度减小的速度减慢。水汽辐合使得水汽密度和大气可降水量增大,风的散度项与水汽通量散度的变化一致,而水汽平流项对水汽辐合贡献较小,水汽的辐合主要由风场辐合造成。     

乌鲁木齐风廓线雷达资料在暴雨天气过程中的应用

利用乌鲁木齐风廓线雷达提供的风场资料、自动站逐时降水量和NCEP/NCAR每6 h再分析资料（1°×1°）,详细分析了2012年5月19日15：00-20日04：00间乌鲁木齐暴雨天气过程。结果表明：高空急流与低空辐合区相配合产生的强垂直上升运动触发了此次暴雨天气;强降雨发生2 h前西北急流迅速下传,引发低层西北急流的加强,低层急流的加强与强降水有较好的对应关系,特别是1 500 m以下的西北急流;低层上升速度2 m/s可作为降水临界值,低层上升速度越大降水越强;强降水阶段整层大气折射率结构常数探测值在-128--120 dB之内,表明整层大气水汽充沛;风廓线雷达产品（垂直速度、折射率结构常数）清楚地反映降水的开始、结束以及降水的强度,可为精细化预报提供参考。