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1.
In this study, the dynamics of track deflection associated with Tropical Cyclone (TC) Sidr (2007) are explored using a numerical weather prediction model. It is found that (a) the simulated track of Sidr is sensitive to flow, orographic, and model vertical structure that change the environmental steering flow leading to the track deflection. In particular, the track of TC Sidr is deflected northwestward for cases with lower domain height, horizontal domain covering only part of Himalaya mountains, and varying mountain heights; (b) the simulated track of TC Sidr, when compared with GFS reanalysis data, is mainly controlled by its deep-layer environmental steering flow as a point vortex; (c) the northwestward deflection with lower domain height is caused by an artificially larger high pressure at lower levels in the vicinity of the Himalayas, due to the upward propagation of wave energy being reflected by the upper domain boundary; (d) the significant northwestward deflection associated with the varying mountain height cases is due to the cyclone vortex being advected by the northeasterly monsoonal flow, which is blocked by the mountains in the corresponding cases with mountains; (e) the northeastward track deflection after the landfall of Sidr is explained by the addition of the frictional force.In summary, the model vertical domain height and the Himalaya mountain representation play key roles in influencing the accuracy of TC Sidr track simulation, compared with other factors, such as the vertical resolution, at least for TC Sidr. 相似文献
2.
The southwestern coast of the Caspian Sea often experiences heavy snowfall during winter season due to the lake effect. The accurate estimation of snowfall in this region is still a challenge for weather forecasters. This study attempts to investigate the simulation of lake-effect snow (LES) event occurring along the southwest coastline of the Caspian Sea from 31 January to 4 February 2014 using Weather Research and Forecasting (WRF) model. The study evaluates the sensitivity of four microphysics (WSM6, Goddard, Morrison, and Thompson) schemes and two planetary boundary layer (PBL) schemes (the Yonsei University (YSU) and the Mellor-Yamada-Janjic (MYJ)), yielding eight distinct combinations. The results indicated that all the simulations overestimated the precipitation. However, the best configurations for estimation of precipitation and snow in terms of their spatiotemporal variation were the Morrison-MYJ and the Goddard-MYJ, respectively. Analyses of the vertical profiles of hydrometeor species showed that the combination of Goddard and MYJ schemes created more snow and graupel than the other configurations. Although the combination of WSM-MYJ schemes revealed the least bias, it was not appropriate for the prediction of snow. A comparison of the two boundary layer schemes showed that the MYJ scheme simulated better intensity and distribution of precipitation than the YSU scheme compared to observations. Also, the maximum radar reflectivity of the model output was useful for identifying the location of maximum precipitation. 相似文献
3.
Based on the NCEP/NCAR reanalysis dataset and in situ meteorological observations of daily precipitation in boreal summer from 1979 to 2008, the features of circulation anomalies have been investigated using the composite analysis for the extreme events and non-extreme events of regional mean daily rainfall(RMDR) occurring over the midand lower- Yangtze valley(MLYV). The extreme RMDR(ERMDR) events are the events at and above the percentile99 in the rearranged time-series of the RMDR with ascending order of rainfall amount. The non-extreme RMDR events are those at the percentiles 90-85 and 80-75 separately. Our results suggest that the threshold value is 25 mm/day for the ERMDR at percentile 99. Precipitation at all the percentiles is found to occur more frequently in the Meiyu rainfall season in MLYV, and the ERMDR events have occurred with higher frequency since the 1990 s. For the percentiles-associated events, the MLYV is under the control of an anomalous cyclonic circulation in the mid- and lower- troposphere with vastly different anomalous circulation at higher levels. However, at both low and high levels, the ERMDR events-related anomalous circulation is stronger compared to that linked to the non-ERMDR events. The dominant sources of water vapor differ between the ERMDR and non-ERMDR events. During the ERMDR events plentiful water vapor is transported from the Bay of Bengal into the MLYV directly by divergence while there is distinctly increased water vapor from the South China Sea(SCS) in non-RMERMDR episodes. The diabatic heating rates < Q1>, < Q2> and< Q1>- < Q2> have their anomalous patterns and are consistent with each other for these percentiles but their strength decreases markedly with the drop of rainfall intensity. For the precipitation at percentiles 99 and 90-85, the sea surface temperature anomalies(SSTA) in the Pacific distribute positively(negatively) in the south(north), and are stronger when the ERMDR emerges, with little or no SSTA as the events at percentile 80-75 occur. Besides, these results suggest that the genesis of the ERMDR event is directly related to intense local circulation anomalies and the circulation anomalies over the Pacific and SCS in tropical to mid-latitudes, and probably linked with the Pacific SSTA closely while the non-ERMDR events are mainly associated with the anomalous circulation on a local basis. The findings here help understand and predict the happening of ERMDR events over the MLYV. 相似文献
4.
The possible changes of tropical cyclone(TC) tracks and their influence on the future basin-wide intensity of TCs over the western North Pacific(WNP) are examined based on the projected large-scale environments derived from a selection of CMIP5(Coupled Model Intercomparison Project Phase 5) models. Specific attention is paid to the performance of the CMIP5 climate models in simulating the large-scale environment for TC development over the WNP. A downscaling system including individual models for simulating the TC track and intensity is used to select the CMIP5 models and to simulate the TC activity in the future.The assessment of the future track and intensity changes of TCs is based on the projected large-scale environment in the21 st century from a selection of nine CMIP5 climate models under the Representative Concentration Pathway 4.5(RCP4.5)scenario. Due to changes in mean steering flows, the influence of TCs over the South China Sea area is projected to decrease,with an increasing number of TCs taking a northwestward track. Changes in prevailing tracks and their contribution to basin-wide intensity change show considerable inter-model variability. The influences of changes in prevailing track make a marked contribution to TC intensity change in some models, tending to counteract the effect of SST warming. This study suggests that attention should be paid to the simulated large-scale environment when assessing the future changes in regional TC activity based on climate models. In addition, the change in prevailing tracks should be considered when assessing future TC intensity change. 相似文献
5.
AbstractThe potential influence of a developing La Niña on Arctic sea-ice annual variability is investigated using both observational data and an atmospheric general circulation model. It is found that during the developing phase of an eastern Pacific (EP) La Niña event in June, July, and August (JJA) and September, October, and November (SON), the sea-ice concentration (SIC) over the Barents–Kara Seas declines more than 15%. The local atmospheric circulation pattern associated with the EP La Niña is characterized as a weak decrease in geopotential height over the Barents–Kara Seas, combined with an anticyclone in the North Atlantic. The corresponding southerly winds push warm waters northward into the key sea-ice reduction region and directly accelerate sea-ice melt. Meanwhile, the abundant moisture contained in the lower troposphere is transported into the Arctic region by winds resulting from the local barotropic structure. The humid atmosphere contributes to both net shortwave and longwave radiation and thus indirectly accelerates the decline in sea ice. Simulations by the European Centre Hamburg Model, version 5.4, are forced by observed sea surface temperature anomalies associated with EP La Niña events. The results of the simulations capture the North Atlantic anticyclone and reproduce the moisture transport, which supports the premise that an EP La Niña plays a crucial role in sea-ice reduction over the Barents–Kara sector from the perspective of atmospheric circulation and net surface heat flux. 相似文献
6.
Time-Expanded Sampling Approach for Ensemble Kalman Filter:Experiment Assimilation of Simulated Soundings 
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下载免费PDF全文 In the Ensemble Kalman Filter (EnKF) data assimilation-prediction system, most of the computation
time is spent on the prediction runs of ensemble members. A limited or small ensemble size does reduce the
computational cost, but an excessively small ensemble size usually leads to filter divergence, especially when
there are model errors. In order to improve the efficiency of the EnKF data assimilation-prediction system
and prevent it against filter divergence, a time-expanded sampling approach for EnKF based on the WRF
(Weather Research and Forecasting) model is used to assimilate simulated sounding data. The approach
samples a series of perturbed state vectors from Nb member prediction runs not only at the analysis time
(as the conventional approach does) but also at equally separated time levels (time interval is △t) before
and after the analysis time with M times. All the above sampled state vectors are used to construct the
ensemble and compute the background covariance for the analysis, so the ensemble size is increased from
Nb to Nb+2M￡Nb=(1+2M)×Nb) without increasing the number of prediction runs (it is still Nb). This
reduces the computational cost. A series of experiments are conducted to investigate the impact of △t (the
time interval of time-expanded sampling) and M (the maximum sampling times) on the analysis. The results
show that if △t and M are properly selected, the time-expanded sampling approach achieves the similar
effect to that from the conventional approach with an ensemble size of (1+2M)×Nb, but the number of
prediction runs is greatly reduced. 相似文献
7.
通过WRF V2.1.2模式数值模拟试验并结合长期观测数据,研究了中国西北半干旱区长期存在和维持的森林山区(兴隆山区,103.84°E、35.86°N)的降水特征及其与周边地区的降水差异,并探讨了造成这种差异的主要原因。结果表明,兴隆山区与周边地区的降水差异主要表现在夏、秋季。在夏、秋季兴隆山区受东南湿润气流的影响,获得较多的水汽输入和较稳定的水汽来源,而山地地形则有利于截留东南气流携带的水汽并形成降水;兴隆山区及其周边地区局地的蒸散差异对二者之间降水差异的贡献不大。另外,兴隆山区土壤堆积覆盖的石质山构造和森林下垫面也有利于降水的截留和贮存以及植被的生长。因此,有利于水汽输入的大尺度环流形势、地形对空中水汽的截留以及特殊的地质因素是兴隆山山区孤立森林岛在半干旱区长期存在和维持的原因。 相似文献
8.
Danni Liu Jinhai He Yonghong Yao Li Qi 《Asia-Pacific Journal of Atmospheric Sciences》2012,48(2):145-152
This study focuses on the evolution of large-scale circulations before and after the beginning of the Meiyu and analyzes the formations of the typical vertical circulation pattern associated with Meiyu and its relationship with the Meiyu rainband. Results show that the typical vertical circulation pattern during the Meiyu season is characterized by the ??two-leg??-type pattern in the vorticity field, a typhoon eye-like structure of the equivalent potential temperature field, as well as the sharp gradient of the equivalent potential temperature zone, i.e., the Meiyu front are typically presented in the Meiyu season. Tracking its evolution process, we find that the typical vertical circulation pattern is built at late March and early April along with the rainband locating at the area south to the Yangtze River. This typical pattern and the rainband both advance northward affecting Jianghuai valley since the beginning of Meiyu. Moreover, the typical vertical circulation pattern derived from Meiyu season has been formed in April and corresponds to the reverse of the land-sea thermal contrast between the Eastern Asia and western Pacific Ocean, demonstrating the close relationship of the movement between the rainband and the march of the East Asian subtropical summer monsoon. 相似文献
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10.
《Dynamics of Atmospheres and Oceans》2005,40(3):151-162
An asymptotic solution of inviscid Boussinesq equations for a ‘dry convective Rankine vortex’ with prescribed buoyant forcing is given. The obtained vortex solution demonstrates monotonic growth with height of the vortex core radius, which becomes infinite at a certain critical altitude, and the corresponding attenuation of the vertical vorticity. This idealized vortex is then embedded in a convectively unstable boundary layer; the resulting approximate vortex solution has been applied to determine the maximum rotational wind speed and diameter of dry convective dust-devil-like vortices. 相似文献