青藏高原與極地氣象研究進(jìn)展

Progress in the Tibetan Plateau and Polar Meteorology Research

1 高原氣象

1 Tibetan Plateau meteorology

1.1 Variability of summertime Tibetan tropospheric temperature and associated precipitation anomalies over the central-eastern Sahel

The relationship between the Tibetan tropospheric thermal condition and West African monsoon precipitation during boreal summer,and the associated physical mechanisms,are investigated.A significant positive correlation exists between the summertime Tibetan tropospheric temperature (TTT) and centraleastern Sahel precipitation.When the TTT increases,higher than-normal precipitation occurs over the centraleastern Sahel,and vice versa.The increased tropospheric temperature over the western Tibetan Plateau (TP)-Mediterranean Sea region associated with a high TTT forms a tropospheric temperature gradient from the middle latitudes to the subtropics,which is accompanied by an anomalous zonal-vertical circulation from the western TP to the Mediterranean Sea.The tropospheric temperature distribution associated with a high TTT over the western TP-Mediterranean Sea region contributes to a tropospheric temperature gradient from the eastern Mediterranean and West Asia to the western Mediterranean,which is accompanied by an anomalous meridional-vertical circulation cell over the Mediterranean Sea-Africa region.The meridional-vertical circulation cell is accompanied by a deepened African continental low,and enhanced low-level westerly and southwesterly winds from the tropical and eastern Atlantic to inland Africa.These conditions favor an increase in precipitation over the central-eastern Sahel.Thus,the relationship between summer TTT and precipitation over the central-eastern Sahel is explained by the anomalous zonal-vertical circulation between the western TP and the Mediterranean Sea,and the anomalous meridional-vertical circulation cell over the Mediterranean Sea-Africa region.The zonal and meridional circulations are connected by the vertical motion over the Mediterranean Sea.Sensitivity experiments with an atmospheric model,in which the surface vegetation type is adjusted,demonstrate the impacts of an increase in the summertime TTT on the anomalous zonal-vertical circulation between the western TP and the Mediterranean Sea,the anomalous meridional-vertical circulation cell over the Mediterranean Sea-Africa region,the deepened African continental low,and the enhanced lowlevel westerly winds from the tropical and eastern Atlantic to inland Africa.Thus,the relationship between summertime TTT and central-eastern Sahelian precipitation reflects an impact of the TTT on central-eastern Sahelian precipitation.(Nan Sulan)

1.2 Observational evidence of particle hygroscopic growth in the upper troposphere-lower stratosphere (UTLS) over the Tibetan Plateau

We measured the vertical profiles of backscatter ratio (BSR) using the balloon-borne,lightweight Compact Optical Backscatter AerosoL Detector (COBALD) instruments above Linzhi,located in the southeastern Tibetan Plateau,in the summer of 2014.An enhanced aerosol layer in the upper troposphere–lower stratosphere (UTLS),with BSR (455 nm)＞1.1 and BSR (940 nm)＞1.4,was observed.The color index (CI) of the enhanced aerosol layer,defined as the ratio of aerosol backscatter ratios (ABSRs) at wavelengths of 940 and 455 nm,varied from 4 to 8,indicating the prevalence of fine particles with a mode radius of less than 0.1 μm.We find that unlike the very small particles (mode radius smaller than 0.04 μm) at low relative humidity (RHi＜40%),the relatively large particles in the aerosol layer were generally very hydrophilic as their size increased dramatically with relative humidity.This result indicates that water vapor can play a very important role in increasing the size of fine particles in the UTLS over the Tibetan Plateau.Our observations provide observation-based evidence supporting the idea that aerosol particle hygroscopic growth is an important factor influencing the radiative properties of the Asian Tropopause Aerosol Layer (ATAL) during the Asian summer monsoon.(Ma Jianzhong)

1.3 Modeling the aerosol chemical composition of the tropopause over the Tibetan Plateau during the Asian summer monsoon

Enhanced aerosol abundance in the upper troposphere and lower stratosphere (UTLS) associated with the Asian summer monsoon (ASM) is referred to as the Asian Tropopause Aerosol Layer (ATAL).The chemical composition,microphysical properties,and climate effects of aerosols in the ATAL have been the subject of discussion over the past decade.In this work,we use the ECHAM/MESSy Atmospheric Chemistry (EMAC) general circulation model at a relatively fine grid resolution (about 1.1°×1.1°) to numerically simulate the emissions,chemistry,and transport of aerosols and their precursors in the UTLS within the ASM anticyclone during the years 2010–2012.We find a pronounced maximum of aerosol extinction in the UTLS over the Tibetan Plateau,which to a large extent is caused by mineral dust emitted from the northern Tibetan Plateau and slope areas,lofted to an altitude of at least 10 km,and accumulating within the anticyclonic circulation.We also find that the emissions and convection of ammonia in the central main body of the Tibetan Plateau make a great contribution to the enhancement of gas-phase NH3in the UTLS over the Tibetan Plateau and ASM anticyclone region.Our simulations show that mineral dust,water-soluble compounds,such as nitrate and sulfate,and associated liquid water dominate aerosol extinction in the UTLS within the ASM anticyclone.Due to shielding of high background sulfate concentrations outside the anticyclone from volcanoes,a relative minimum of aerosol extinction within the anticyclone in the lower stratosphere is simulated,being most pronounced in 2011,when the Nabro eruption occurred.In contrast to mineral dust and nitrate concentrations,sulfate increases with increasing altitude due to the larger volcano effects in the lower stratosphere compared to the upper troposphere.Our study indicates that the UTLS over the Tibetan Plateau can act as a well-defined conduit for natural and anthropogenic gases and aerosols into the stratosphere.(Ma Jianzhong)

1.4 Estimation of surface heat fluxes over the central Tibetan Plateau using the maximum entropy production model

Surface heat fluxes over the central Tibetan Plateau have been estimated using the maximum entropy production (MEP) model with the surface energy balance and the observation data from the Third Tibetan Plateau Atmospheric Scientific Experiment (TIPEX-III).The MEP surface heat fluxes are highly correlated with those of the TIPEX-III observations.The agreement between the MEP and TIPEX-III heat fluxes is higher when the observed surface energy balance closure is better.The errors of the MEP heat fluxes are smaller compared to those of the fluxes derived from the bulk transfer method,the Land Data Assimilation Systems,and the Simple Biosphere Model version 2 reported in the previous studies.The values of MEP sensible and latent heat fluxes tend to be less than those of the previous bulk heat fluxes.Thus,the MEP model can reasonably estimate surface heat fluxes over the central Tibetan Plateau.(Zhao Ping)

2 極地氣象

2 Polar meteorology

2.1 Distribution and temporal trends of temperature extremes over Antarctica

The spatiotemporal characteristics of temperature extremes over Antarctica remain largely unknown.Here,we use quality-controlled daily datasets from Antarctic weather stations to show that the annual maximum and minimum temperatures exhibit a decreasing pattern over Antarctica from the coast to inland regions.This feature holds for the warmest daily maximum and coldest daily minimum temperatures,which define the intensity of extremes,but not for the number of warm (cold) days measuring the frequency of extremes,which show limited dependence on latitude or elevation.During 1970–2000,the temperature extremes in the South Orkney islands and on the margins of East Antarctica show opposite trends,especially with a significant increasing and decreasing trend in warm events,respectively.During 1999–2013,the intensity and frequency of extreme temperatures decrease significantly over West Antarctica,but the trends vary greatly across subregions of Antarctica.Despite the limited number of stations and the potential time dependence of trends,these results not only help to decipher the climate regimes of Antarctica and fill current gaps in the map of global climate extremes,but also may guide the future design of Antarctic observational networks and be used to assess the capability of reanalysis datasets and climate models.(Wei Ting)

2.2 Surface energy balance on the Antarctic plateau as measured with an automatic weather station during 2014

AWS data during 2014 collected at PANDA-N station,on the East Antarctica Plateau,are analysed.Net Short Wave Radiation (QSWR),net Long Wave Radiation (QLWR),sensible (QH),latent (QL) and subsurface or ground (QG) heat fluxes are computed.Annual averages forQSWR,QLWR,QH,QLandQGof 19.65,?49.16,26.40,?0.77 and 3.86 W m?2were derived based on an albedo value of 0.8.QSWRandQHare the major sources of heat gain to the surface andQLWRis the major component of heat loss from the surface.An iterative method is used to estimate surface temperature in this paper; surface temperature of snow/ice is gradually increased or decreased,thereby changing longwave radiation,sensible,latent and subsurface heat fluxes,so that the net energy balance becomes zero.Mass loss due to sublimation at PANDA-N station for 2014 is estimated to be 12.18 mm w.e.a?1; and mass gain due to water vapour deposition is estimated to be 3.58 mm w.e.a?1.Thus the net mass loss due to sublimation/deposition is 8.6 mm w.e.a?1.This study computes surface energy fluxes using a model,instead of direct measurements.Also there are missing data especially for wind speed,though 2 m air temperature data is almost continuously available throughout the year.The uncertainties of albedo,wind speed and turbulent fluxes cause the most probable error in monthly values ofQLWR,QH,QL,QGand surface temperature of about ±4%,±20%,±50%,±11% and ±0.74 K respectively.(Ding Minghu)

2.3 2017年夏季北冰洋浮冰區(qū)海霧特征分析

首先利用中國(guó)第八次北極考察隊(duì)期間獲取的走航觀測(cè)數(shù)據(jù)分析了環(huán)北極考察的海霧特征。接下來利用在北冰洋密集浮冰區(qū)海霧加密觀測(cè)試驗(yàn)期間獲取的GPS探空觀測(cè)數(shù)據(jù)NCEP再分析資料，研究了北冰洋浮冰區(qū)海霧生成的氣象要素特征、邊界層特征和大氣環(huán)流形勢(shì)特征，發(fā)現(xiàn)浮冰區(qū)冰雪面之上的冷空氣穹丘使得大氣易于達(dá)到飽和，為北冰洋不同種類海霧的出現(xiàn)制造了有利條件。平流霧、輻射霧和蒸汽霧生消的機(jī)理和影響期間邊界層氣象特征各不相同，并且根據(jù)特定環(huán)流形勢(shì)對(duì)3種海霧進(jìn)行預(yù)報(bào)是可行的。（陳志昆，丁明虎）

2.4 極地業(yè)務(wù)服務(wù)

圓滿完成了南極和北極考察任務(wù)，進(jìn)行了黃河站梯度觀測(cè)系統(tǒng)的維護(hù)工作，在中山站增設(shè)LGRCO/N2O監(jiān)測(cè)儀、太陽光度計(jì)，實(shí)現(xiàn)了中山站數(shù)據(jù)的自動(dòng)化采集與部分?jǐn)?shù)據(jù)的實(shí)時(shí)顯示。

3 模型和預(yù)測(cè)

3 Model and prediction

3.1 A statistical forecast model for the Chinese winter temperature based on autumn SST anomalies

This study investigates the impacts of the autumn sea surface temperature anomalies (SSTAs) on the following winter (DJF) 2-m air temperature anomalies (TSAs) in China and discusses the potential predictability of the DJF TSAs based on the intimate link between the DJF TSAs and autumn global SSTAs.The empirical orthogonal function (EOF) analysis suggests that the three leading EOF modes jointly account for 80% of the total TSA variances and are characterized by a homogeneous spatial pattern,a north-south seesaw pattern and a cross structure pattern.These EOFs are temporally stable and suggest the potential predictability of the DJF TSAs.The EOF1 mode is influenced by changes in the intensities of the Siberian high,East Asian winter monsoon (EAWM) and East Asian trough related to a Eurasian teleconnection pattern,which can be tracked back to the autumn (SON) SSTAs.The Arctic Oscillation (AO) exerts a strong influence on the EOF2 mode.The configuration of the global SON SSTAs induces the AO signal and causes a TSA oscillation between the northern and southern parts of China.The EOF3 mode is associated with the western pathway of the EAWM and the westward shift of the Siberian high,which are attributed to two SON SSTA patterns.The multiple correlation coefficients between the SSTA indices and winter atmospheric circulations suggest the cooperative contribution of the autumn global SSTAs to the DJF TSAs.Therefore,a physically motivated statistical model is established based on the autumn SSTA indices.Cross validation suggests that this statistical forecast model shows a good performance in predicting the DJF TSAs wave dynamics and then be amplified because of the involvement of transient eddy feedbacks.(Lyu Junmei)

3.2 A study of objective prediction for summer precipitation patterns over the eastern China based on a multinomial logistic regression model

The prediction of summer precipitation patterns (PPs) over the eastern China is an important and topical issue in China.Predictors that are selected based on historical information may not be suitable for the future due to non-stationary relationships between summer precipitations and corresponding predictors,and might induce the instability of prediction models,especially in cases with few predictors.This study aims to investigate how to learn as much information as possible from various and numerous predictors reflecting different climate conditions.An objective prediction method based on the multinomial logistic regression (MLR) model is proposed to facilitate the study.The predictors are objectively selected from a machine learning perspective.The effectiveness of the objective prediction model is assessed by considering the influence of collinearity and number of predictors.The prediction accuracy is found to be comparable to traditionally estimated predictability,ranging between 0.6 and 0.7.The objective prediction model is capable of learning the intrinsic structure of the predictors,and is significantly superior to the prediction model with randomly-selected predictors and the single best predictor.A robust prediction can be generally obtained by learning information from plenty of predictors,although the most effective model may be constructed with fewer predictors through proper methods of predictor selection.In addition,the effectiveness of objective prediction is found to generally improve as observation increases,highlighting its potential for improvement during application as time passes.(Gao Lihao)

3.3 Simulation of the Northern and Southern Hemisphere annular modes by CAMS-CSM

As leading modes of the planetary-scale atmospheric circulation in the extratropics,the Northern Hemisphere (NH) annular mode (NAM) and Southern Hemisphere (SH) annular mode (SAM) are important components of global circulation,and their variabilities substantially impact the climate in mid-high latitudes.A 35-year (1979?2013) simulation by the climate system model developed at the Chinese Academy of Meteorological Sciences (CAMS-CSM) was carried out based on observed sea surface temperature and sea ice data.The ability of CAMS-CSM in simulating horizontal and vertical structures of the NAM and SAM,relation of the NAM to the East Asian climate,and temporal variability of the SAM are examined and validated against the observational data.The results show that CAMS-CSM captures the zonally symmetric and outof-phase variations of sea level pressure anomaly between midlatitudes and polar zones in the extratropics of the NH and SH.The model has also captured the equivalent barotropic structure in tropospheric geopotential height and the meridional shifts of the NH and SH jet systems associated with the NAM and SAM anomalies.Furthermore,the model is able to reflect the variability of northern and southern Ferrel cells corresponding to the NAM and SAM anomalies.The model reproduces the observed relationship of the boreal winter NAM with the East Asian trough and air temperature over East Asia.It also captures the upward trend of the austral summer SAM index during recent decades.However,compared with the observation,the model shows biases in both the intensity and center locations of the NAM’s and SAM’s horizontal and vertical structures.Specifically,it overestimates their intensities.(Nan Sulan)

3.4 A skilful prediction scheme for West China autumn precipitation

West China is one of the country’s largest precipitation centres in autumn.This region’s agriculture and people are highly vulnerable to the variability in the autumn rain.This study documents that the water vapour for West China autumn precipitation (WCAP) is from the Bay of Bengal,the South China Sea and the Western Pacific.A strong convergence of the three water vapour transports (WVTs) and their encounter with the cold air from the northern trough over Lake Barkersh?Lake Baikal result in the intense WCAP.Three predictors in the preceding spring or summer are identified for the interannual variability of WCAP:(1) sea surface temperature in the Indo-Pacific warm pool in summer; (2) soil moisture from the Hexi Corridor to the Hetao Plain in summer; and (3) snow cover extent over East Europe and West Siberian in spring.The cold SSTAs contribute to an abnormal regional meridional circulation and intensified WVTs.The wet soil results in greater air humidity and anomalous southerly emerging over East Asia.Reduced snow cover stimulates a Rossby wave train that weakens the cold air,favouring autumn rainfall in West China.The three predictors,which demonstrate the influences of air-sea interaction,land surface processes and the cryosphere on the WCAP,have clear physical significance and are independent with each other.We then develop a new statistical prediction model with these predictors and the multilinear regression analysis method.The predicted and observed WCAP shows high correlation coefficients of 0.63 and 0.51 using cross-validation tests and independent hind casts,respectively.(Wei Ting)

3.5 降雨型地質(zhì)災(zāi)害預(yù)報(bào)方法研究

開展降雨型地質(zhì)災(zāi)害預(yù)報(bào)是減少災(zāi)害損失的有效方法。該文在討論降雨型地質(zhì)災(zāi)害預(yù)報(bào)相關(guān)概念的基礎(chǔ)上，結(jié)合國(guó)內(nèi)外已有的研究成果，系統(tǒng)總結(jié)了隱式統(tǒng)計(jì)模型、顯式統(tǒng)計(jì)模型和動(dòng)力模型等降雨型地質(zhì)災(zāi)害預(yù)報(bào)模型的特點(diǎn)和適用條件。近幾年區(qū)域降雨型地質(zhì)災(zāi)害的預(yù)報(bào)技術(shù)研究有以下新特點(diǎn)：統(tǒng)計(jì)模型簡(jiǎn)單實(shí)用，目前已經(jīng)由單一考慮降雨特征的第1代隱式統(tǒng)計(jì)模型，進(jìn)一步發(fā)展為考慮地質(zhì)、地貌等靜態(tài)因子的顯示統(tǒng)計(jì)模型；動(dòng)力模型逐漸由基于垂直入滲理論的邊坡穩(wěn)定性模型開始向基于水土耦合機(jī)制的復(fù)雜預(yù)報(bào)模型發(fā)展；降雨型地質(zhì)災(zāi)害業(yè)務(wù)預(yù)報(bào)預(yù)警的核心是地質(zhì)災(zāi)害預(yù)報(bào)模型的本地化運(yùn)行，我國(guó)已經(jīng)基于統(tǒng)計(jì)模型搭建了服務(wù)于不同區(qū)域的業(yè)務(wù)預(yù)報(bào)預(yù)警系統(tǒng)。結(jié)合多源預(yù)報(bào)降雨資料，搭建基于水土耦合機(jī)制的降雨型地質(zhì)災(zāi)害集合預(yù)報(bào)預(yù)警系統(tǒng)是未來可能的發(fā)展方向。（陳悅麗）

4 其他

4 Others

4.1 Seasonal variation in surface ozone and its regional characteristics at global atmosphere watch stations in China

We investigated the seasonal and spatial ozone variations in China by using three-year surface ozone observation data from the six Chinese Global Atmosphere Watch (GAW) stations and tropospheric column ozone data from satellite retrieval over the period 2010?2012.It is shown that the seasonal ozone variations at these GAW stations are rather different,particularly between western and eastern locations.Compared with the western China,the eastern China has lower background ozone levels.However,the Asian summer monsoon (ASM) can transport photochemical pollutants from the southern to the northern areas in the eastern China,leading to a northward gradual enhancement of background ozone levels at the eastern GAW stations.Over China,the tropospheric column ozone densities peak during spring and summer in the areas that are directly and/or indirectly affected by the ASM,and the peak time lags from the south to the north in the eastern China.We also investigated the regional representativeness of seasonal variations of ozone at the six Chinese GAW stations using the yearly maximum tropospheric column month as an indicator.The results show that the seasonal variation characteristics of ozone revealed by the Chinese GAW stations are typical,with each station having a considerable large surrounding area with the ozone maximum occurring at the same month.Ozone variations at the GAW stations are influenced by many complex factors and their regional representativeness needs to be investigated further in a broader sense.(Ma Jianzhong)

4.2 Precipitation data and their uncertainty as input for rainfall induced shallow landslide models

Physical models used to forecast the temporal occurrence of rainfall-induced shallow land slides are based on deterministic laws.Owing to the existing measuring technology and our knowledge of the physical laws controlling landslide initiation,model uncertainties are due to an inability to accurately quantify the model input parameters and rainfall forcing data.An uncertainty analysis of slope instability prediction provides a rationale for refining the geotechnical models.The Transient Rainfall Infiltration and Grid-based Regional Slope Stability-Probabilistic (TRIGRS-P) model adopts a probabilistic approach to compute the changes in the Factor of Safety (FS) due to rainfall infiltration.Slope Infiltration Distributed Equilibrium (SLIDE) is a simplified physical model for landslide prediction.The new code (SLIDE-P) is also modified by adopting the same probabilistic approach to allow values of the SLIDE model input parameters to be sampled randomly.This study examines the relative importance of rainfall variability and the uncertainty in the other variables that determine slope stability.The precipitation data from weather stations,China Meteorological Administration Land Assimilation System 2.0 (CLDAS2.0),China Meteorological Forcing Data set precipitation (CMFD),and China geological hazard bulletin are used to drive TRIGRS,SLIDE,TRIGRS-P and SLIDE-P models.The TRIGRS-P and SLIDE-P models are used to generate the input samples and to calculate the values of FS.The outputs of several model runs with varied input parameters and rainfall forcings are analyzed statistically.A comparison suggests that there are significant differences in the simulations of the TRIGRS-P and SLIDE-P models.Although different precipitation data sets are used,the simulation results of TRIGRS-P are more concentrated.This study can inform the potential use of numerical models to forecast the spatial and temporal occurrence of regional rainfall-induced shallow landslides.(Chen Yueli)

4.3 Tropospheric NO2 vertical column densities retrieved from ground-based MAX-DOAS measurements at Shangdianzi regional atmospheric background station in China

Ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements were performed at the Shangdianzi (SDZ) regional atmospheric background station in the northern China from March 2009 to February 2011.The tropospheric NO2vertical column densities (VCDs) were retrieved to investigate the background condition of the Beijing-Tianjin-Hebei region developed economic circle in China.The seasonal variation of mean NO2tropospheric VCDs (VCDTrop) at SDZ is apparent,with the maximum (1.3 × 1016molec cm?2) in February and the minimum (3.5 × 1015molec cm?2) in August,much lower than those observed at the Beijing city center.The average daytime diurnal variations of NO2VCDTrop are rather consistent for all four seasons,presenting the minimum at noon and the higher values in the morning and evening.The largest and lowest amplitudes of NO2VCDTrop diurnal variation appear in winter and in summer,respectively.The diurnal pattern at the SDZ station is similar to those at other less polluted stations,but distinct from the ones at the urban or polluted stations.Tropospheric NO2VCDs at SDZ are strongly dependent on the wind,with the higher values being associated with the pollution plumes from Beijing.Tropospheric NO2VCDs derived from ground-based MAX-DOAS at SDZ show to be well correlated with corresponding OMI (Ozone Monitoring Instrument) satellite products with a correlation coefficientR= 0.88.However,the OMI observations are on average higher than MAX-DOAS NO2VCDs by a factor of 28%,probably due to the OMI grid cell partly covering the south of SDZ,which is influenced more by the pollution plumes from the urban areas.(Cheng Siyang)

4.4 Seasonal and interannual variability of the subsurface velocity profile of the Indonesian throughflow at the Makassar Strait

Although there have been many previous studies that have interpreted the variability of Indonesian throughflow (ITF) transport,the processes that determine its vertical structure have rarely been investigated due to limited observations and model inefficiencies.In this study,a regionally coupled model is developed to address this issue.The model adopts an unstructured model grid,with an 3 km resolution within the Indonesian seas and straits and reveals somewhat inconsistent results compared to previous models with coarser resolutions.The results suggest that the seasonal variability of the depth of the ITF velocity maximum is partially controlled by the seasonally reversed Karimata throughflow,while the remainder primarily originated from the Mindanao-Sulawesi inflow rather than the Sibutu Strait throughflow.The Mindanao-Sulawesi inflow possesses a subsurface velocity core similar to that of the Makassar Strait,with a deep (shallow) maximum during El Ni?o (La Ni?a) years that plays a crucial role in determining the downstream Makassar throughflow profile in the interannual timescale.A sensitivity experiment by fixing the Indian Ocean boundary condition shows that the impacts from the Indian Ocean might be significant only within the intra-seasonal to seasonal timescales.(Jiang Guoqing)

4.5 四川地形擾動(dòng)對(duì)降水分布影響

引入一維加權(quán)平均的譜分析方法定量研究四川地形強(qiáng)迫對(duì)該區(qū)域降水分布的影響。結(jié)果表明：緯向地形和冬季降水譜峰鎖相于同一波長(zhǎng)（475.8 km），呈共振關(guān)系，地形與其他季節(jié)降水呈漂移關(guān)系，這與經(jīng)向和緯向上環(huán)流變動(dòng)有關(guān)，即冬季緯向環(huán)流占主導(dǎo)，緯向地形觸發(fā)的大氣波動(dòng)對(duì)冬季降水策動(dòng)作用大；夏季降水是各種不同尺度系統(tǒng)相互作用的結(jié)果，地形是重要因素之一。經(jīng)向和緯向地形特征尺度分別為296.8 km和475.8 km，反映了地形強(qiáng)迫的中尺度特征，且緯向地形譜峰比經(jīng)向大1個(gè)數(shù)量級(jí)，緯向強(qiáng)迫更明顯。夏季降水譜峰比冬季大2個(gè)數(shù)量級(jí)，降水系統(tǒng)緯向特征尺度比冬季小約150 km，說明夏季在緯向地形強(qiáng)迫下，降水系統(tǒng)尺度減小的同時(shí)其強(qiáng)度大大增加，這在一定程度上可以解釋中尺度對(duì)流性降水在夏季偏多。四川夏季最大降水位于雅安地區(qū)，其地形擾動(dòng)比四川整體擾動(dòng)更明顯，故產(chǎn)生的降水也更大。夏季降水和經(jīng)向地形鎖相于同一波長(zhǎng)（37.1 km），經(jīng)向地形對(duì)雅安夏季強(qiáng)降水起關(guān)鍵作用。（王成鑫）