Altai-Sayan lee cyclogenesis: Numerical simulations

Summary In this paper we compare and analyse two cases of lee cyclogenesis observed near the Altai-Sayan massif (14–16 April 1988; 3–5 March 1986). The interpretation of the events is based on numerical simulations with an isentropic primitive equation model, where runs without orography are compared to those with realistic orography. The runs with mountains are successful in depicting the observed developments, so that the no-mountain experiments can be assumed to provide further information on the dynamics of both cases.The case of 14–16 April 1988 is dominated by rapid cyclogenesis and cut-off low formation aloft linked to the southward motion of a potential vorticity maximum towards the Altai-Sayan massif. Cyclone formation would have occurred without mountains as well. However, cyclogenesis at low levels is enhanced by the presence of the mountains through blocking of the cold air and corresponding deformation of the surface cold front. So this is a case of orographically modified cyclogenesis whereas the cyclogenesis of 3–5 March 1986 is induced by the mountain. In that case a broad and almost steady trough moves eastward over the Altai-Sayan region. Cyclone formation is absent in the no-mountain run. However, both cases exhibit some similarities with respect to the low-level developments induced by the mountains. A conceptual model is presented in order to further elucidate the key features of both cases.With 11 Figures     

A three-dimensional atmospheric trajectory model: Application to a case study of alpine lee cyclogenesis

Summary In this paper the problem of computing large-scale trajectories of atmospheric parcels is examined in the framework of a numerical limited-area model used for weather prediction. The parcel of air is considered to be a completely passive tracer, having no interaction with the environment, even when going through an area of deep atmospheric convection. The usual method of computing the trajectory after the integration, using model-produced wind fields every six hours, is compared with the method of computing the trajectory in an almost continuous fashion, during the integration of the model itself, providing in this way an estimate of the time-truncation errors associated to the first, more usual, method. This second method is found to be quite superior and can, in selected instances, lead to largely different results, as documented. Examples of beams of trajectories going through a strongly diffluent flow-field, as well as through an area of strong convection are shown and found to be very realistic, within the limitations imposed by model resolution. In addition, the sensitivity of trajectories to the definition of the vertical coordinate is explored and found to be large.     

Frontal modification and lee cyclogenesis in the Alps: A case study using the ALPEX reanalysis data set

Summary The interaction of a cold front with the Alps is studied by means of real-case numerical simulations for a case occurring at the end of the Alpine Experiment (ALPEX) on 28 April–2 May, 1982. Simulations are performed with the numerical weather prediction model chain Europa-Modell (EM) and its one-way nested high-resolution model (HM) of the German and Swiss Weather Services. The outer EM simulation (56 km horizontal resolution) uses initial and lateral boundary fields taken from the ALPEX-IIIb reanalysis data set. This reanalysis data set is based on the operational EM analysis scheme, but takes into consideration a wide range of field phase data taken during the ALPEX field campaign. A comparison of simulations driven by the ALPEX and ECMWF reanalysis (ERA) data is performed. The simulation driven by the former captures the intensity and vertical depth of the developing lee cyclone substantially better than the corresponding run driven by the ERA. The transient development of the impingement of a cold front on the Alps induces a wide range of mesoscale phenomenon such as flow splitting, mistral, north foehn, cyclogenesis, anticyclonic vortex and bora. These flow evolutions are analyzed using HM simulations with a horizontal resolution of 14 km and visualized performing comprehensive trajectory calculations. Received September 9, 2000/Revised November 28, 2000     

Numerical simulations on the explosive cyclogenesis over the kuroshio current

In this paper, the Pennsylvania State University-NCAR Mesoscale Model (MM4) is used to investigate the explosive oceanic cyclone of 14-15 March 1988 over the warm Kuroshio Current. A series of numerical simulations on this cyclogenesis indicates that the favorable weather condi-tions and strong baroclinity in the low- and middle-level are essential to its explosive development. The explosive cyclogenesis occurred over a wide range of sea surface temperatures (SST’s), which was then characterized by strong baroclinity, the low-level jet (LLJ) was initially formed under the favorable atmospheric circulation and then this LLJ advected the moisture and heat northward for the explosive development of the cyclone, the LLJ played an important role in the process of cyclogenesis. Sensitivity experiments show that the latent heating was a key factor to explosive cyclogenesis, the latent heating deepened the short-wave trough, which resulted in the rapid intensification of the cyclone; while in the explosive intensification stage and continuous de-velopment stage, there was less contribution of local surface processes for the explosion of the cy?clone.     

Normal mode theory of lee cyclogenesis and the initial value problem

Summary An initial value problem is proposed to study the influence of mountains upon atmospheric flows. The normal mode, quasigeostrophic theory of lee cyclogenesis is revisited; in order to model Alpine cyclogenesis, the flow, upon which the mountain acts, should represent a cyclone wave propagating eastward in a sheared zonal current. Two different models are considered in order to describe cyclone waves: an unstable Eady normal mode and a finite amplitude, neutral Rossby mode in the two layer model. It is shown that the topographic modifications proper of normal modes emerge very rapidly from the integration of the initial value problem. It is also shown that an enhanced deepening of the upper level trough can occur when finite amplitude effects are taken into account.With 8 Figures     

A numerical case study of a cold air outbreak over East Asia during late spring

Summary A case study of a late spring cold air outbreak associated with heavy rainfall and snow storms over East Asia is carried out with a numerical model. This was the most severe case over northern China in 50 years. In the lower troposphere, the evolution of the edge anticyclone and the northerly low-level orographic jet east of Qinghai-Tibetan Plateau are well simulated in the ECMWF limited area model but the associated cold surge is under predicted. In the experiments with envelope and valley filled orography, results are improved. Neither cold surge nor strong frontogenesis is simulated in a no latent heating experiment, indicating that the feedback of latent heat release is quite essential to the burst of cold air in this case.With 9 Figures     

Diagnosis of mesoscale structures in cases of lee cyclogenesis during ALPEX

Summary Several cases of lee cyclogenesis that occurred during ALPEX-SOP have been analyzed, with the aim of separating the large scale structures from the subsynoptic/meso-a scale features that are characteristic of this meteorological phenomenon. The results presented here are mainly based on composites of the analyzed cases. We assume that the deformation caused by the orography can be, at least to a reasonable extent, isolated from the undisturbed state using scale separation. The analysis technique we employ provides the scale separation as built-in in the interpolation algorithm. The scale separation error due to the large inhomogeneities of the data density distribution is partly corrected using a method described in the text. The orographic disturbance appears in different mesoscale fields as a quasiantisymmetric dipolar structure. For example, high/low pressure, cold/warm temperature and anticyclonic/cyclonic couplets characterize the mesoscale fields near the Alps. A qualitative agreement is found with the structure of the orographically induced perturbations predicted by the normal mode theory of lee cyclogenesis.With 7 Figures     

Numerical study of 1998 late summer flood in East Asia

The Purdue Regional Model (PRM) is applied to study the evolution of regional climate and weather systems during the heavy precipitation over Korea and China between 30 July and 18 August 1998. The results show that heavy rainfall along the Mei-yu and Changma front was due to the combination of: (1) an anomalous 850 hPa subtropical high, (2) a stronger baroclinicity around 40°N over eastern Asia and a low pressure located to the north of the front, and (3) an excessive evaporation from abnormal wet, warm land. The precipitation ended by 18 August when the subtropical high had retreated and the low pressure in Mongolia moved away from Asia continent. The model reproduced in great detail the observed baroclinic waves to the north, subtropical high and low-level jet to the south, and the front with heavy precipitation extending from southern China, and the Korean peninsula to Japan. High correlations are found for mass, momentum, and moisture fields between model simulation and the European Center for Medium Range Weather Forecast (ECMWF) reanalysis for the 20-day means.     

Synergism of upper-level potential vorticity and mountains in Genoa lee cyclogenesis – A numerical study

Summary The interaction of topography and upper-level potential vorticity (PV) anomaly in intensive case of Alpine lee cyclogenesis (3–6 March 1982) is investigated. The factor separation method is used in conjunction with the PV inversion technique to isolate individual roles of topography and upper-level PV as well as their synergic nonlinear effect. The application of the factor separation method allows to separate low and upper tropospheric dynamics in the real case of lee cyclogenesis and to estimate quantitatively the pure and interactive contributions of topography and upper tropospheric PV anomaly to the pressure deepening in the lee of the Alps. The PV-topography interactive effect was found to be strong and comparable to the pure PV advection contribution. It is shown that the synergic contribution is responsible for the dipole structure oriented exactly as predicted by theory and as found in the observations. The “pure” topography contribution is small during the “trigger” phase but becomes strongly cyclolytic (i.e., assisting cyclone decay) in the second phase of the lee development. Superposition of the pressure change patterns produced by the two factors along with their interaction, results in a strong deepening in the right location. The joint cyclogenetic action is proposed as the explanation for the fast pressure fall during the “trigger” phase. Received July 31, 2000/Revised June 4, 2001     

A numerical study on the winter monsoon and cold surge over East Asia

By using the improved regional climate model （RegCM_NCC）, a numerical study has been undertaken for the East Asia region over a period of 5 years （1998-2002） in an effort to evaluate the model＇s ability to reproduce the winter monsoon conditions that were observed. The results showed that the model can successfully simulate the basic characteristics of the winter monsoon circulations, including the location and intensity of the cold-surface, high-pressure system, as well as the wind patterns and the intensity of the winter monsoon. The simulated occurrence frequency and regions of the cold surge were consistent with the observations. The simulated rainfall distribution over China was consistent with the observations collected in South China. The features of the simulated moisture transport were also in good agreement with the observations that were derived from the NCEP reanalysis data, indicating that moisture transport coming from the Bay of Bengal trough plays a crucial role in supplying moisture needed for precipitation in South China. In addition, the moisture transport coming from the near-equatorial west-Pacific was also important. These two branches of moisture transport converged in South China, as a prerequisite for occurrence of the precipitation that was observed there. Heat budgets have shown that the development of a heat sink over the East Asian continent was remarkable and its thermal contrast relative to the neighboring seas was the important forcing factor for the winter monsoon activity. The simulation also indicated that the significant differences in circulation patterns and rainfalls during the winters of 1997/98 and 1998/99 were affected by cold and warm ENSO events, respectively. The above analysis demonstrated the model＇s ability to simulate the East Asian winter monsoon.     

A mechanism of Alpine lee cyclogenesis as revealed by a quasigeostrophic variational filter

Summary Mechanisms associated with Alpine lee cyclogenesis during the early phase of their generation are investigated using a variational quasigeostrophic filter technique. It was possible to extract the quasigeostrophic signal from the available analyzed real data set.The results presented here are for the 11–12 March 1982, an example of so-called orographically induced lee cyclogenesis. Non-quasigeostrophic fields, calculated as a difference between observations and the quasigeostrophic fields, show significant magnitudes indicating the possible importance of non-quasigeostrophic processes. A dipole structure in the residual geopotential field was observed, similar to the results of numerical model experiments. Also, a strong upper-level non-quasigeostrophic divergence was found in the Alpine region 24 hours prior to lee cyclogenesis, lasting for 6–12 hours. On the other hand, quasigeostrophic results indicate only a local effect of mountain slopes, suggesting possibly a dominant role of the low-level blocking. A hypothetical scenario of Alpine lee cyclogenesis is proposed, based on results obtained here.With 14 Figures     

Numerical analysis of the influence of jets,fronts, and mountains on alpine lee cyclogenesis: More cases from the ALPEX SOP

Summary In a recently published work it was shown that Alpine cyclone development depends on a short wave trough upstream of the Alps, the presence of an upper-level jet streak in this trough, and a low-level front interacting with the mountain barrier. Specifically, by modifying the strength of the upper baroclinic zone (potential vorticity) or by decreasing the strength of the low-level baroclinic zone impinging on the Alps in an initial field, the intensity of the resulting numerically predicted cyclogenesis could be modulated. The major finding of the work was that the ALPEX cyclones reacted differently to upper-level and lower-level modification, providing a basis for broadly classifying storms: that is, dependence on upper-level processes and on low-level processes.The present work extends this study by considering additional cyclones that occurred during the ALPEX Special Observing Period (SOP), and describes the influence of upper-level and lower-level processes on each. An index (I) discussed in the previous paper is examined in terms of its relative value from case to case, and in terms of its value in a time-dependent sense during the lifetime of the storm. The results show that the most powerful lee cyclones depended more on the strength of the upper-level jet or potential vorticity than on the strength of the low-level front of baroclinic zone. In most cases the time evolution ofI showed the influence of the upper-level jet to be important during the early phase of development. The front is increasingly important in midlife as intensification takes place, and the jet is increasingly important during late life. These conclusions were reached during the earlier study but are supported by the additional SOP cyclone cases.Cyclone steering flow and the impinging of the storm on the Alps had a great impact on defining its character: storms with NW-N trajectories were heavily dependent on low level frontal strength and WSW-WNW trajectories were dominated by effects of the upper level jet.With 16 Figures     

东亚地区春季硫氧化物浓度分布及其收支研究

利用一个耦合的区域化学输送模式系统,模拟了2001年3~4月间东亚区域二氧化硫(SO2)和硫酸盐(SO42-)气溶胶浓度的时空分布,并分析了它们的来源、输送、转化和沉降过程在浓度形成中的作用。为了检验模拟结果的合理性,侧重分析了4月9~12日SO2的输送和转化过程。在这一期间,SO2浓度观测资料显示,日本的观测站Happo、Oki和Hedo周围及日本海部分海域先后经历了一次SO2浓度高值。模拟结果表明,日本东京附近的三宅岛火山喷发严重影响了日本列岛和周边海域SO2和SO42-气溶胶的分布,是引起观测站周围及日本海部分海域SO2浓度高值的主要原因。3月1日至4月31日间的硫收支分析显示,气相与液相化学过程在SO2的氧化过程中具有同等的重要性,模拟区域排放的硫氧化物中有约42%(其中约25%为SO2)被输送到模拟区域以外,约57%(约35%为湿沉降)沉积在模拟区域内。     

A modeling study of the effects of aerosols on clouds and precipitation over East Asia

The National Center for Atmospheric Research Community Atmosphere Model (version 3.5) coupled with the Morrison?CGettelman two-moment cloud microphysics scheme is employed to simulate the aerosol effects on clouds and precipitation in two numerical experiments, one representing present-day conditions (year?2000) and the other the pre-industrial conditions (year?1750) over East Asia by considering both direct and indirect aerosol effects. To isolate the aerosol effects, we used the same set of boundary conditions and only altered the aerosol emissions in both experiments. The simulated results show that the cloud microphysical properties are markedly affected by the increase in aerosols, especially for the column cloud droplet number concentration (DNC), liquid water path (LWP), and the cloud droplet effective radius (DER). With increased aerosols, DNC and LWP have been increased by 137% and 28%, respectively, while DER is reduced by 20%. Precipitation rates in East Asia and East China are reduced by 5.8% and 13%, respectively, by both the aerosol??s second indirect effect and the radiative forcing that enhanced atmospheric stability associated with the aerosol direct and first indirect effects. The significant reduction in summer precipitation in East Asia is also consistent with the weakening of the East Asian summer monsoon, resulting from the decreasing thermodynamic contrast between the Asian landmass and the surrounding oceans induced by the aerosol??s radiative effects. The increase in aerosols reduces the surface net shortwave radiative flux over the East Asia landmass, which leads to the reduction of the land surface temperature. With minimal changes in the sea surface temperature, hence, the weakening of the East Asian summer monsoon further enhances the reduction of summer precipitation over East Asia.