Evaluating and Improving Wind Forecasts over South China: The Role of Orographic Parameterization in the GRAPES Model

Unresolved small-scale orographic（SSO） drags are parameterized in a regional model based on the Global/Regional Assimilation and Prediction System for the Tropical Mesoscale Model（GRAPES TMM）. The SSO drags are represented by adding a sink term in the momentum equations. The maximum height of the mountain within the grid box is adopted in the SSO parameterization（SSOP） scheme as compensation for the drag. The effects of the unresolved topography are parameterized as the feedbacks to the momentum tendencies on the first model level in planetary boundary layer（PBL）parameterization. The SSOP scheme has been implemented and coupled with the PBL parameterization scheme within the model physics package. A monthly simulation is designed to examine the performance of the SSOP scheme over the complex terrain areas located in the southwest of Guangdong. The verification results show that the surface wind speed bias has been much alleviated by adopting the SSOP scheme, in addition to reduction of the wind bias in the lower troposphere. The target verification over Xinyi shows that the simulations with the SSOP scheme provide improved wind estimation over the complex regions in the southwest of Guangdong.

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.

Evolution and Frontogenesis of an Imbalanced Flow- the Influence of Vapor Distribution and Orographic Forcing

If the initial fields are not in geostrophic balance, the adjustment and evolution will occur in the stratified fluid. and the frontogenesis will occur under suitable conditions. The evolution is studied here with a nonhydrostatic fully compressible meso-scale model (Advanced Regional Prediction System, ARPS). Four cases are designed and compared: (i) control experiment: (ii) with different initial temperature gradient; (iii) with vapor distribution; (iv) with orographic forcing. The results show that: (1) there is an inertial oscillation in the evolution of the imbalanced flow with the frequency of the local Coriolis f, and with its amplitude decreasing with time. The stationary balanced state can only be approached as it cannot be reached in the limit duration of time. The energy conversion ratio varies in the range of [0, 1; 3]; (2) the stronger initial temperature gradient can make the final energy conversion ratio higher. and vice versa; (3) suitable vapor distribution is favorable for the frontogenesis. It will bring forward the time of the frontogenesis, strengthen the intensity of the cold front, and influence the final energy conversion ratio; (4) the orographic forcing has an evidently strengthening effect on the frontogenesis. The strengthening effect on the frontogenesis and the influence on the final energy conversion ratio depend on the relative location of the mountain to the cold front.

Analysis of the Characteristics of Inertia-Gravity Waves during an Orographic Precipitation Event

A numerical experiment was performed using the Weather Research and Forecasting（WRF） model to analyze the generation and propagation of inertia-gravity waves during an orographic rainstorm that occurred in the Sichuan area on 17 August 2014. To examine the spatial and temporal structures of the inertia-gravity waves and identify the wave types, three wavenumber-frequency spectral analysis methods（Fourier analysis, cross-spectral analysis, and wavelet cross-spectrum analysis）were applied. During the storm, inertia-gravity waves appeared at heights of 10-14 km, with periods of 80-100 min and wavelengths of 40-50 km. These waves were generated over a mountain and propagated eastward at an average speed of 15-20 m s^（-1）. Meanwhile, comparison between the reconstructed inertia-gravity waves and accumulated precipitation showed there was a mutual promotion process between them. The Richardson number and Scorer parameter were used to demonstrate that the eastward-moving inertia-gravity waves were trapped in an effective atmospheric ducting zone with favorable reflector and critical level conditions, which were the primary causes of the long lives of the waves. Finally, numerical experiments to test the sensitivity to terrain and diabatic heating were conducted, and the results suggested a cooperative effect of terrain and diabatic heating contributed to the propagation and enhancement of the waves.

Planetary Stationary Waves in the Atmosphere Part I: Orographic Stationary Waves

It is proposed that the orographic stationary waves are required by long-term balance of momentum in the atmosphere with zonally asymmetric orographic forcing. This hypothesis may be confirmed successfully with the theoretical model of geostrophic waves. In the Part I, we will explain the observed phase distributions of orographic stationary waves at middle and high latitudes of the Northern Hemisphere, according to the long-term balance of zonal momentum over the stationary orographic forcing. It is revealed that the geographic distribution of stationary waves depends not only on local topgraphy but also on mean circulation fields and angular momentum flux in the atmosphere. So these waves cannot be simulated by the models in a restricted area.

Airborne Measurements of the Impact of Ground-based Glaciogenic Cloud Seeding on Orographic Precipitation

Data from in situ probes and a vertically-pointing ram-wave Doppler radar aboard a research aircraft are used to study the cloud microphysical effect of glaciogenic seeding of cold-season orographic clouds. A previous study （Geerts et al., 2010） has shown that radar reflectivity tends to be higher during seeding periods in a shallow layer above the ground downwind of ground-based silver iodide （AgI） nuclei generators. This finding is based on seven flights, conducted over a mountain in Wyoming （the Unites States）, each with a no-seeding period followed by a seeding period. In order to assess this impact, geographically fixed flight tracks were flown over a target mountain, both upwind and downwind of the AgI generators. This paper examines data from the same flights for further evidence of the cloud seeding impact. Com- posite radar data show that the low-level reflectivity increase is best defined upwind of the mountain crest and downwind of the point where the cloud base intersects the terrain. The main argument that this increase can be attributed to AgI seeding is that it is confined to a shallow layer near the ground where the flow is turbulent. Yet during two flights when clouds were cumuliform and coherent updrafts to flight level were recorded by the radar, the seeding impact was evident in the flight-level updrafts （about 610 m above the mountain peak） as a significant increase in the ice crystal appears short-lived as it is not apparent just downwind of concentration in all size bins. The seeding effect the crest.

Interaction of Orographic Disturbance with Front

The interaction of orographic disturbance with front is investigated with a nonhydrostatic fully compressible mesoscale model (ARPS). It is shown that the front is dominated mainly by the orographic disturbance if the front is weak. Firstly, because the stratified airstream is forced to flow along the topographic surface, the topographic surface almost coincides with the lowest isentrope for the barotropic flow. The potential temperature gradients are opposite on upwind slope and downwind slope. As the cold front moves across the mountain, its intensity decreases on the upwind side and increases on the downwind side due to the thermal superposition. Conversely, the warm front is strengthened on the upwind slope and weakened on the downwind slope. This is the thermal superposition effect. Secondly, the mountain-forced circulation and orographic waves, which depend on the shape and size of topography and characteristics of airflow, contribute to frontogenesis and / or frontolysis. This is referred as dynamical action. For the mesoscale mountain ridge of gentle slope, the dynamical action weakens the cold front on the upwind slope, and strengthens the cold front on the lee side. While for the mesoscale mountain of steep slope, the dynamical effect weakens the cold front on the upwind side and strengthens the cold front on the mountain top, the frontal intensity is decreased when front moves downslope rapidly. As front moves into the convergent zone near the mountain base, its intensity is enhanced severely. If the front is intensive, there is strong interaction between the orographic disturbance and the front. The cold front dramatically increases downslope wind and lee side gravity wave activity. And these in turn act upon the frontal intensity and frontal structure. For the baroclinic basic flow, the southerly warm advection on the upwind side makes the cold front less frontolysis; the northerly on the lee side violently intensifies the clod front.

Preliminary Results of the Ground-Based Orographic Snow Enhancement Experiment for the Easterly Cold Fog (Cloud) at Daegwallyeong during the 2006 Winter

The snow enhancement experiments, carried out by injecting AgI and water vapor into orographically enhanced clouds （fog）, have been conducted to confirm Li and Pitter＇s forced condensation process in a natural situation. Nine ground-based experiments have been conducted at Daegwallyeong in the Taebaek Mountains for the easterly foggy days from January-February 2006. We then obtained the optimized conditions for the Daegwallyeong region as follows： the small seeding rate （1.04 g min-1） of AgI for the easterly cold fog with the high humidity of Gangneung. Additional experiments are needed to statistically estimate the snowfall increment caused by the small AgI seeding into the orographical fog （cloud） over the Taeback Mountains.

Effect of Aerosol Particles on Orographic Clouds:Sensitivity to Autoconversion Schemes

Aerosol particles can serve as cloud condensation nuclei(CCN)to influence orographic clouds.Autoconversion,which describes the initial formation of raindrops from the collision of cloud droplets,is an important process for aerosol-cloud-precipitation systems.In this study,seven autoconversion schemes are used to investigate the impact of CCN on orographic warm-phase clouds.As the initial cloud droplet concentration is increased from 100 cm^(-3)to 1000 cm^(-3)(to represent an increase in CCN),the cloud water increases and then the rainwater is suppressed due to a decrease in the autoconversion rate,leading to a spatial shift in surface precipitation.Intercomparison of the results from the autoconversion schemes show that the sensitivity of cloud water,rainwater,and surface precipitation to a change in the concentration of CCN is different from scheme to scheme.In particular,the decrease in orographic precipitation due to increasing CCN is found to range from-87%to-10%depending on the autoconversion scheme.Moreover,the surface precipitation distribution also changes significantly by scheme or CCN concentration,and the increase in the spillover(ratio of precipitation on the leeward side to total precipitation)induced by increased CCN ranges from 10%to 55%under different autoconversion schemes.The simulations suggest that autoconversion parameterization schemes should not be ignored in the interaction of aerosol and orographic cloud.

NUMERICAL STUDY OF OROGRAPHIC INFLUENCE ON LOW FREQUENCY OSCILLATION

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Orographic Effects on Extreme Rainfall at Different Durations: A Case Study in Campania Region (Southern Italy)

Long-term probabilistic prediction of extreme rainfall at the regional scale is a significant tool in the mitigation of hydro-geological disasters: it actually provides the starting point in the design of strategic hydraulic infrastructures and emergency plans. A crucial task of regional estimation of extreme rainfall is how to include the complex effects of orographic barriers in a mathematical model for Intensity-Duration-Frequency (IDF) curves. Here, an analysis of how orography can affect extreme rainfall at different durations is presented for three orographic systems that are very relevant for hydrological risk assessment in the Campania Region in Southern Italy. Then, we introduce a power law model to link the amplification factor to the duration, thus allowing a simple and effective enhancement of the IDF model in mountainous areas.

Improved Simulation of Summer Heavy Rainfall over Beijing and Henan by the YHGSM with Updated Subgrid Orographic Parameters

In numerical weather prediction(NWP),the parameterization of orographic drag plays an important role in representing subgrid orographic effects.The subgrid orographic parameters are the key input to the parameterization of orographic drag.Currently,the subgrid orographic parameters in most NWP models were produced based on elevation datasets generated many years ago,with a coarse resolution and low quality.In this paper,using the latest high-quality elevation data and considering the applicable scale range of the subgrid orographic parameters,we construct the orographic parameters,including the subgrid orographic standard deviation,anisotropy,orientation,and slope,that are required as input to the orographic gravity wave drag(OGWD)parameterization.Finally,we introduce the newly constructed orographic parameters into the Yin-He Global Spectral Model(YHGSM),optimize the description of the orographic effect in the model,and improve the simulation of two typical heavy rainfall events in Beijing and Henan.

Antecedent snowmelt and orographic precipitation contributions to water supply of Pakistan disastrous floods,2022

In 2022,the Pakistan witnessed the hottest spring and wettest summer in history.And devastating floods inundated a large portion of Pakistan and caused enormous damages.However,the primary water source and its contributions to these unprecedented floods remain unclear.Based on the reservoir inflow measurements,Multi-Source Weighted-Ensemble Precipitation(MSWEP),the fifth generation ECMWF atmospheric reanalysis(ERA5)products,this study quantified the contributions of monsoon precipitation,antecedent snow-melts,and orographic precipitation enhancement to floods in Pakistan.We found that the Indus experienced at least four inflow up-rushes,which was mainly supplied by precipitation and snowmelt;In upper Indus,abnormally high temperature continued to influence the whole summer and lead to large amounts of snowmelts which not only was a key water supply to the flood but also provided favorable soil moisture conditions for the latter precipitation.Before July,the snowmelt has higher contributions than the precipitation to the streamflow of Indus River,with contribution value of more than 60%.Moreover,the snowmelt could still supply 20%-40%water to the lower Indus in July and August;The leading driver of 2022 mega-floods over the southern Pakistan in July and August was dominated by the precipitation,where terrain disturbance induced precipitation account to approximately 33%over the southern Pakistan.The results help to understand the mechanisms of flood formation,and to better predict future flood risks over complex terrain regions.

Importance of orographic gravity waves over the Tibetan Plateau on the spring rainfall in East Asia

The springtime persistent rainfall(SPR)is the major rainy period before the onset of summer monsoon in East Asia,which profoundly affects the regional and even global hydrological cycle.Despite the great importance of the mechanical and thermal effects of the Tibetan Plateau(TP)large-scale orography on the formation of SPR,the impact of small-scale orography over the TP remains poorly understood.Here we show that upward-propagating orographic gravity waves(OGWs),which occur as the subtropical westerlies interact with the TP's small-scale orography,contribute importantly to the SPR.The breaking of OGWs induces a large zonal wave drag in the middle troposphere,which drives a meridional circulation across the TP.The rising branch of the meridional circulation acts to lower the pressure and increase the meridional pressure gradient to the south of the TP by dynamically pumping the lower-tropospheric air upwards.The southwesterly monsoonal flow on the southeastern flank of the TP thus intensifies and transports more water vapor to East Asia,resulting in an enhancement of the SPR.This finding helps more completely understand the impacts of TP's multiscale orography on the SPR and provides a new perspective on the westerly-monsoon synergy in East Asia.

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.

NUMERICAL SIMULATION OF MESOSCALE STRUCTURES OF TYPHOON AND OROGRAPHIC EFFECTS USING MM5

The track, landfall, dynamic and thermodynamic and cloud-rain physical mesoscale structures and their evolution of typhoon HERB 1996 in 36 h from 0000 UTC 31 July to 1200 UTC 1 August 1996 were simulated by using the non-hydrostatic mesoscale model MM5. This period covered the process of typhoon HERB landfall at Taiwan and Fujian Provinces. Results show that the model successfully simulated the landfall process of typhoon HERB, revealed the most important characteristics of the mesoscale dynamic and thermodynamic and cloud-rain physical structure during its landfall. The simulated typhoon track was close to the observation. The center of cyclonic circulation simulated at 0000 UTC on 1 August 1996 (24 h integration) was located in shore near Fuqing, Fujian Province at which the typhoon was reported to landfall two hours later. It shows that strong upward motion formed by low level convergence existed in the eye-wall and subsidence at the eye. The wind field shows clear asymmetrical structure near the typhoon center. The cloud and rainband was screw-typed distributed around typhoon center, and consisted of meso-β scale rain cores. During the period of typhoon HERB staying near and passing over Taiwan, the lower cloud was developed in the eye region so that the previous clear typhoon eye on the satellite pictures became fuzzy. Observation shows that the typhoon center was 'warm', but the model simulations with higher space resolution show that in the mid-troposphere the region of eye-wall with stronger upward motion and more cloud-and rain- water was warmer than the eye. During the period of typhoon passing over Taiwan and its following landfall at Fujian, the track of model typhoon deviated about 30 km northward (i. e., rightward) because of the orographic effects of Taiwan Island, but the strength of the typhoon was not affected remarkably. The amount of rainfall on Taiwan in the 36 h simulations was enhanced more than six times by the orographic lifting of Taiwan Mountain.

IMPACTS OF SURFACE FRICTION AND OROGRAPHICAL FORCING ON THE EAST ASIA COASTAL CYCLOGENESIS

Some numerical simulations from real data were carried out to examine the impacts of surface friction and orographic forcing on the East Asia coastal cyclogenesis.The results show that the decreasing of the surface friction over the ocean is essential for the cyclone development and the mechanical forcing of Qinghai-Xizang Plateau acts a damping effect in the initial stage of the cyclone.

CAS FGOALS-f3-L Model Datasets for CMIP6 GMMIP Tier-1 and Tier-3 Experiments

The Chinese Academy of Sciences(CAS)Flexible Global Ocean Atmosphere Land System(FGOALS-f3-L)model datasets prepared for the sixth phase of the Coupled Model Intercomparison Project(CMIP6)Global Monsoons Model Intercomparison Project(GMMIP)Tier-1 and Tier-3 experiments are introduced in this paper,and the model descriptions,experimental design and model outputs are demonstrated.There are three simulations in Tier-1,with different initial states,and five simulations in Tier-3,with different topographies or surface thermal status.Specifically,Tier-3 contains four orographic perturbation experiments that remove the Tibetan Iranian Plateau,East African and Arabian Peninsula highlands,Sierra Madre,and Andes,and one thermal perturbation experiment that removes the surface sensible heating over the Tibetan Iranian Plateau and surrounding regions at altitudes above 500 m.These datasets will contribute to CMIP6’s value as a benchmark to evaluate the importance of long-term and short-term trends of the sea surface temperature in monsoon circulations and precipitation,and to a better understanding of the orographic impact on the global monsoon system over highlands.

Impacts of the Thermal Effects of Sub-grid Orography on the Heavy Rainfall Events Along the Yangtze River Valley in 1991

A P - σ regional climate model using a parameterization scheme to account for the thermal effects of the sub-grid scale orography was used to simulate the three heavy rainfall events that occurred within the Yangtze River Valley during the mei-yu period of 1991. The simulation results showed that by considering the sub-grid scale topography scheme, one can significantly improve the performance of the model for simulating the rainfall distribution and intensity during these three heavy rainfall events, most especially the second and third. It was also discovered that the rainfall was mainly due to convective precipitation. The comparison between experiments, either with and without the sub-grid scale topography scheme, showed that the model using the scheme reproduced the convergence intensity and distribution at the 850 hPa level and the ascending motion and moisture convergence center located at 500 hPa over the Yangtze River valley. However, some deviations still exist in the simulation of the atmospheric moisture content, the convergence distribution and the moisture transportation route, which mainly result in lower simulated precipitation levels. Further analysis of the simulation results demonstrated that the sub-grid topography scheme modified the distribution of the surface energy budget components, especially at the south and southwest edges of the Tibetan Plateau, leading to the development and eastward propagation of the negative geopotential height difference and positive temperature-lapse rate difference at 700 hPa, which possibly led to an improved precipitation simulation over eastern China.

Spatio-temporal variability in sea surface wind stress near and off the east coast of Korea

Sea surface wind stress variabilities near and off the east coast of Korea, are examined using 7 kinds of wind datasets from measurements at 2 coastal (land) stations and 2 ocean buoys,satellite scatterometer (QuikSCAT), and global reanalyzed products (ECMWF,NOGAPS,and NCEP/NCAR). Temporal variabilities are analyzed at 3 frequency bands; synoptic (2-20 d), intra-seasonal (20-90 d),and seasonal (>90 d).Synoptic and intra-seasonal variations are predominant near and off the Donghae City due to the passage of the mesoscale weather system. Seasonal variation is caused by southeastward wind stress during Asian winter monsoon. The sea surface wind stress from reanalyzed datasets.QuikSCAT and KMA-B measurements off the coast show good agreement in the magnitude and direction,which are strongly aligned with the alongshore direction.At the land-based sites,wind stresses are much weaker by factors of 3-10 due to the mountainous landmass on the east parts of Korea Peninsula.The first EOF modes(67%-70%) of wind stresses from reanalyzed and QuikSCAT data have similar structures of the strong southeastward wind stress in winter along the coast but show different curl structures at scales less than 200 km due to the orographic effects.The second EOF modes (23%-25%) show southwestward wind stress in every September along the east coast of the North Korea