Mechanism of Diabatic Heating on Precipitation and the Track of a Tibetan Plateau Vortex over the Eastern Slope of the Tibetan Plateau

Existing studies contend that latent heating(LH)will replace sensible heating(SH)to become the dominant factor affecting the development of the Tibetan Plateau vortex(TPV)after it moves off the Tibetan Plateau(TP).However,in the process of the TPV moving off the TP requires that the airmass traverse the eastern slope of the Tibetan Plateau(ESTP)where the topography and diabatic heating(DH)conditions rapidly change.How LH gradually replaces SH to become the dominant factor in the development of the TPV over the ESTP is still not very clear.In this paper,an analysis of a typical case of a TPV with a long life history over the ESTP is performed by using multi-sourced meteorological data and model simulations.The results show that SH from the TP surface can change the TPV-associated precipitation distribution by temperature advection after the TPV moves off the TP.The LH can then directly promote the development of the TPV and has a certain guiding effect on the track of the TPV.The SH can control the active area of LH by changing the falling area of the TPV-associated precipitation,so it still plays a key role in the development and tracking of the TPV even though it has moved out of the main body of the TP.

Interannual Variability of Late-spring Circulation and Diabatic Heating over the Tibetan Plateau Associated with Indian Ocean Forcing

The thermal forcing of the Tibetan Plateau （TP） during boreal spring, which involves surface sensible heating, latent heating released by convection and radiation flux heat, is critical for the seasonal and subseasonal variation of the East Asian summer monsoon. Distinct from the situation in March and April when the TP thermal forcing is modulated by the sea surface temperature anomaly （SSTA） in the North Atlantic, the present study shows that it is altered mainly by the SSTA in the Indian Ocean Basin Mode （IOBM） in May, according to in-situ observations over the TP and MERRA reanalysis data. In the positive phase of the IOBM, a local Hadley circulation is enhanced, with its ascending branch over the southwestern Indian Ocean and a descending one over the southeastern TP, leading to suppressed precipitation and weaker latent heat over the eastern TP. Meanwhile, stronger westerly flow and surface sensible heating emerges over much of the TP, along with slight variations in local net radiation flux due to cancellation between its components. The opposite trends occur in the negative phase of the IOBM. Moreover, the main associated physical processes can be validated by a series of sensitivity experiments based on an atmospheric general circulation model, FAMIL. Therefore, rather than influenced by the remote SSTAs of the northern Atlantic in the early spring, the thermal forcing of the TP is altered by the Indian Ocean SSTA in the late spring on an interannual timescale.

RELATIONSHIPS BETWEEN THE POSITION VARIATION OF THE WEST PACIFIC SUBTROPICAL HIGH AND THE DIABATIC HEATING DURING PERSISTENT INTENSE RAIN EVENTS IN YANGTZE-HUAIHE RIVERS BASIN

By using NCEP/NCAR daily reanalysis data and daily precipitation data of 740 stations in China, relationships between the position variation of the West Pacific subtropical high (WPSH) and the diabatic heating during persistent and intense rains in the Yangtze-Huaihe Rivers basin are studied. The results show that the position variation of WPSH is closely associated with the diabatic heating. There are strong apparent heating sources and moisture sinks in both the basin (to the north of WPSH) and the north of Bay of Bengal (to the west of WPSH) during persistent and intense rain events. In the basin, Q 1z begins to increase 3 days ahead of intense rainfall, maximizes 2 days later and then reduces gradually, but it changes little after precipitation ends, thus preventing the WPSH from moving northward. In the north of Bay of Bengal, 2 days ahead of strong rainfall over the basin, Q 1z starts to increase and peaks 1 day after the rain occurs, leading to the westward extension of WPSH. Afterwards, Q 1z begins declining and the WPSH makes its eastward retreat accordingly. Based on the complete vertical vorticity equation, in mid-troposphere, the vertical variation of heating in the basin is favorable to the increase of cyclonic vorticity north of WPSH, which counteracts the northward movement of WPSH and favors the persistence of rainbands over the basin. The vertical variation of heating in the north of Bay of Bengal is in favor of the increase of anti-cyclonic vorticity to the west of WPSH, which induces the westward extension of WPSH.

Global Response to Tropical Diabatic Heating Variability in Boreal Winter

Global teleconnections associated with tropical convective activities were investigated, based on monthly data of 29 Northern Hemisphere winters： December, January, February, and March （DJFM）. First, EOF analyses were performed on the outgoing longwave radiation （OLR） data to characterize the convective ac tivity variability in the tropical Indian Ocean and the western Pacific. The first EOF mode of the convective activity was highly correlated with the ENSO. The second EOF mode had an east–west dipole structure, and the third EOF mode had three convective activity centers. Two distinct teleconnection patterns were identified that were associated, respectively, with the second and third EOF modes. A global primitive equation model was used to investigate the physical mechanism that causes the global circulation anoma lies. The model responses to anomalous tropical thermal forcings that mimic the EOF patterns matched the general features of the observed circulation anomalies well, and they were mainly controlled by linear processes. The importance of convective activities in the tropical Indian Ocean and western Pacific to the extended and longrange forecasting capability in the extratropics is discussed.

Can Current AGCMs Reproduce Historical Changes in the Atmospheric Diabatic Heating over the Tibetan Plateau?

Recent studies have demonstrated a persistent decreasing trend in the spring sensible heat(SH) source over the Tibetan Plateau(TP) during the past three decades. By comparing simulations from nine state-of-the-art atmospheric general circulation models(AGCMs) driven by historical forcing fields with both observational data and five reanalysis datasets, the authors found that the AGCMs are unable to reproduce the change in the SH flux over the TP. This deficiency arises because the observed decreasing trend in SH flux depends primarily on the change in surface wind speed according to the bulk formula, whereas in the models it is also influenced largely by changes in the land-air temperature difference related to the systematic cold bias. In addition, an obvious discrepancy exists in other aspects of the diabatic heating simulated by the models, suggesting that a significant improvement is required in the physical schemes associated with land surface processes and diabatic heating over the complicated topography.

Atmospheric diabatic heating–induced wave train from the Caspian Sea to South and East Asia during the summer monsoon season

During the summer monsoon season,the authors observe a wave train that stretches from the northern Arabian Peninsula and Caspian Sea to the Indo-Gangetic plains along the foothills of the Himalaya and extending further east of the Tibetan Plateau.The trend analysis between 1979 and 2018 with NCEP–NCAR reanalysis data show that the diabatic heating flux(averaged over 1000 to 500 hPa)tends to decrease significantly over the Caspian Sea and its surrounding regions.In addition,the sea level pressure is increasing by^0.1 hPa yr-1 over the Caspian Sea,forming a high-pressure divergent center over there.The divergent center is collocated with an anticyclonic circulation trend at 850 hPa over the Caspian Sea.This decreasing diabatic heating flux modulates the local atmospheric circulation by increasing the surface pressure around the center of divergence,which further facilitates a wave train to propagate towards South and East Asia.This wave train transports the moisture fluxes at 925 h Pa from the Caspian Sea,southeastward towards the South and East Asian monsoon region.

Atmospheric Diabatic Heating and Summertime Circulation in Asia-Africa Area

Utilizing data from NCEP/ NCAR reanalysis, the summertime atmospheric diabatic heating due to different physical processes is investigated over the Sahara desert, the Tibetan Plateau, and the Bay of Bengal. Atmospheric circulation systems in summer over these three areas are also studied. Thermal adaptation theory is employed to explain the relationship between the circulation and the atmospheric diabatic heating. Over the Sahara desert, heating resulting from the surface sensible heat flux dominates the near-surface layer, while radiative cooling is dominant upward from the boundary layer. There is positive vorticity in the shallow boundary layer and negative vorticity in the middle and upper troposphere. Downward motion prevails over the Sahara desert, except in the shallow near—surface layer where weak ascent exists in summer. Over the Tibetan Plateau, strong vertical diffusion resulting from intense surface sensible heat flux to the overlying atmosphere contributes most to the boundary layer heating, condensation associated with large—scale ascent is another contributor to the lower layer heating. Latent heat release accompanying deep convection is critical in offsetting longwave radiative cooling in the middle and upper troposphere. The overall diabatic heating is positive in the whole troposphere in summer, with the most intense heating located in the boundary layer. Convergence and positive vorticity occur in the shallow near—surface layer and divergence and negative vorticity exist deeply in the middle and upper troposphere. Accordingly, upward motion prevails over the Plateau in summer, with the most intense rising occurring near the ground surface. Over the Bay of Bengal, summertime latent heat release associated with deep convection exceeds longwave radiative cooling, resulting in intense heating in almost the whole troposphere. The strongest heating over the Bay of Bengal is located around 400 hPa, resulting in the most intense rising occurring between 300 hPa and 400 hPa, and producing positive vorticity in the lower troposphere and negative vorticity in the upper troposphere. It is also shown that the divergent circulation is from a heat source region to a sink region in the upper troposphere and vice versa in lower layers. Key words Atmospheric diabatic heating - Summer - Circulation This work was jointly supported by “ National Key Program for Developing Basic Sciences” G1998040904 by NSFC projects 49805003, 49635170, 49823002, and 49825504.

Relationship between cross-equatorial flows over the Bay of Bengal and Australia in boreal summer:Role of tropical diabatic heating

The interannual variability of cross-equatorial flows(CEFs)over the Asian–Australian monsoon(AAM)region during boreal summer was analyzed by applying the empirical orthogonal function(EOF)method to the meridional wind at 925 h Pa.The first mode(EOF1)exhibits an in-phase relationship among different CEF channels over the AAM region,which has received much attention owing to its tight linkage with ENSO.By contrast,the second mode(EOF2)possesses an out-of-phase relationship between the Bay of Bengal(BOB)CEF(90°E)and Australian CEF,among which the New Guinea CEF near 150°E shows the most significant opposite correlation with the BOB CEF.Observational and numerical model results suggest that the equatorially asymmetric heat source(sink)over the western(eastern)Maritime Continent,closely associated with the in-situ sea surface temperature anomaly,can induce cross-equatorial northerly(southerly)flow into the heating hemisphere,which dominates the out-of-phase relationship between the BOB and New Guinea CEFs.Furthermore,an equatorially symmetric heating over the central Pacific may indirectly change the CEFs by modulating the zonal atmospheric circulation near the Maritime Continent.

The Role of Topography and Diabatic Heating in the Formation of Dipole Blocking in the Atmosphere

In this paper, the nonlinear stationary waves forced by topography and diabatic heating are investigated. It is pointed out that (1) the nonlinear interaction of different stationary waves forced only by topography might form dipole blocking in the atmosphere, this might explain the dipole blocking appeared in the Pacific and Atlantic regions; (2) the dipole blocking could not be caused by the nonlinear interaction of the different stationary waves forced by the diabatic heating alone; (3) the nonlinear interaction of the diffferent stationary waves forced by both topography and diabatic heating could initiate dipole blocking in the atmosphere. In winter, the dipole blocking mainly occurs in the west regions of the Pacific and the Atlantic, and the heat source over the western part of the two oceans is advantageous to the formation of dipole blocking in the west of two oceans. However, in summer, the dipole blocking could be formed in the east part of the two oceans, and the heat source over the eastern part of two continents is favourable for the formation of dipole blocking in the east regions of two oceans.

Influence of rainfall-induced diabatic heating on southern rainfall-northern haze over eastern China in early February 2023

In early February 2023,there was severe haze on the North China Plain(NCP)that was contemporaneous with heavy rainfall over southern China,which was known as southern rainfall-northern haze(SR-NH).Based on observational and reanalysis data,the meteorological causes of this SR-NH event are investigated in this study using correlation analysis,dynamic diagnostics and numerical experiments.The results show that the anticyclonic anomaly in the Pacific Northwest(also referred to as the northeast Asian anomalous anticyclone)is responsible for the SR-NH.On the one hand,this anticyclonic anomaly leads to persistent rainfall over southern China by causing strong ascending motion in conjunction with an anomalous cyclone over the Chinese mainland and transporting large amounts of water vapor there.On the other hand,it weakens the climatological northerly winds of the NCP through the southeasterly flow,worsening the horizontal diffusion conditions of pollutants.Additionally,the atmospheric stability and relative humidity over the NCP are significantly increased by this anticyclonic anomaly.These conditions result in higher PM2.5concentrations over the NCP.Additional results suggest that this anticyclonic anomaly is related to diabatic heating released by rainfall in southern China,which not only intensifies the rainfall process there(with a contribution of 11.5%)but also induces an anticyclonic anomaly in the upper troposphere of the Pacific Northwest(i.e.,200 hPa).The rainfall-related anticyclonic anomaly reinforces the anticyclonic anomaly in the Pacific Northwest caused by large-scale circulation(with a contribution of 27%)and thus affects haze over the NCP.This study provides a new reference for understanding the contribution of rainfall in southern China to haze over the NCP.

Study of a Horizontal Shear Line over the Qinghai–Tibetan Plateau and the Impact of Diabatic Heating on Its Evolution

Based on the 4 times daily 0.75°× 0.75° ERA-Interim data, the structural evolution of a Qinghai-Tibetan Plateau horizontal （east-west-oriented） shear line （TSL） during 15-19 August 2015 and the effect of diabatic heating on its evolution were analyzed. The results show that the TSL possessed a vertical thickness of up to 1.5 km （approxim-ately 600-450 hPa）, and was baroclinic in nature. Weak ascending motions occurred near the TSL, accompanied with more significant gradients in dew point temperature than in temperature. The TSL was characterized by diurnal vari- ations in its appearance and structure. It was relatively full in shape （broken） and was the lowest （highest） in vertical extent at 0000 （1800） UTC, and veered clockwise （anticlockwise） dttring 0000--0600 （1200-1800） UTC. When the north-south span of the TSL increased, it was prone to fracturing; and it disappeared when the dew point temperat-ure gradients to its either side decreased. When the TSL moved northward （southward）, its western （eastern） section broke up, while the eastern （western） section inclined to regenerate or merge. The TSL tended to move towards the positive vorticity areas with significant increases in vorticity. When the positive vorticity center moved down, the height of TSL decreased. Further analysis shows that the plateau surface heating dominated the vorticity attribute of the TSL and its movement, with different contributions from local variation, horizontal advection, and vertical advec-tion of the diabatic heating to the TSL at different heights.

The Diabatic Heating and the Generation of Available Potential Energy: Results from NCEP Reanalysis

In the existing studies on the atmospheric energy cycle, the attention to thegeneration of available potential energy (APE) is restricted to its global mean value. Thegeographical distributions of the generation of APE and its mechanism of formation are investigatedby using the three-dimensional NCEP/NCAR diabatic heating reanalysis in this study. The results showthat the contributions from sensible heating and net radiation to the generation of zonal andtime-mean APE (G_Z) are mainly located in high and middle latitudes with an opposite sign, while thelatent heating shows a dominant effect on G_Z mainly in the tropics and high latitudes where thecontributions from the middle and upper tropospheres are also contrary to that from the lowtroposphere. In high latitudes, the G_Z is much stronger for the Winter Hemisphere than for theSummer Hemisphere, and this is consistent with the asymmetrical feature shown by the reservoir ofzonal and time-mean APE in two hemispheres, which suggests that the generation of APE plays afundamental role in maintaining the APE in the global atmospheric energy cycle. The samecontributions to the generation of stationary eddy APE (G_(SE)) from the different regions relatedto the maintenance of longitudinal temperature contrast are likely arisen by different physics.Specifically, the positive contributions to G_(SE) from the latent heating in the western tropicalPacific and from the sensible heating over land are dominated by the heating at warm regions,whereas those from the latent heating in the eastern tropical Pacific and from the sensitive heatingover the oceans are dominated by the cooling at cold regions. Thus, our findings provide anobservational estimate of the generation of eddy APE to identify the regional contributions in theclimate simulations because it might be correct for the wrong reasons in the general circulationmodel (GCM). The largest positive contributions to the generation of transient eddy APE (G_(TE)) arefound to be at middle latitudes in the middle and upper tropospheres, where reside the strong localcontributions to the baroclinic conversion from transient eddy APE to transient eddy kinetic energyand the resulting transient eddy kinetic energy.

Relationship Between the Western Pacific Subtropical High and the Subtropical East Asian Diabatic Heating During South China Heavy Rains in June 2005

Based on the daily NCEP/NCAR reanalysis data,the position variation of the western Pacific subtropical high（WPSH） in June 2005 and its relation to the diabatic heating in the subtropical East Asia are analyzed using the complete vertical vorticity equation.The results show that the position variation of the WPSH is indeed associated with the diabatic heating in the subtropical East Asian areas.In comparison with June climatology,stronger heating on the north side of the WPSH and relatively weak ITCZ（intertropical convergence zone） convection on the south side of the WPSH occurred in June 2005.Along with the northward movement of the WPSH,the convective latent heating extended northward from the south side of the WPSH.The heating to the west of the WPSH was generally greater than that inside the WPSH,and each significant enhancement of the heating field corresponded to a subsequent westward extension of the WPSH.In the mid troposphere,the vertical variation of heating on the north of the WPSH was greater than the climatology,which is unfavorable for the northward movement of the WPSH.On the other hand,the vertical variation of heating south of the WPSH was largely smaller than the climatology,which is favorable for the anomalous increase of anticyclonic vorticity,leading to the southward retreat of the WPSH.Before the westward extension of the WPSH in late June 2005,the vertical variation of heating rates to（in） the west（east） of the WPSH was largely higher（lower） than the climatology,which is in favor of the increase of anticyclonic（cyclonic） vorticity to（in） the west（east） of the WPSH,inducing the subsequent westward extension of the WPSH.Similar features appeared in the lower troposphere.In a word,the heating on the north-south,east-west of the WPSH worked together,resulting in the WPSH extending more southward and westward in June 2005,which is favorable to the maintenance of the rainbelt in South China.

GLOBAL ATMOSPHERIC SEASONAL-MEAN HEATING:DIABATIC VERSUS TRANSIENT HEATING

With the ERA40 reanalysis daily data for 1958-2001, the global atmospheric seasonal-mean diabatic heating and transient heating are computed by using the residual diagnosis of the thermodynamic equation. The three-dimensional structures for the two types of heating are described and compared. It is demonstrated that the diabatic heating is basically characterized by strong and deep convective heating in the tropics, shallow heating in the midlatitudes and deep cooling in the subtropics and high-latitudes. The tropical diabatic heating always shifts towards the summer hemisphere, but the midlatitude heating and high-latitude cooling tend to be strong in the winter hemisphere. On the other hand, the transient heating due to transient eddy transfer is characterized by a meridional dipole pattern with cooling in the subtropics and heating in the mid- and high-latitudes, as well as by a vertical dipole pattern in the midlatitudes with cooling at lower levels and heating in the mid- and higher-levels, which gives rise to a sloped structure in the transient heating oriented from the lower levels in the high latitudes and higher levels in the midlatitudes. The transient heating is closely related to a storm track along which the transient eddy activity is much stronger in the winter hemisphere than in the summer hemisphere. In Northern Hemisphere, the transient heating locates in the western oceanic basin, while it is zonally-oriented in Southern Hemisphere, for which the transient heating and cooling are far separated over South Pacific during the cold season. The transient heating tends to cancel the diabatic heating over most of the globe. However, it dominates the mid-tropospheric heating in the midlatitudes. Therefore, the atmospheric transient processes act to help the atmosphere gain more heat in the high-latitudes and in the mid-troposphere of midlatitudes, reallocating the atmospheric heat obtained from the diabatic heating.

Effect of Horizontal Nonuniformity of Diabatic Heating on Tropical Cyclone Intensity and Structure

The effect of the horizontal variation of diabatic heating on the tropicalcyclone intensity and structure is studied in this paper. According to the potential vorticity (PV)equation in axis-symmetric cylindrical coordinates, PV disturbance caused by the radial differenceof diabatic heating is positive (negative) inside (outside) the maximum heating radius, implyingthat the radial nonuniformity of diabatic heating should contribute positively to the intensity of atropical cyclone while negatively to its size. A primitive equation model is then used to get somequantitative ideas on the problem. Results show that the modeled tropical cyclone weakens by about20% but is larger in size if the effect of horizontal variety of convective heating is excluded inthermodynamic and dynamic equations. The PV disturbance originated from the horizontal nonuniformityof diabatic heating is positive inside the maximum heating radius and negative outside, inconsistent with the PV equation analyses. The maximum disturbance (both negative and positive)appears around the maximum heating level and their magnitude is comparable to that generated byvertical variance of heating. It is concluded that the effect of the horizontal heat nonuniformityon the intensity and structure of TC cannot be neglected.

DYNAMICAL STUDY OF DIABATIC HEATING INFLUENCE ON RADIAL INHOMOGENEOUS STRUCTURE OF TROPICAL CYCLONE

Based on the tropical cyclone(TC)asymmetric disturbance as the superposition of the symmetric environmental circulation,the analytical solution of travelling wave is given by using the barotropical nondivergent model with diabatic heating forcing and non-friction in a plane polar coordinate.Then,the TC radial inhomogeneous structure is analyzed on radial/tangential velocity and geopotential height.It is found that the different kinds of structures are influenced by the Coriolis parameter(f),TC intensity(Ω),disturbance circular frequency(ω),and TC angular wavenumber(m).And,the diabatic heating(Q_1)has significant impacts on the radial/tangential velocity distribution shaped like the inner-tight and outer-relaxed.

Diagnostic Study of an Extreme Explosive Cyclone over the Kuroshio/Kuroshio Extension Region

Explosive cyclones(ECs)occur frequently over the Kuroshio/Kuroshio Extension region.The most rapidly intensified EC over the Kuroshio/Kuroshio Extension region during the 42 years(1979-2020)of cold seasons(October-April)was studied to reveal the variations of the key factors at different explosive-developing stages.This EC had weak low-level baroclinicity,mid-level cyclonic-vorticity advection,and strong low-level water vapor convergence at the initial explosive-developing stage.The low-level baroclinicity and mid-level cyclonic-vorticity advection increased substantially during the maximum-deepening-rate stage.The diagnostic analyses using the Zwack-Okossi equation showed that diabatic heating was the main contributor to the initial rapid intensification of this EC.The cyclonic-vorticity advection and warm-air advection enhanced rapidly in the middle and upper troposphere and contributed to the maximum rapid intensification,whereas the diabatic heating weakened slightly in the mid-low troposphere.The relative contribution of the diabatic heating decreased from the initial explosive-developing stage to the maximum-deepening-rate stage due to the enhancement of other factors(the cyclonic-vorticity advection and warm-air advection).Furthermore,the physical factors contributing to this EC varied with the explosive-developing stage.The non-key factors at the initial explosive-developing stage need attention to forecast the rapid intensification.

An Analysis of the Low Moving Speed of Landfalling Typhoon In-Fa in 2021

The movement speed of Typhoon In-Fa(2021)was notably slow,at 10 km h-1or less,for over 20 hours following its landfall in Zhejiang,China,in contrast to other typhoons that have made landfall.This study examines the factors contributing to the slow movement of Typhoon In-Fa,including the steering flow,diabatic heating,vertical wind shear(VWS),and surface synoptic situation,by comparing it with Typhoons Yagi(2018)and Rumbia(2018)which followed similar tracks.The findings reveal that the movement speed of Typhoons Yagi and Rumbia is most closely associated with their respective 500 h Pa environmental winds,with a steering flow of 10^(-12)m s^(-1).In contrast,Typhoon InFa’s movement speed is most strongly correlated with the 850 h Pa environmental wind field,with a steering flow speed of only 2 m s^(-1).Furthermore,as Typhoon In-Fa moves northwest after landfall,its intensity is slightly greater than that of Typhoons Yagi and Rumbia,and the pressure gradient in front of Typhoon In-Fa is notably smaller,leading to its slow movement.Additionally,the precipitation distribution of Typhoon In-Fa differs from that of the other two typhoons,resulting in a weak asymmetry of wavenumber-1 diabatic heating,which indirectly affects its movement speed.Further analysis indicates that VWS can alter the typhoon’s structure,weaken its intensity,and ultimately impact its movement.

Comparative Analysis of Energy Characteristics of Two Southwest Vortices in Sichuan Under Similar Circulation Backgrounds

Based on ERA5 reanalysis data,the present study analyzed the thermal energy development mechanism and kinetic energy conversion characteristics of two extreme rainstorm processes in relation to the shallow southwest vortex in the warm-sector during a“rain-generated vortex”process and the deep southwest vortex in a“vortex-generated rain”process.The findings were as follows:(1)During the extreme rainstorm on August 11,2020(hereinafter referred to as the“8·11”process),intense surface heating and a high-energy unstable environment were observed.The mesoscale convergence system triggered convection to produce heavy rainfall,and the release of latent condensation heat generated by the rainfall promoted the formation of a southwest vortex.The significant increase(decrease)in atmospheric diabatic heating and kinetic energy preceded the increase(decrease)in vorticity.By contrast,the extreme rainstorm on August 16,2020(hereinafter referred to as the“8·16”process)involved the generation of southwest vortex in a low-energy and highhumidity environment.The dynamic uplift of the southwest vortex triggered rainfall,and the release of condensation latent heat from rainfall further strengthened the development of the southwest vortex.The significant increase(decrease)in atmospheric diabatic heating and kinetic energy exhibited a delayed progression compared to the increase(decrease)in vorticity.(2)The heating effect around the southwest vortex region was non-uniform,and the heating intensity varied in different stages.In the“8·11”process,the heating effect was the strongest in the initial stage,but weakened during the vortex's development.On the contrary,the heating effect was initially weak in the“8·16”process,and intensified during the development stage.(3)The available potential energy of the“8·11”process significantly increased in kinetic energy converted from rotational and divergent winds through baroclinic action,and the divergent wind energy continued to convert into rotational wind energy.By contrast,the“8·16”process involved the conversion of rotational wind energy into divergent wind energy,which in turn converted kinetic energy back into available potential energy,thereby impeding the further development and maintenance of the southwest vortex.

Seasonal Prediction of Indian Summer Monsoon Using WRF: A Dynamical Downscaling Perspective

Seasonal forecasting of the Indian summer monsoon by dynamically downscaling the CFSv2 output using a high resolution WRF model over the hindcast period of 1982-2008 has been performed in this study. The April start ensemble mean of the CFSv2 has been used to provide the initial and lateral boundary conditions for driving the WRF. The WRF model is integrated from 1st May through 1st October for each monsoon season. The analysis suggests that the WRF exhibits potential skill in improving the rainfall skill as well as the seasonal pattern and minimizes the meteorological errors as compared to the parent CFSv2 model. The rainfall pattern is simulated quite closer to the observation (IMD) in the WRF model over CFSv2 especially over the significant rainfall regions of India such as the Western Ghats and the central India. Probability distributions of the rainfall show that the rainfall is improved with the WRF. However, the WRF simulates copious amounts of rainfall over the eastern coast of India. Surface and upper air meteorological parameters show that the WRF model improves the simulation of the lower level and upper-level winds, MSLP, CAPE and PBL height. The specific humidity profiles show substantial improvement along the vertical column of the atmosphere which can be directly related to the net precipitable water. The CFSv2 underestimates the specific humidity along the vertical which is corrected by the WRF model. Over the Bay of Bengal, the WRF model overestimates the CAPE and specific humidity which may be attributed to the copious amount of rainfall along the eastern coast of India. Residual heating profiles also show that the WRF improves the thermodynamics of the atmosphere over 700 hPa and 400 hPa levels which helps in improving the rainfall simulation. Improvement in the land surface fluxes is also witnessed in the WRF model.