The Modified Envelope Orography and the Air Flow over and around Mountains

By use of the two-layer adiabatic globe spectral model and the zonally averaged climatic data of winter season as initial values, 10-day integrations are carried out based on three kinds of model topography (i.e., (1) the averaged topography; (2) the envelope topography; (3) the modified envelope topography). The results show that the orography of the Northern Hemisphere plays an important role in the simulation of large-scale weather patterns in winter season. The simulation based on the envelope topography developed by Wallace et al. has some improvements in the Rocky Mountains area. But this scheme causes very serious horizontal expansion around the Tibetan Plateau (hereafter referred to as the TV). A modified envelope topography scheme has been worked out that increases the slope of the TP by decreasing the horizontal expansion while keeping the maximum altitude. The results show some improvements of the scheme around the TP. By analysis of the mechanical effects of the large-scale orography on the currents, the different forcings of the air flow over and around the TP and the Rocky Mountain (the RM) are investigated.

The Influence of Weakly-Nonlinear Vertical Advection on the Wind Field of PBL with Large-Scale Orography

The analytical solutions of the PBL wind distribution under the equilibrium of four forces including both horizontal and vertical advections are obtained in this paper using small parameter method. Utilizing this simple PBL model, we also compute the wind spirals of a certain circular vortex over level ground and over a west-east ranged mountain, respectively. The results show that when there exsits large-scale orographic forcing,vertical advection may exert considerable influence on the wind distribution of PBL, the angle between geostrophic flow and surface wind, and the height of the top of PBL.

A Study on Frontal Motion over Orography

In order to study the characteristic features of the frontal motion over mountains, a frontal model is designed inthis paper. The analytical solution of the model with the assumption of semigeostrophic approximation and no frichon is obtained and compared with numerical solution without the assumption. It assumes that the front is a free surface betWeen two nuid layers with different densihes. It also assumes that the mountain and the front are indefinitelylong. Therefore, the motion of such a front can be determined by the one-dimensional shallow water equationsystem. By making a series of experiments, we confirm the rule further that front can be retarded when climbing themountain and accelerated when going down the mountains. We also found some interesting characteristics of frontalstructure influenced by topography in addihon.

EFFECTS OF OROGRAPHY AND AIR FLOW IN EKMAN BOUNDARY LAYER

It is convenient to use σ-coordinates to discuss the dynamic effect of orography and the flow in Ekman boundary layer.In this paper,the theory of mixing length is generalized to the σ-coordinate system. Then the governing equations,describing the motion in the boundary layer over the mountain regions are derived.The features of flow in the boundary layer,especially the effects of Ekman pumping,are discussed in details.It is pointed out that there are three factors affecting the vertical motion at the top of the boun- dary layer:(1)vorticity distribution in the boundary layer,which is directly related to the divergence and convergence of air flow caused by friction,(2)the upslope or downslope motion of flow over the mountain slopes,and(3)the mutual effect of orography and friction induced by the ageostrophic component climbing up- ward or downward in the boundary layer over mountain regions.

SIMULATION STUDY OF THE EFFECT ON COLD SURGE OF THE QINGHAI-XIZANG PLATEAU AS A HUGE OROGRAPHY

Using a limited-area P-σ incorporated coordinate five-level primitive equation model fed with the ECMWF 5°×5°grid data,a simulation is made of a large-scale cold surge of late December 1982.Results show that the Qinghai-Xizang Plateau has no profound thermal but dynamic effect on the East-Asian winter monsoon that forces cold air to go southwards by its east side,exciting Kelvin waves behind the cold front to result in the maximum NE wind.Besides,gravitational waves independent of the Plateau occur ahead of the front advancing towards the south.They may be due to the excitation involved in the front itself. The cold surge is propagated under the Hadley cell,making it strengthened and moved southwards.

On the Key Dynamical Processes Supporting the 21.7 Zhengzhou Record-breaking Hourly Rainfall in China

An extremely heavy rainfall event occurred in Zhengzhou,China,on 20 July 2021 and produced an hourly rainfall rate of 201.9 mm,which broke the station record for China's Mainland.Based on radar observations and a convection-permitting simulation using the WRF-ARW model,this paper investigates the multiscale processes,especially those at the mesoscale,that support the extreme observed hourly rainfall.Results show that the extreme rainfall occurred in an environment characteristic of warm-sector heavy rainfall,with abundant warm moist air transported from the ocean by an abnormally northward-displaced western Pacific subtropical high and Typhoon In-Fa(2021).However,rather than through back building and echo training of convective cells often found in warm-sector heavy rainfall events,this extreme hourly rainfall event was caused by a single,quasi-stationary storm in Zhengzhou.Scale separation analysis reveals that the extreme-rainproducing storm was supported and maintained by the dynamic lifting of low-level converging flows from the north,south,and east of the storm.The low-level northerly flow originated from a mesoscale barrier jet on the eastern slope of the Taihang Mountain due to terrain blocking of large-scale easterly flows,which reached an overall balance with the southerly winds in association with a low-level meso-β-scale vortex located to the west of Zhengzhou.The large-scale easterly inflows that fed the deep convection via transport of thermodynamically unstable air into the storm prevented the eastward propagation of the weak,shallow cold pool.As a result,the convective storm was nearly stationary over Zhengzhou,resulting in record-breaking hourly precipitation.

The Influence of Tibetan Plateau on the Interannual Variability of Asian Monsoon

Forced by the realistic SST, an atmospheric general circulation model (AGCM) with 9 sigma levels in vertical and rhomboidal truncation at wave number 15 in the horizontal is run for 16 years with and without the Tibetan Plateau respectively(called TP and NTP experiment). The result simulated is used to investigate the influence of the Tibetan Plateau on the interannual variability of Asian monsoon. It is found that the interannual variability of Asian monsoon associated with El Nino/La Nina in NTP experiment is quite different from that in TP experiment. With the Tibetan Plateau included, the results are consistent with the observation very well. To a great extent, the anomalous variation of Asian monsoon during El Nino/La Nina period in observation is due to the existence of the Tibetan Plateau. Therefore, the topography of the Tibetan Plateau is an important factor to the interannual variability of Asian monsoon.

Treatment of LBCs in 2D Simulation of Convection over Hills

A series of idealized model simulations are analyzed to determine the sensitivity of model results to different configurations of the lateral boundary conditions (LBCs) in simulating mesoscale shallow convection over hilly terrain. In the simulations with steady thermal forcing at the model surface, a radiation condition at both boundaries is the best choice under high wind conditions, and the best results are produced when both the normal velocities and the temperature are treated with the radiation scheme in which the phase speed is the same for different variables. When the background wind speed is reasonably small, the LBC configuration with either the radiation or the zero gradient condition at both boundaries tends to make the numerical solution unstable. The choice of a constant condition at the inflow boundary and a radiation outflow boundary condition is appropriate in most cases. In the simulations with diurnal thermal forcing at the model surface, different LBC schemes are combined together to reduce spurious signals induced by the outflow boundary. A specification inflow boundary condition, in which the velocity fields at the inflow boundary are provided using the time-dependent results of a simulation with periodic LBCs over a flat domain, is tested and the results indicate that the specification condition at the inflow boundary makes it possible to use a smaller model domain to obtain reasonable results. The model horizontal domain length should be greater than a critical length, which depends on the domain depth H and the angle between gravity wave phase lines and the vertical. An estimate of minimum domain length is given by , where N and U are the background stability and wind speed, respectively, Lx is the typical gravity wavelength scale, and Zi is the convective boundary layer (CBL) depth.

Formation and Development of a Mountain-induced Secondary Center inside Typhoon Morakot(2009)

The structural evolution of Typhoon Morakot（2009） during its passage across Taiwan was investigated with the WRF model. When Morakot approached eastern Taiwan, the low-level center was gradually filled by the Central Mountain Range（CMR）, while the outer wind had flowed around the northern tip of the CMR and met the southwesterly monsoon to result in a strong confluent flow over the southern Taiwan Strait. When the confluent flow was blocked by the southern CMR, a secondary center（SC） without a warm core formed over southwestern Taiwan. During the northward movement of the SC along the west slope of the CMR, the warm air produced within the wake flow over the northwestern CMR was continuously advected into the SC, contributing to the generation of a warm core inside the SC. Consequently, a well-defined SC with a warm core, closed circulation and almost symmetric structure was produced over central western Taiwan, and then it coupled with Morakot＇s mid-level center after crossing the CMR to reestablish a new and vertically stacked typhoon. Therefore, the SC inside Morakot was initially generated by a dynamic interaction among the TC＇s cyclonic wind, southwesterly wind and orographic effects of the CMR, while the thermodynamic process associated with the downslope adiabatic warming effect documented by previous studies supported its development to be a well-defined SC. In summary, the evolution of the SC in this study is not in contradiction with previous studies, but just a complement, especially in the initial formation stage.

Local meteorology in a northern Himalayan valley near Mount Everest and its response to seasonal transitions

An automatic weather station(AWS) has been installed at the Qomolangma Station of the China Academy of Sciences(QOMS) since 2005, in a northern Himalayan valley near Mount Everest, with an altitude of 4,270 m a.s.l.. Nine years of meteorological records(2006–2014) from the automatic weather station(AWS) were analyzed in this study, aiming to understand the response of local weather to the seasonal transition on the northern slopes of Mount Everest, with consideration of the movement of the subtropical jet(STJ) and the onset of the Indian Summer Monsoon(ISM). We found:(1) Both the synoptic circulation and the orography have a profound influence on the local weather, especially the local circulation.(2) Southwesterly(SW) and southeasterly(SE) winds prevail alternately at QOMS in the afternoon through the year. The SW wind was driven by the STJ during the non-monsoon months, while the SE was induced by the trans-Himalayan flow through the Arun Valley, a major valley to the east of Mount Everest, under a background of weak westerly winds aloft.(3) The response of air temperature(T) and specific humidity(q) to the monsoon onset is not as marked as that of the nearsurface winds. The q increases gradually and reaches a maximum in July when the rainy period begins.(4) The alternation between the SW wind at QOMS and the afternoon SE wind in the pre-monsoon season signals the northward shift of the STJ and imminent monsoon onset. The average interval between these two events is 14 days.

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.

Hydrometeorological Modeling Study of Tropical Cyclone Phet in the Arabian Sea in 2010

Tropical cyclone Phet is the second strongest tropical cyclone ever recorded in the Arabian Sea. Phet made landfall in the northeast mountainous area of Oman in early morning on 4 June in 2010, causing a breaking record rainfall in this arid region of 488 mm/48 h. The cyclone heavy rainfall triggered flash floods causing enormous losses in lives and infrastructure in northeast Oman. The state of the art Advanced Research WRF model is used to study the atmospheric circulation and to reproduce the heavy rainfall over Oman. Three one-way nested domains with 32 vertical layers with terrain following sigma coordinate are used to setup eight numerical experiments aiming to investigate the effect of initialization time, horizontal grid resolution and terrain elevations on reproducing the cyclone track, intensity and heavy rainfall. Simulation results show negligible effect of model initialization time on cyclone track, intensity and rainfall. In contrast, the orographic effect played a substantial role in rainfall simulation over northeast Oman. The heavy rainfall was a combination of the cyclone circulation effect and the orographic lifting in the mountains. The northeasterly cyclone moist-warm wind was lifted in the Omani mountains releasing its potential energy and enhancing further thermal convection. The numerical experiment with the highest terrain elevation (RUN3.3-C) resulted in overestimation of observed rainfall due to the enhanced topographic lifting of the saturated cyclone wind. Experiment with similar horizontal grid resolution but smoother terrain elevation (RUN3.3-TER) resulted in much less rainfall amount comparable to the observed values. The increased precipitation in RUN3.3-C is due to the increase in the rain- water and cloud water and graupel of the explicit moisture scheme.

Mesoscale Numerical Study of Quasi-Stationary Convective System over Jeddah in November 2009

25 November 2009 is an unforgettable day for the people in Jeddah, the second largest city in the Kingdom of Saudi Arabia (KSA). On that day, Jeddah turned into a disaster zone following a short heavy rainfall event that triggered flash floods leaving 122 fatalities and considerable losses. Numerical experiments using the Pennsylvania State University-National Center for Atmospheric research mesoscale meteorological model (MM5) have been performed to investigate the event. It was caused by a short quasi-stationary mesoscale convective system that developed over Jeddah and lasted for about 8 hours. Rainfall totals computed by the model exceeded 400 mmin some localities in the southern part of Jeddah city and to the north of Jeddah in Thuwal city. The limited available observed rainfall totals, atKingAbdulAzizInternationalAirportand wadiQaws rain gauges, and Jeddah’s weather radar observations corroborates the ability of the model to reproduce the spatial and temporal characteristics of the rainfall event. A synoptic environment characterized by warmRed Seasurface temperatures and high humidity in the low levels of the troposphere. A stationary anticyclone centered over the southeast of theArabian Peninsulaconcentrated the water vapour flow to a narrow passage over Jeddah. Simulation results suggested that the development of a mesolow by latent heat release, as well as cyclogenesis induced by Al Hejaz escarpments, could have played an important role in enhancing the event by providing low-level convergence and enhanced upslope winds, and upper level atmospheric instability.

THE EFFECTS OF TIBETAN PLATEAU ON THE ANOMALOUS VARIATION OF ASIAN MONSOON IN A COUPLED OCEAN-ATMOSPHERE SYSTEM

Experimental predictions with a hybrid coupled ocean-atmosphere model(L9R15 AGCM-ZC ocean model)were performed for the 1986/87 El Nino event and the 1988/89 La Nina event with and without the Tibetan Plateau respectively(called TP FORC and NTP FORC hereinafter). Comparison shows that,to some extent,the existence of the Tibetan Plateau orography weakens or restrains(strengthens or facilitates)the formation of the anomalous circulation of Asian monsoon during El Nino(La Nina)period.Opposite results are found in the uncoupled AGCM simulation.

ANALYSES OF THE DYNAMIC EFFECTS ON WINTER CIRCULATION OF THE TWO MAIN MOUNTAINS IN THE NORTHERN HEMISPHERE——Ⅰ.RELATIONSHIP AMONG GENERAL CIRCULATION,TELECONNECTION AND STATIONARY WAVES

The distribution of troughs and ridges of geopotential height,the teleconnection patterns and the propagation pat- terns of stationary waves are the main features of the January mean geopotential height field at 500hPa.Data analyses and numerical experiments indicate that these three characteristics are associated to one another and closely related to the mechanical forcing of the Rocky Mountains and Tibetan Plateau.There exists a prominent negative correlation in the intensity variation between the American trough and the Asian trough at high and middle latitudes.Such negative correlation,in connection with the interannual variation of the intensity of the jets in front of the two troughs,leads to the existence of similar teleconnection patterns in North America and East Asia.On the other hand,the different propa- gation behaviour of quasi-stationary waves downstream of the two main mountains results in a fundamental difference in the distribution of correlation chains in North America and East Asia.

DESIGN AND TEST OF AN IMPROVED SCHEME FOR GLOBAL SPECTRAL MODEL WITH REDUCED TRUNCATION ERROR

Ⅰ. INTRODUCTIONA scheme suitable for the effects of large scale orography is designed and tested. In the improved scheme a reference atmosphere which is only a function of pressure p is introduced in the governing equations of the spectral model. The

A THREE-DIMENSIONAL ELASTIC NESTED-GRID MESO-(β-γ) SCALE ATMOSPHERIC MODEL——PART Ⅱ:MODEL VALIDATION

By using the model developed in Part Ⅰ of the sister papers,simulations of severe storm,winter airflow and snowfall over a mountain,as well as mountain-valley winds and sea breeze are performed.Different functions of the model are verified through the computation and special emphasis is put on some important properties of the meso-(β-γ) systems frequently occurring.The simulation results of the cases are agreeable to field observations,conceptual models,and pre- vious numerical studies.It is shown that the model is applicable for the researches and prediction of various local and/or regional weather processes,and capable of describing their macro- and micro-structures,influences of orographic and underlying-surface forcing,and interaction between meso-β and meso-γ scales.In addition,the evolutional characteristics and mechanisms of the modeled processes are also analyzed.

INFLUENCE OF TIBETAN PLATEAU ON AIR-SEA INTERACTION OVER TROPICAL PACIFIC IN OCEAN-ATMOSPHERIC COUPLED SYSTEM

A hybrid coupled ocean-atmosphere model is designed,which consists of a global atmospheric general circulation model(L9R15 AGCM)and a simple ocean model(ZC ocean model over tropical Pacific).Using the model,experimental predictions are performed for the 1986/87 El Nino event and the 1988/89 La Nina event with and without the Tibetan Plateau respectively(called TP FORC and NTP FORC hereinafter).It is found as follows:(1)The coupled system can successfully predict the El Nino or La Nina event even if the Tibetan Plateau orography is not included in the model.The patterns of SSTA and wind anomalies in the model without the Tibetan Plateau are similar to those with the Tibetan Plateau,which further verifies the fact that ENSO process is mainly caused by the air-sea interaction in tropical Pacific.(2)However.the existence of the Tibetan Plateau exerts its influences on the intensity and duration of El Nino(or La Nina). It is unfavorable to the development and maintenance of westerly anomalies,so to some extent, restrains the development of El Nino,but favors the development of La Nina.(3)Effects of the Tibetan Plateau orography on the wind anomalies in the coupled system are different from those in uncoupled AGCM simulation.

THE LIMITED AREA ANALYSIS AND FORECAST SYSTEM AND ITS OPERATIONAL APPLICATION

The limited area analysis and forecast system(LAFS)was developed and has been put into operational use at National Meteorological Center since January 1991.This system can be regarded as a branch system attached to the global assimilation and medium-range forecast system which is based on a spectral model T42L9.The main advancements as an upgrade operational system are as follows:the use of a regional fine mesh optimum interpolation(OI)analysis scheme:the realiza- tion of the nonlinear normal mode initialization for the regional model:the development of a 15L- spherical grid primitive equation model(with real topography and enstrophy conservation)and its nesting forecast with the spectral model T42L9.

NUMERICAL EXPERIMENTS IN SYMMETRIC AND ASYMMETRIC MOTIONS IN THE BAROTROPIC PRIMITIVE EQUATION MODEL ATMOSPHERE

Three numerical experiments have been carried out by using a spectral barotropic primitive equation mo- del.It is found that the results obtained are quite similar to those with the barotropic filtered model.The main results read as follows: (1)In the case with symmetric orography or without orography,if the motion is symmetric(with re- spect to the equator,the same is true hereafter)at the initial instant,then it would be symmetric afterwards. (2)The antisymmetric orography distribution could cause antisymmetric motion,and the original sym- metric motion might become asymmetric. In order to explain the above results,it has been proved that they are theoretically valid.And it is found that if the motion is antisymmetric at the initial instant,then it would become asymmetric.Therefore,no pure antisymmetric motion could be maintained.