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1.
The objective of this study is to describe how a monsoon depression in the western North Pacific, which typically has a diameter of 1000 km, may be transitioned into a tropical cyclone with an inner core of strong winds and deep convection on the order of 100 km. Our previous case study of the pre-Typhoon Man-Yi monsoon depression formation is extended to show that the same cross-equatorial airstream continued and led to enhanced equatorial westerlies on the equatorward side of the pre-Man-Yi circulation, and a surge in the trade easterlies was also present on the poleward side. As these surges in the near-equatorial flow are inertially unstable, inward-directed wave-activity fluxes then led to flux convergence over the eastern vorticity maximum of the monsoon depression, which resulted in a scale contraction to that of a pretropical cyclone seedling. Eight case studies of the transitions of monsoon depressions during 2009 are presented that document persistent inward-directed wave-activity fluxes over a vorticity maximum within the monsoon depression is a key feature of each transition. In some transitions, the same cross-equatorial airstream as led to the monsoon depression formation continues as the primary airstream, and in other transitions another airstream to the west or enhanced tropical easterlies become the primary airstream. Analysis of 10 non-transitioning monsoon depressions during 2009 indicated the airstream wave-activity flux did not persist after the formation of the monsoon depression. In another 11 non-transitioning monsoon depressions, the inward-directed wave-activity flux was small and no region of wave-activity flux convergence was associated with the vorticity maximum in the monsoon depression.  相似文献   

2.
Summary In this paper we address the issue of monsoon forecasts in relation to the organization of convection. Given a physical initialization procedure, within a data assimilation, it is possible to use the detailed distribution of rainfall from mesoconvective precipitating elements to define the initial state of a global model. If that is carried out using a very high resolution model then the initial state can carry within it an organization of convection within the resolvable scales. Then the impact of physical initialization on the maintenance and prediction of tropical weather such as the monsoon can be determined. Lacking such an initialization, one can expect the convectively driven energetics to be biased, and a slow degradation of the forecasts can follow. Several examples of forecasts at different resolutions are discussed here. The main findings of this study are that improved forecast results are obtained when physical initialization is invoked where the observed rain and the model resolution are comparable, i.e. the footprint of the highest resolutions rainfall estimates obtained from satellite based data sets (principally we use the SSM/I instrument over the oceans). At this resolution, we note that the model is able to carry an organization of convection in the initialization and in the forecasts through the medium-range time scale.We have compared our results of monsoon studies at a resolution T255 with those at resolution T62. The transform grid separation at the resolution T255 is approximately 50 km and at the resolution T62, it is approximately 200 km. We find that the model at the higher resolution (T255) performs better and has more realistic energy conversions for the convectively driven synoptic scale monsoon.An organization of convection, at the synoptic scales, is not seen in the forecasts at lower resolutions, T62, where the rainfall patterns are generally much broader and tend to be more zonal. Such organization appears more realistic at the resolution T255. Variances of the energy conversion, calculated in the two-dimensional spectral space, from physically initialized short range forecasts at the higher resolution are seen to be largest on the scales of the monsoon. Similar calculations for the reanalyzed fields at lower resolutions show the spectral distribution of variances to be biased towards local Hadley scale overturnings.With 12 Figures  相似文献   

3.
对2016-2020年全球模式ECMWF和区域模式GZ_GRAPES、基于模式的解释应用和广东省气象局发布的定量降水预报(QPF)进行检验和评估.结果表明:ECMWF和GZ_GRAPES模式对一般性降水预报技巧在逐年提升,对大雨或以上的降水预报技巧的提升缓慢.GZ_GRAPES对大雨以上降水的预报技巧和定量降水预报的精...  相似文献   

4.

The role of the Madden–Julian Oscillation (MJO) in producing active and break periods of the South American (SA) monsoon and the performance of the ECMWF and NCEP models in predicting these periods at multiweek lead times are assessed. Two monsoon indices, based on precipitation and wind, are proposed to characterize these periods. The models represent well the observed association of active and break monsoon days with large scale convection and circulation anomalies. Although reproducing approximately the distribution of active and break days proportions in each phase of the MJO cycle, models produce a phase shift between observed and simulated distributions because they establish the teleconnection between Central Pacific and South America, as well as its impacts, sooner than in observations. The predictive skill of both rainfall and wind anomalies is limited to about 2 weeks, with the monsoon wind index displaying higher correlation score till week 3. The forecast performance is apparently not affected by initialization on active or break monsoon days. However, it is higher for prediction of lower precipitation in break days than heavier rainfall in active days. Wind is much better predicted than rainfall for active days, which could be used for extreme rainfall events forecast. Although relatively small at shorter lead times, the MJO contribution is the major source of rainfall predictability after week 3. To improve the multiweek prediction of SA monsoon, models need not only to predict correctly the MJO phase, but also to reproduce in the right phase the MJO-related SA rainfall anomalies.

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5.
The real-time multi-model ensemble (MME)-based extended range (up to 3 weeks) forecast of monsoon rainfall over India during the 2012 monsoon season is analyzed using the outputs of European Centre for Medium Range Weather Forecasts (ECMWF) monthly forecast coupled model, National Centre for Environmental Prediction (NCEP) Climate Forecast System version 2 coupled model and Japan Meteorological Agency (JMA)’ ensemble prediction system. Although the individual models show useful skill in predicting the extended range forecast of monsoon, the MME forecast is found to be superior compared to these. For the country as a whole, the correlation coefficient (CC) between the observed and MME forecast rainfall departure is found to be statistically significant (99 % level) at least for 2 weeks (up to 18 days). Over the four homogeneous regions of India, the CC is found to be significant (above 95 % level) up to 2 weeks except in case of northeast India, which shows significant CC for week 1 (days 5–11) only. On the meteorological subdivision level (India is divided into 36 meteorological subdivisions) the mean percentage of correct forecast is found to be much higher than the climatology forecast. Considering the complex problem of forecasting of monsoon in the extended range timescales, the MME-based predictions for 2–3 weeks provide skillful results and useful guidance for application in agriculture and other sectors in India.  相似文献   

6.
Performance of national centers for environmental prediction based global forecast system (GFS) T574/L64 and GFS T382/L64 over Indian region has been evaluated for the summer monsoon season of 2011. The real-time model outputs are generated daily at India Meteorological Department, New Delhi for the forecasts up to 7 days. Verification of rainfall forecasts has been carried out against observed rainfall analysis. Performance of the model is also examined in terms of lower tropospheric wind circulation, vertical structure of specific humidity and precipitable water content. Case study of a monsoon depression is also illustrated. Results obtained show that, in general, both the GFS T382 and T574 forecasts are skillful to capture climatologically heavy rainfall regions. However, the accuracy in prediction of location and magnitude of rainfall fluctuates considerably. The verification results, at the spatial scale of 50 km resolution, in a regional spatial scale and country as a whole, in terms of continuous skill score, time series and categorical statistics, have demonstrated superiority of GFS T574 against T382 over Indian region. Both the model shows bias of lower tropospheric drying and upper tropospheric moistening. A bias of anti-cyclonic circulation in the lower tropospheric level lay over the central India, where rainfall as well as precipitable water content shows negative bias. Considerable differences between GFS T574 and T382 are noticed in the structure of model bias in terms of lower tropospheric wind circulation, vertical structure of specific humidity and precipitable water contents. The magnitude of error for these parameters increases with forecast lead time in both GFS T574 and T382. The results documented are expected to be useful to the forecasters, monsoon researchers and modeling community.  相似文献   

7.
A low pressure system that formed on 21 September 2006 over eastern India/Bay of Bengal intensified into a monsoon depression resulting in copious rainfall over north-eastern and central parts of India. Four numerical experiments are performed to examine the performance of assimilation schemes in simulating this monsoon depression using the Fifth Generation Mesoscale Model (MM5). Forecasts from a base simulation (with no data assimilation), a four-dimensional data assimilation (FDDA) system, a simple surface data assimilation (SDA) system coupled with FDDA, and a flux-adjusting SDA system (FASDAS) coupled with FDDA are compared with each other and with observations. The model is initialized with Global Forecast System (GFS) forecast fields starting from 19 September 2006, with assimilation being done for the first 24 hours using conventional observations, sounding and surface data of temperature and moisture from Advanced TIROS Operational Vertical Sounder satellite and surface wind data over the ocean from QuikSCAT. Forecasts are then made from these assimilated states. In general, results indicate that the FASDAS forecast provides more realistic prognostic fields as compared to the other three forecasts. When compared with other forecasts, results indicate that the FASDAS forecast yielded lower root-mean-square (r.m.s.) errors for the pressure field and improved simulations of surface/near-surface temperature, moisture, sensible and latent heat fluxes, and potential vorticity. Heat and moisture budget analyses to assess the simulation of convection revealed that the two forecasts with the surface data assimilation (SDA and FASDAS) are superior to the base and FDDA forecasts. An important conclusion is that, even though monsoon depressions are large synoptic systems, mesoscale features including rainfall are affected by surface processes. Enhanced representation of land-surface processes provides a significant improvement in the model performance even under active monsoon conditions where the synoptic forcings are expected to be dominant.  相似文献   

8.
Mathew Roxy 《Climate Dynamics》2014,43(5-6):1159-1169
Over the tropical oceans, higher sea surface temperatures (SST, above 26 °C) in summer are generally accompanied by increased precipitation. However, it has been argued for the last three decades that, any monotonic increase in precipitation with respect to SST is limited to an upper threshold of 28–29.5 °C, and beyond this, the relationship fails. Based on this assessment it has often been presumed that, since the mean SSTs over the Asian monsoon basins (Indian Ocean and north-west Pacific) are mostly above the threshold, SST does not play an active role on the summer monsoon variability. It also implies that increasing SSTs due to a changing climate need not result in increasing monsoon precipitation. The current study shows that the response of precipitation to SST has a time lag, that too with a spatial variability over the monsoon basins. Taking this lag into account, the results here show that enhanced convection occurs even up to the SST maxima of 31 °C averaged over these basins, challenging any claim of an upper threshold for the SST-convection variability. The study provides us with a novel method to quantify the SST-precipitation relationship. The rate of increase is similar across the basins, with precipitation increasing at ~2 mm day?1 for an increase of 1 °C in SST. This means that even the high SSTs over the monsoon basins do play an active role on the monsoon variability, challenging previous assumptions. Since the response of precipitation to SST variability is visible in a few days, it would also imply that including realistic ocean–atmosphere coupling is crucial even for short term monsoon weather forecasts. Though recent studies suggest a weakening of the monsoon circulation over the last few decades, results here suggest an increased precipitation over the tropical monsoon regions, in a global warming environment with increased SSTs. Thus the signature of SST is found to be significant for the Asian summer monsoon, in a quantifiable manner, seamlessly through all the timescales—from short-term intraseasonal to long-term climate scales.  相似文献   

9.
We have evaluated the simulation of Indian summer monsoon and its intraseasonal oscillations in the National Centers for Environmental Prediction climate forecast system model version 2 (CFSv2). The dry bias over the Indian landmass in the mean monsoon rainfall is one of the major concerns. In spite of this dry bias, CFSv2 shows a reasonable northward propagation of convection at intraseasonal (30–60 day) time scale. In order to document and understand this dry bias over the Indian landmass in CFSv2 simulations, a two pronged investigation is carried out on the two major facets of Indian summer monsoon: one, the air–sea interactions and two, the large scale vertical heating structure in the model. Our analysis shows a possible bias in the co-evolution of convection and sea surface temperature in CFSv2 over the equatorial Indian Ocean. It is also found that the simulated large scale vertical heat source (Q1) and moisture sink (Q2) over the Indian region are biased relative to observational estimates. Finally, this study provides a possible explanation for the dry precipitation bias over the Indian landmass in the simulated mean monsoon on the basis of the biases associated with the simulated ocean–atmospheric processes and the vertical heating structure. This study also throws some light on the puzzle of CFSv2 exhibiting a reasonable northward propagation at the intraseasonal time scale (30–60 day) despite a drier monsoon over the Indian land mass.  相似文献   

10.
Two numerical experiments were performed by using ECMWF limited area model in order to assess theimpact of radiative processes on the onset of summer monsoon.The control experiment includes all model physicsand the other experiment is exclusive of radiative processes.The simulated results show that the radiativeprocesses enhanced the monsoon circulation and the associated vertical circulation over South Asia.Especiallyover the Qinghai-Xizang Plateau,the build-up of the plateau monsoon depression and the beginning of the plat-eau rainy season are highly dependent upon the radiative processes.Through the radiation-cloud interaction,the “moist processes” were enhanced over land.More latent heatwas released.The feedback of the physical processes caused the strengthening of the monsoon circulation oversouthern Asia.  相似文献   

11.
The forecast of summertime afternoon convection by numerical weather prediction models is highly challenging because of its weak dynamical forcing, small scale, and low predictability. To assess such an ability for future improvement, we evaluated the performance of the 2.5-km Cloud-Resolving Storm Simulator (CReSS) in predicting afternoon convection in Taiwan under weak synoptic conditions during the summers (May–October) of 2011 and 2012. For a total of 89 target days, daily CReSS forecasts in three ranges, starting at 0000 UTC (0800 LST) on the same day (D0), the day before (D-1), and 2 days before (D-2), were examined. With regard to the occurrence of afternoon convection anywhere in Taiwan, the predictive skill of the model was considerably high, as evidenced by the threat score (TS) and post-agreement (PA) of D0 (D-1) forecasts reaching 0.75 and 0.90 (0.50 and 0.79), respectively. While the score values decrease when Taiwan is divided into four regions (i.e., the forecast must be in the correct region to be considered a hit), the TS and PA for D0 (D-2) forecasts remain respectable at 0.44 and 0.73 (0.29 and 0.59). Among the four regions, the TS (0.48–0.65) and PA (0.77–0.83) for Central Taiwan are the highest with the best predictive skill. Overall, while the prediction of afternoon convection in the correct region is challenging, the 2.5-km CReSS model has considerable skill (TS ~ 0.30) even 2 days in advance, and can provide useful guidance for afternoon convection in Taiwan.  相似文献   

12.
基于1958~2002年ECMWF再分析资料,我国160个台站降水和气温资料,从夏季高原季风环流系统特点出发,定义了能较好表征高原夏季风环流变化的特征指数,分析了高原夏季风年际、年代际变化特征,并揭示了高原夏季风强弱异常时的环流特征及其与中国夏季降水和气温的关系,主要结论为:(1)用6~8月600hPa(27.5~30°N,80~100°E)范围内平均的西风分量距平与(35~37.5°N,80~100°E)范围内平均的东风分量距平差定义了高原夏季风指数(PM I)。该指数计算简单,意义清楚,代表性好。(2)1958~2002年高原夏季风整体呈增强趋势,在20世纪60年代中期之前是高原夏季风的强盛期,之后是高原夏季风弱期,在80年代以后又转为季风强期。(3)高原夏季风与中国夏季降水和气温相关很好。将该指数与之前汤懋苍定义的指数进行性能综合比较后,发现该指数对川渝地区的夏季降水及气温有更好的指示意义。  相似文献   

13.
The performance of the ECMWF 32-day ensemble predictions of western North Pacific tropical cyclone events (formation plus track) made once a week during the 2009 season is evaluated with the same procedures as for the 2008 season. Seventeen of the 23 tropical cyclones during the 2009 season occurred during multiple storm scenarios that are more difficult to predict, and many of the deficient track predictions involved unusual and rapidly changing tracks that typically involve interactions with adjacent synoptic circulations that are not predictable on intraseasonal timescales (10–30 days). Such incorrect predictions of the duration and tracks of these multiple cyclones were found to degrade the performance in predicting subsequent tropical cyclone formations and tracks during the 32-day integration. Predominantly northward tracks throughout the life cycle tended to be less predictable on intraseasonal timescales. Given these caveats, the overall performance of the ECMWF ensemble for the 12 typhoons was more successful than during the 2008 season. However, the performance for three tropical storms during the 2009 season was less successful due to the difficult track forecast scenarios. A surprisingly good performance was found in predicting the formation location and early track segments of eight minimal tropical storms or tropical depressions. The less satisfactory aspect for many of the late season tropical depressions was that the ECMWF ensemble continued to predict member vortices for extended periods after the system had actually dissipated.  相似文献   

14.
李崇银  阎敬华 《气象学报》1992,50(2):129-139
本文利用广东热带所的数值预报模式和ECMWF的格点资料,对两个南海低压进行了对比数值分析。其中一个低压在24小时内发展为台风,而另一个不发展。分析发现,二者初始场的温、湿、风、压结构都存在着显著差异。最为明显的是,发展者低层有明显的暖心结构和深厚的高湿层,使得条件不稳定层深厚;而不发展者暖心在高层明显,低层无暖心,而且,中低层湿度较小,所以条件不稳定层较浅薄。两个低压的不同发展趋势都得到了很好的数值模拟结果。利用模式输出结果的对比分析,讨论了南海热带气旋发展与不发展的最基本物理过程及其作用。表明对流层中低层的深厚高温层的存在及相应的温度结构和风场所导致的较强的对流加热的维持和加强,对低压的发展极为重要。  相似文献   

15.
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16.
Summary An intercomparison of the characteristic features of the Indian summer monsoon has been carried out for the monsoon months (June to September) of 1995 using the mean monthly analyses/forecasts from the operational centres of ECMWF, JMA, UKMO and NCMRWF. This exercise was undertaken to determine how well the large scale monsoon features over India were reproduced in the operational output in 1995 and also to assess the performance of the NCMRWF assimilation/forecast system. For this purpose, precipitation, mean sea level pressure, circulation features in the lower (850 hPa) and upper (200 hPa) troposphere, mid-tropospheric (500 hPa) temperature, and latent heat flux were examined.It is found that all the dominant features of the Indian summer monsoon are fairly well represented in the analysis and medium range forecasts of the ECMWF, JMA and UKMO. The NCMRWF output agrees well with those from other centres except for a sharp gradient in precipitation across the west coast which was not captured well in the forecasts due to the relatively coarse horizontal resolution of the model compared to that used at other operational centres. Other important features of the southwest monsoon, like the heat low over the northwestern part of the country, the lower level westerly jet and upper level easterly jet etc. are found to be reasonably well represented in the output of all operational centres. The JMA analyses and forecasts possessed greater levels of moisture compared to the NCMRWF output possibly due to the synthetic moisture information used at JMA. The evolution characteristics of the summer monsoon onset over the southern tip of India are found to be comparable in the output of JMA and NCMRWF.With 13 Figures  相似文献   

17.
The objective of this study is to provide evidence of predictability on intraseasonal time scales (10–30 days) for western North Pacific tropical cyclone formation and subsequent tracks using the 51-member ECMWF 32-day forecasts made once a week from 5 June through 25 December 2008. Ensemble storms are defined by grouping ensemble member vortices whose positions are within a specified separation distance that is equal to 180 n mi at the initial forecast time t and increases linearly to 420 n mi at Day 14 and then is constant. The 12-h track segments are calculated with a Weighted-Mean Vector Motion technique in which the weighting factor is inversely proportional to the distance from the endpoint of the previous 12-h motion vector. Seventy-six percent of the ensemble storms had five or fewer member vortices. On average, the ensemble storms begin 2.5 days before the first entry of the Joint Typhoon Warning Center (JTWC) best-track file, tend to translate too slowly in the deep tropics, and persist for longer periods over land. A strict objective matching technique with the JTWC storms is combined with a second subjective procedure that is then applied to identify nearby ensemble storms that would indicate a greater likelihood of a tropical cyclone developing in that region with that track orientation. The ensemble storms identified in the ECMWF 32-day forecasts provided guidance on intraseasonal timescales of the formations and tracks of the three strongest typhoons and two other typhoons, but not for two early season typhoons and the late season Dolphin. Four strong tropical storms were predicted consistently over Week-1 through Week-4, as was one weak tropical storm. Two other weak tropical storms, three tropical cyclones that developed from precursor baroclinic systems, and three other tropical depressions were not predicted on intraseasonal timescales. At least for the strongest tropical cyclones during the peak season, the ECMWF 32-day ensemble provides guidance of formation and tracks on 10–30 day timescales.  相似文献   

18.
The mechanism responsible for high rainfall over the Indian west coast region has been investigated by studying dynamical, thermodynamical and microphysical processes over the region for the monsoon season of 2009. The European Centre for Medium-Range Weather Forecasts wind and NCEP flux data have been used to study the large scale dynamical parameters. The moist adiabatic and multi-level inversion stratifications are found to exist during the high and low rainfall spells, respectively. In the moist adiabatic stratification regime, shallow and deep convective clouds are found coexisting. The Cloud Aerosol Interaction and Precipitation Enhancement EXperiment aircraft data showed cloud updraft spectrum ranging from 1 to 10 m s?1 having modal speed 1–2.5 m s?1. The low updrafts rates provide sufficient time required for warm rain processes to produce rainfall from shallow clouds. The low cloud liquid water is observed above the freezing level indicating efficient warm rain process. The updrafts at the high spectrum end go above freezing level to generate ice particles produced due to mixed-phase rainfall process from deep convective clouds. With aging, deep convection gets transformed into stratiform type, which has been inferred through the vertical distribution of the large scale omega and heating fields. The stratiform heating, high latent heat flux, strong wind shear in the lower and middle tropospheric levels and low level convergence support the sustenance of convection for longer time to produce high rainfall spell. The advection of warm dry air in the middle tropospheric regions inhibits the convection and produce low rainfall spell. The mechanisms producing these spells have been summarized with the block diagram.  相似文献   

19.
北半球七层原始方程谱模式ff考虑了较完整的物理过程:（1）地形作用（采用了σ坐标系）和地面摩擦；（2）动量、热量，水汽的水平扩散过程和垂直扩散过程（其中包括了下垫面感热和水汽作用）；（3）水汽凝结潜热作用；（4）云和辐射作用；（5）积云对流过程等。上述模式采用了非线性平衡方程初值化方法。并利用逐日客观分析资料，制作5天中期数值预报试验。本文给出了1982年冬季和夏季的20个例子的五天数值预报验证结果和一个例子的中期数值预报图和对应的实况图。预报效果比较好。在数值预报试验中，虽然纬向波数只取15个波，但所取得的短期、中期的预报效果能和一些较高分辨率模式的预报效果相比较。  相似文献   

20.
The three-dimensional variational data assimilation (3DVAR) technique in the advanced weather research and forecast model is used to study the impact of assimilating Moderate Resolution Spectroradiometer (MODIS) retrieved temperature and humidity profiles on the dynamic and thermodynamic features for three monsoon depressions over the Bay of Bengal, India. For better understanding of the role of various physical processes in the evolution of monsoon depression, a detailed diagnostic study is performed on all the three depression cases. Numerical experiments were conducted in a system of two-way nested domains with a horizontal resolution of 36 and 12 km, respectively. The assimilation of MODIS data did improve the mean sea level pressure patterns and spatial distribution of rainfall patterns in all the three monsoon depression cases studied. Higher values of equitable threat score and lower bias values are seen consistently for the entire rainfall threshold range and for all the three depression cases with 3DVAR assimilation of MODIS temperature and humidity profiles. The current operational regional models in India do not ingest the MODIS temperature and humidity profiles and hence the present study is particularly relevant to the operational forecasting community in India in their ongoing efforts to improve weather forecasting over India.  相似文献   

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