ANALYSIS OF INDICES FOR SEVERE CONVECTIVE WEATHER UNDER THE INFLUENCE OF TROPICAL CYCLONES IN JIANGSU

By statistical research on the occurrence pattern of severe convective weather in Jiangsu province under the influence of tropical cyclones within a 10-year period(from 2001 to 2010),this paper discovers that among different severe convective weather,the occurrence frequency of short-range heavy precipitation is the highest,thunderstorms and gales come in second,and general thunderstorms rarely happen,while hailstorms and tornadoes never occur.The statistical results also showed that within the research period there are 21 tropical cyclones(TCs) affecting the Jiangsu area and most of them are in the stage of weakening to tropical depressions.Moreover,through studying indices for relevant cases of severe convection,it is discovered that their thresholds are lower than that of previous research,which indicated that convective instability and energy accumulation can easily lead to severe convective weather eventually due to the influence of TCs.

A Physics-informed Deep-learning Intensity Prediction Scheme for Tropical Cyclones over the Western North Pacific

Accurate prediction of tropical cyclone(TC)intensity is challenging due to the complex physical processes involved.Here,we introduce a new TC intensity prediction scheme for the western North Pacific(WNP)based on a time-dependent theory of TC intensification,termed the energetically based dynamical system(EBDS)model,together with the use of a long short-term memory(LSTM)neural network.In time-dependent theory,TC intensity change is controlled by both the internal dynamics of the TC system and various environmental factors,expressed as environmental dynamical efficiency.The LSTM neural network is used to predict the environmental dynamical efficiency in the EBDS model trained using besttrack TC data and global reanalysis data during 1982–2017.The transfer learning and ensemble methods are used to retrain the scheme using the environmental factors predicted by the Global Forecast System(GFS)of the National Centers for Environmental Prediction during 2017–21.The predicted environmental dynamical efficiency is finally iterated into the EBDS equations to predict TC intensity.The new scheme is evaluated for TC intensity prediction using both reanalysis data and the GFS prediction data.The intensity prediction by the new scheme shows better skill than the official prediction from the China Meteorological Administration(CMA)and those by other state-of-art statistical and dynamical forecast systems,except for the 72-h forecast.Particularly at the longer lead times of 96 h and 120 h,the new scheme has smaller forecast errors,with a more than 30%improvement over the official forecasts.

MICROPHYSICAL CHARACTERISTICS OF THE RAINDROP SIZE DISTRIBUTION IN TYPHOON MORAKOT(2009)

Microphysical characteristics of the raindrop size distribution(RSD)in Typhoon Morakot(2009) have been studied through the PARSIVEL disdrometer measurements at one site in Fujian province,China during the passage of the storm from 7 to 10 August 2009.The time evolution of the RSD reveals different segments of the storm.Significant difference was observed in the microphysical characteristics between the outer rainband and the eyewall;the eyewall precipitation had a broader size distribution(a smaller slope) than the outer rainband and eye region.The outer rainband and the eye region produced stratiform rains while the eyewall precipitation was convective or mixed stratiform-convective.The RSD was typically characterized by a single peak distribution and well represented by the gamma distribution.The relations between the shape(μ)and slope(Λ)of the gamma distribution and between the reflectivity(Z)and rainfall rate(R)have been investigated.Based on the NW-Dm relationships,we suggest that the stratiform rain for the outer rainband and the eye region was formed by the melting of graupel or rimed ice particles,which likely originated from the eyewall clouds.

A Simulation Study of the Mesoscale Convective Systems Associated with a Meiyu Frontal Heavy Rain Event

In this study, evolution of the mesoscale convective systems （MCSs） within a Meiyu front during a particularly heavy rainfall event on 22 June 1999 in East China was simulated by using a nonhydrostatic numerical model ARPS （Advanced Regional Prediction System）. Investigations were conducted with emphasis on the impact of the interaction among multi-scale weather systems （MWSs） on the development of MCSs in the Meiyu frontal environment. For this case, the development of MCSs experienced three different stages. （1） The convections associated with MCSs were firstly triggered by the eastward-moving Southwest Vortex （SWV） from the Sichuan Basin, accompanying the intensification of the upper-level jet （ULJ） and the low-level jet （LLJ） that were approaching the Meiyu front. （2） Next, a low-level shear line （LSL） formed, which strengthened and organized the MCSs after the SWV decayed. Meanwhile, the ULJ and LLJ enhanced and produced favorable conditions for the MCSs development. （3） Finally, as the MCSs got intensified, a mesoscale convective vortex （MCV）, a mesoscale LLJ and a mesoscale ULJ were established. Then a coupled-development of MWSs was achieved through the vertical frontal circulations, which further enhanced the MCV and resulted in the heavy rainfall. This is a new physical mechanism for the formation of Meiyu heavy rainfall related to the SWV during the warm season in East China. In the three stages of the heavy rainfall, the vertical frontal circulations exhibited distinguished structures and played a dynamic role, and they enhanced the interaction among the MWSs. A further examination on the formation and evolution of the MCV showed that the MCV was mainly caused by the latent heat release of the MCSs, and the positive feedback between the MCSs and MCV was a key characteristic of the scale interaction in this case.

Evaluation of Two Momentum Control Variable Schemes and Their Impact on the Variational Assimilation of Radar Wind Data:Case Study of a Squall Line

Different choices of control variables in variational assimilation can bring about different influences on the analyzed atmospheric state. Based on the WRF model＇s three-dimensional variational assimilation system, this study compares the be- havior of two momentum control variable options-streamfunction velocity potential （ψ-χ） and horizontal wind components （U-V）-in radar wind data assimilation for a squall line case that occurred in Jiangsu Province on 24 August 2014. The wind increment from the single observation test shows that the ψ-χ control variable scheme produces negative increments in the neighborhood around the observation point because streamfunction and velocity potential preserve integrals of velocity. On the contrary, the U-V control variable scheme objectively reflects the information of the observation itself. Furthermore, radial velocity data from 17 Doppler radars in eastern China are assimilated. As compared to the impact of conventional observation, the assimilation of radar radial velocity based on the U-V control variable scheme significantly improves the mesoscale dynamic field in the initial condition. The enhanced low-level jet stream, water vapor convergence and low-level wind shear result in better squall line forecasting. However, the ψ-χ control variable scheme generates a discontinuous wind field and unrealistic convergence/divergence in the analyzed field, which lead to a degraded precipitation forecast.

Effects of Surface Flux Parameterization on the Numerically Simulated Intensity and Structure of Typhoon Morakot(2009)

The effects of surface flux parameterizations on tropical cyclone（TC） intensity and structure are investigated using the Advanced Research Weather Research and Forecasting（WRF-ARW） modeling system with high-resolution simulations of Typhoon Morakot（2009）.Numerical experiments are designed to simulate Typhoon Morakot（2009） with different formulations of surface exchange coefficients for enthalpy（C_K） and momentum（C_D） transfers,including those from recent observational studies based on in situ aircraft data collected in Atlantic hurricanes.The results show that the simulated intensity and structure are sensitive to C_K and C_D,but the simulated track is not.Consistent with previous studies,the simulated storm intensity is found to be more sensitive to the ratio of C_K/C_D than to C_K or C_D alone.The pressure-wind relationship is also found to be influenced by the exchange coefficients,consistent with recent numerical studies.This paper emphasizes the importance of C_D and C_K on TC structure simulations.The results suggest that C_D and C_K have a large impact on surface wind and flux distributions,boundary layer heights,the warm core,and precipitation.Compared to available observations,the experiment with observed C_D and C_K generally simulated better intensity and structure than the other experiments,especially over the ocean.The reasons for the structural differences among the experiments with different C_D and C_K setups are discussed in the context of TC dynamics and thermodynamics.

ON THE OPTIMAL BACKGROUND ERROR COVARIANCES: DIFFERENT SCALE ERRORS' CONTRIBUTION

The large-scale and small-scale errors could affect background error covariances for a regional numerical model with the specified grid resolution.Based on the different background error covariances influenced by different scale errors,this study tries to construct a so-called"optimal background error covariances"to consider the interactions among different scale errors.For this purpose,a linear combination of the forecast differences influenced by information of errors at different scales is used to construct the new forecast differences for estimating optimal background error covariances.By adjusting the relative weight of the forecast differences influenced by information of smaller-scale errors,the relative influence of different scale errors on optimal background error covariances can be changed.For a heavy rainfall case,the corresponding optimal background error covariances can be estimated through choosing proper weighting factor for forecast differences influenced by information of smaller-scale errors.The data assimilation and forecast with these optimal covariances show that,the corresponding analyses and forecasts can lead to superior quality,compared with those using covariances that just introduce influences of larger-or smallerscale errors.Due to the interactions among different scale errors included in optimal background error covariances,relevant analysis increments can properly describe weather systems(processes)at different scales,such as dynamic lifting,thermodynamic instability and advection of moisture at large scale,high-level and low-level jet at synoptic scale,and convective systems at mesoscale and small scale,as well as their interactions.As a result,the corresponding forecasts can be improved.

Re-examination of Trends Related to Tropical Cyclone Activity over the Western North Pacific Basin

In order to re-examine some trends related to tropical cyclones（TCs） over the western North Pacific since 1949,the unreliable maximum sustained wind(V;) recorded in the 1949-1978 TC best-track data from the Shanghai Typhoon Institute was modified based on the wind-pressure relationships（WPRs） in this study. Compared to the WPR scheme based on the cyclostrophic balance,the WPR scheme based on the gradient balance could give a better fit to TCs under higher wind speeds and could introduce smaller estimated errors for TCs locating at higher latitudes as well as TCs landing on the continent.After the V;modification based on minimum sea-level pressure and TC center latitude,the revised annual number of category 4-5 typhoons shows no long-term trend,while the potential destructiveness measured by power-dissipation index decreases slightly,and this trend is not significant in the period 1949-2008.

Impact of Surface Exchange Coefficients and Sea Surface Cooling on Tropical Cyclone Simulation

The surface flux exchange associated with the exchange coefficients and upper ocean conditions is essential to the development of tropical cyclones(TCs).Using the Weather Research and Forecasting(WRF)model,the present study has investigated the impact of exchange coefficients and ocean coupling during Super Typhoon Saomai(2006).Firstly,two experiments with different formula of roughness are conducted.The experiment with the Donelan formula for drag coefficient(C_(d))and ramped formula for enthalpy coefficient(C_(k))can simulate stronger intensity compared to other experiments due to the increased surface wind and enthalpy fluxes.That is because the new formulas allows for a smaller C_(d)and larger C_(k)in the high wind regime than the former formulas did.Moreover,two coupled simulations between WRF and a one-dimensional ocean model are conducted to examine the feedback of sea surface cooling to the TC.In the experiments with a horizontal uniform mixed layer depth of 70 m,the sea surface cooling is too weak to change the evolution of TC.While in the experiment with an input mixed layer calculated using the Hybrid Coordinate Ocean Model(HYCOM)data,the significant sea surface cooling induces obvious impact on TC intensity and structure.Under the negative feedback of sea surface cooling,the sensible and latent heat fluxes decreases,especially in the right part of Saomai(2006).The negative feedback with coupled ocean model plays a vital role in simulating the intensity and structure of TC.

Challenges in the Development of Film-Forming Additives for Lithium Ion Battery: A Review

Electrolytes additives are ubiquitous and indispensable in all electrochemical devices. In this sense, the principle and the classification of film-forming additives for lithium ion secondary batteries are described. The film formation mechanism and research progress of the pyrazole derivatives, organic halogenide, esters and derivatives, boron compounds and inorganic compounds are introduced. Emphasis is focused on the principles and film-forming mechanisms of each additive. The development of film-forming additives is forecasted and prospected.

Impact of Multi-Scale Oscillations at High and Low Latitudes on Two Persistent Heavy Rainfall Events in the Middle and Lower Reaches of the Yangtze River

To investigate the multi-scale features in two persistent heavy rainfall （PHR） events in the middle and lower reaches of the Yangtze River （MLRYR） in June of 1982 and 1998, this study examines the impact of multi-scale oscillations in the north and south of 30°N on the PHR events by performing sensitivity experiments with the Weather Research and Forecast （WRF） model. It is found that the 60-day low- pass perturbation made a trivial contribution to the MLRYR precipitation during the PHR event in 1982. This PHR event resulted mainly from the combined effects of 30-60-day oscillation at low latitudes and 10-30-day oscillation at both high and low latitudes. The southwesterly anomalies associated with the 30- 60-day anticyclonic anomaly over the northwestern Pacific facilitated moisture transport from the ocean to the MLRYR and enhanced the low-level convergence and ascending motion in the MLRYR. This similarly occurred in the 10-30-day oscillation as well. Moreover, the 10 30-day anomalies at high latitudes played a role in strengthening the large-scale low-level convergence over the MLRYR. The PHR event in 1998 was mainly related to the 60-day oscillation at both high and low latitudes and 30-60-day oscillation at low latitudes. The 60-day low-pass filtered anomalous cyclone at high latitudes in the north of 30°N contributed to the development of low-level convergence and ascending motion in northern MLRYR while the anomalous anticyclone at low latitudes in the south of 30°N not only increased the moisture in the MLRYR but also preconditioned the dynamical factors favorable for PHR over the whole area. The 30-60-day perturbations located north and south of 30°N worked together producing positive moisture anomaly in the MLRYR. In addition, the anomalous circulation in the south of 30°N tended to favor the development of ascending motion and low-level convergence in the MLRYR.

A New Mechanism of Convective Cell Regeneration and Development Within a Two-Dimensional Multicell Storm

In this study, based on simulations of a two-dimensional multicell storm under a ground-layer upshear （Uz〈0） by a mesoscale numerical model, a new mechanism of cell regeneration and development within the multicell storm at the ＂less than optimal shear＂ state.is proposed. In the presence of a ground-layer upshear, the circulation associated with the surface cold pool is not counteracted by that associated with the ambient wind sl^ear, and the density current extends out faster, making the multicell storm stay at the ＂less than optimal shear＂ state. As a result, a new cell is triggered by the strong vertical perturbation ahead of the mature convection, rather than by the split-up from the updraft at the leading edge of the surface cold pool as well as the gust front. The latter is the mechanism at the ＂optimal＂ state proposed by Lin et al. in 1998. In the new mechanism, the regenerated cell grows fast with the incident warm moist air from the upstream of the multicell storm, and tends to cut off the moist airflow into the mature convection at its western sector. Consequently, the mature convection would weaken, be replaced, and eventually decay. Actually, these two different mechanisms come into play in a way depending on the relationship between the circulation of the low-level shear and that of the cold pool. When the circulation of the cold pool is stronger than that of the wind shear, the multicell storm is at the ＂less than optimal shear＂ state, and the new convective cell is produced by the disturbance ahead of the mature cell. When the circulation of the cold pool is weaker, the cell regeneration is dominated by the mechanism at the ＂optimal＂ state, and the new cell is split from the gust front updraft. Therefore, these two mechanisms are not contradictive. With a moderate ground-layer upshear, they can alternately operate within a multicell storm.

Nonlinear Ekman Layer Theories and Their Applications

Based on the classical Ekman theory, a series of intermediate boundary layer models, which retain the nonlinear advective process while discard embellishments, have been proposed with the intention to understand the complex nonlinear features of the atmospheric boundary layer and its interaction with the free atmosphere. In this paper, the recent advances in the intermediate boundary-layer dynamic models are reviewed. Several intermediate models such as the boundary-layer models incorporating geostrophic momentum approximation, Ekman momentum approximation, and the weak nonlinear Ekman-layer model are a major theme. With inspection of the theoretical frameworks, the physical meaning and the limitations of each intermediate model axe discussed. It is found that the qualitative descriptions of the nonlinear nature in Ekman layer made by the intermediate models are fairly consistent though the details may be different. As the application of the intermediate models is concerned, the application of the intermediate models to the study of the topographic boundary layer, frontogenesis, low-level frontal structure, and low-level jet are especially summarized in this paper. It is shown that the intermediate boundary-layer models have great potential in illustrating the low-level structures of the weather and climate systems as they are coupled with the free atmospheric models. In addition, the important remaining scientific challenges and a prospectus for future research on the intermediate model are also discussed.

The Effect of Asymmetric Wind Structures of Tropical Cyclones on Their Tracks

A model is proposed relating a variety of factors of tropical cyclones (TCs)to their tracks, and attentions are paid to the influence of the asymmetric wind structures of TCs.Ideal numerical calculation shows that the asymmetric wind structures of TCs have conspicuousinfluence on their motion tracks. When moving due westward initially, an axisymmetric TC willdeflect right, and the overall trend is eastward. When it is asymmetric but the asymmetry is notsharp, wherever the area of maximum wind is, the TC deflects northwest first, and then, to variousareas depending on the positions of the areas of maximum wind, i.e. for the area of maximum wind inQuadrant Ⅰ, to the southeast; for in Quadrant Ⅱ, to the northeast; for in Quadrant Ⅲ, to thenorthwest; for in Quadrant IV, to the southwest. And in the above four cases, the TC tracks are allaccompanied by clockwise looping motions. With the asymmetry of the TC intensifiying, the track ofthe TC motion will be stretched further. For the area of maximum wind in Quadrants Ⅰ and Ⅱ, the TCtracks are still accompanied by clockwise looping motions; for the area of maximum wind inQuadrants Ⅲand Ⅳ, the TC tracks become oscillatory, without clockwise looping motions.

A Two-Wave Scheme for Orographic Gravity Wave Drag Parameterization

When the magnitude of sub-scale ographic forcing is comparable with explicitly ordinary dynamic forcing, the drag effect reduced by ographic gravity wave is to be significant for maintaining dynamic balance of atmospheric circulation, as well as the momentum and energy transport. Such sub-scale ographic forcing should be introduced into numerically atmospheric model by means of drag being parameterized. Furthermore, the currently mature ographic gravity wave drag （OGWD） parameterization, i.e., the so-called first-generation （based on lineal single-wave theoretical framework） or the second-generation drag parameterization （including an important extra forcing by the contribution of critical level absorption）, cannot correctly and effectly describe the vertical profile of wave stress under the influence of ambient wind shearing. Based on aforementioned consideration, a new two-wave scheme was proposed to parameterize the ographic gravity wave drag by means of freely propagating gravity waves. It starts with a second order WKB approximation, and treats the wave stress attenuations caused by either the selective critical level absorption or the classical critical level absorption explicitly; while in the regions where critical levels are absent, it transports the wave stress vertically by two sinusoidal waves and deposits them and then damps them according to the wave saturation criteria. This scheme is thus used to conduct some sample computations over the Dabie Mountain region of East China, as an example. The results showed that the new two-wave scheme is able to model the vertical distribution of the wave stress more realistically.