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1.
Summary An attempt has been made to simulate the unprecedented heavy precipitation of 94.4 cm in a day over Santacruz, Mumbai during
0300 UTC 26 July to 0300 UTC 27 July 2005. Three experiments have been conducted using Advanced Regional Prediction System
model developed by Center for Analysis and Prediction of Storms of Oklahoma University, USA. In first experiment the model
input at large domain size has been obtained using NCEP/NCAR reanalysis data at 2.5° × 2.5° lat.–lon. resolution. In other
two experiments model input at large as well as at small domain sizes, have been obtained from NCEP/NCAR FNL data of 1° ×
1° lat.–lon. resolution. In all three experiments model’s horizontal resolution is 40 km and integration period is 30 hours
from 0000 UTC 26 July 2005. Based on the temporal distribution of observed rainfall rates it is considered that the rainfall
of 38.1 cm during 0900–1200 UTC on 26 July could be due to cloud burst phenomenon and 56.3 cm from 1200 UTC of 26 July to
0300 UTC of 27 July has been due to continuous regeneration of thunderstorm activity under influence of mesoscale cloud complex.
It is found that model forecast of rainfall in first experiment was qualitatively as well as quantitatively very poor. Among
other two, experiment with large domain size has predicted better rainfall values and location compared to the experiment
with small domain size. The larger domain has produced rainfall of 41 cm as against observed rain rate of 56.3 cm. during
1200 UTC of 26 July to 0300 UTC of 27 July. Divergence, vorticity, vertical velocity and moisture parameters are examined
in relation with the various stages of the event. The maximum values of convergence, vorticity and moisture fluxes precede
the initial phase of mature stage, however vertical velocity follows the later phase of mature stage. Vorticity budget over
the location of maximum rainfall, revealed the significant role of tilting term in maintenance and dissipation of the cloud
complex responsible for the event. The model has simulated mixing ratios of ice, snow and hail up to height of 15 km which
matches with the observations that clouds reaching up to 15 km were present at the time of event of heavy precipitation. 相似文献
2.
L. Juneng F. T. Tangang C. J. C. Reason S. Moten W. A. W. Hassan 《Meteorology and Atmospheric Physics》2007,97(1-4):273-290
Summary In this study, a rare tropical cyclone Vamei was simulated using the non-hydrostatic version 3.6 of the Penn State University
(PSU) – National Center for Atmospheric Research (NCAR) mesoscale model MM5. This unusual cyclone was generated on 26 December
2001 in an area close to the equator in the southern part of the South China Sea. The model was integrated for 80 h from 0000
UTC 26 December 2001 to 1800 UTC 29 December 2001. To examine the model performance, several important simulated fields including
sea-level pressure, surface wind speed and precipitation were compared to observations. The model simulated track of the cyclone
was also compared to the best track provided by the Joint Typhoon Warning Center (JWTC). Overall, the model performed reasonably
well, particularly in simulating the cyclone track and precipitation amount and spatial distribution. The analysis of the
model output indicated the important role of the latent heat flux in the genesis and intensification of tropical cyclone Vamei. 相似文献
3.
Summary A series of numerical experiments on an f plane are conducted using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale
Model, version 3 (MM5) to investigate how environmental vertical wind shear affects the motion, structure, and intensity of
a tropical cyclone. The results show that a tropical cyclone has a motion component perpendicular to the vertical shear vector,
first to the right of the shear and then to the left. An initially axisymmetric, upright tropical cyclone vortex develops
a downshear tilt and wavenumber-one asymmetry when embedded in environmental vertical wind shear.
In both small-moderate shears, a storm weakens slightly compared to that in a quiescent environment. The circulation centers
between 300 hPa and the surface varies from 20 km to over 80 km. The secondary circulation becomes quite asymmetric about
the surface cyclone center. As a result, convection on the upshear-right quadrant diminishes, limiting the upward heat transport
in the eyewall and thus lowering the warm core and leading to a weakening of the storm. In strong vertical shear (above 12 m s−1), the vertical tilt exceeds 160 km in 48 h of simulation and the secondary circulation on the upshear side is completely
destroyed with low-level outflow. The axisymmetric component of eyewall convection weakens remarkably and becomes much less
penetrative. As a result, the warm core becomes weak and appears at lower levels and the storm weakens rapidly accordingly.
This up-down weakening mechanism discussed in this study is different from those previously discussed. It emphasizes the penetrative
role of eyewall convection in transporting heat from the ocean to the mid-upper troposphere, maintaining the warm core structure
of the tropical cyclone. The vertical shear is found negative to eyewall penetrative convection. 相似文献
4.
Summary Three cyclones developing between 28 August and 6 September 1995 were studied with respect to the temporal evolution of their
water budget components. The cyclones were simulated with the regional model REMO. Water budget values were determined from
hourly model output for circle areas with 500 km radius around the pressure minimum. The results show a maximum liquid water
path of about 0.12 kg m−2 and a maximum ice water path of 0.16 kg m−2. In the vertical cloud structure the medium cloud layer disappears at the end of the life cycle for all three cyclones. The
release of precipitation onto the Baltic Sea drainage basin is different for each cyclone. It lies between 13 and 22 · 1012 kg. This is about 50% of the total precipitation in the whole area for the strongest cyclone and 65% for the others. The
P — E (precipitation minus evaporation) is 15 · 1012 kg for two of the cyclones and 10 · 1012 kg for the third one.
Received August 7, 2000 Revised March 19, 2001 相似文献
5.
利用中国地面加密自动站观测资料、北京地区雷达探测资料、NCEP （1°×1°） FNL资料、ECMWF ERA Interim （0.125°×0.125°）逐日再分析资料等,对造成2016年7月19-20日华北极端暴雨中的低涡系统发展演变的结构特征和加强机制进行了研究。华北地区这次特大暴雨过程出现了3个阶段降水,其中与低涡系统强烈发展对应的第2阶段降水是本次华北暴雨过程的主要降水阶段。针对该低涡的分析表明：（1）850 hPa以西南低涡为中心的低压带中,在河南西北部新生低涡系统,并且其在向华北地区移动过程中显著加强,该低涡系统在空间结构上,从倾斜涡柱逐渐发展成近乎直立的、贯穿整个对流层的深厚低涡系统;（2）中低层低涡系统快速发展过程与高低空系统构成耦合作用有关：低层低涡系统显著加强之前,对流层上层（300-200 hPa）首先出现高空槽异常加深并向南发展,该高空槽发展的开始阶段与其本身冷暖平流造成的斜压发展过程对应;而后,随着高纬度平流层高位涡沿等熵面向南运动,造成华北地区对流层上层涡度增强,形成正位涡异常区;当这一正位涡异常区叠加在对流层中低层锋区上空时,造成对流层中低层气旋快速发展并向下伸展,诱发河南西北部的新生气旋;低涡系统的发展进一步强化了低空暖平流,促使低空气旋向东北方向发展"移动"（本质上是暖平流前端造成的气旋发展）,这一动力学过程反过来使高层的涡度增强;这一正反馈过程形成的耦合环流不仅造成了整个涡度柱强度增强,而且垂直结构上逐渐由倾斜涡柱演变为近乎于直立的涡柱;（3）随着低涡系统增强,极大地加强了垂直上升运动并触发了对流,形成大范围的强降水,大量的凝结潜热释放,造成了低层低涡系统在强降水开始阶段的快速发展和增强;20日00时（世界时）以后,虽然对流活动显著减弱,但低涡系统的加深维持了大范围强降水过程的持续。强降水与低涡发展的正反馈过程是这次华北暴雨得以长时间维持的重要机制之一,这一过程形成的持续性潜热释放也是对流层中上层低涡系统热力结构发生改变的重要原因。 相似文献
6.
利用美国国家环境预报中心（NCEP）的FNL格点资料和大气数值模式WRF,对2012年1月11—13日发生于西北太平洋上的一次爆发性气旋过程进行诊断分析和数值模拟。气旋于1月11—12日在日本以东海域爆发性发展,经历2次转向后于13日在堪察加半岛附近减弱。研究表明,气旋发展中存在明显锋面结构,对流层高层的高位涡下传对气旋发展非常有利,气旋发展过程中伴随着一支低空急流的生成和发展,使辐合抬升更加明显。利用WRF模式对10日1800 UTC至13日0000 UTC气旋过程进行海温敏感性试验。结果表明,海温变化对气旋发展强度影响明显,但对气旋路径影响较小。 相似文献
7.
Numerical simulation of a heavy rainfall event in China during July 1998 总被引：16，自引：0，他引：16
Summary A detailed analysis associated with this case has been carried out (Zhao et al., 2001). In order to conduct further research
on the meso-β scale system, which is the directly influencing system, the heavy rainfall that occurred in Wuhan (Station no.:
57494) and Huangshi (Station no.: 58407), Hubei Province during July 1998 are simulated using higher resolution and more complete
initial data, after the large scale fields and rainfall areas have been simulated successfully. The simulation results indicate
that there are meso-β scale weather systems which developed and dissipated near Wuhan and Huangshi during 1800 UTC 20 July
to 0600 UTC 21 July and 1800 UTC 21 July to 0600 UTC 22 July in 1998, respectively. The life cycle of the meso-scale system
is about 12 hours and its horizontal scale is from 100 to 200 km. These are characteristic of a typical meso-β scale system.
By analyzing the vertical section of wind field and other physical variables during the mentioned-above two periods, it is
found that horizontal convergence, ascending motion and positive vorticity of the middle and lower troposphere are strengthened
during the heavy rainfall periods near the above mentioned two places. In addition, the wind disturbance in middle and lower
troposphere may be a possible triggering mechanism for the occurrence of the meso-β weather system. A budget analysis of the
meso-scale system indicates that the sources of moisture and positive vorticity are different during the different stages
of the meso-scale systems. Finally, a three dimensional conceptual model of the meso-β scale systems causing the sudden heavy
rainfall in Wuhan and Huangshi is suggested.
Received November 4, 2001 Revised December 28, 2001 相似文献
8.
Summary The Mediterranean basin experiences considerable cyclone activity mostly during fall, winter and spring and diminished activity
during summer. In this study we present results of synoptic disturbance track analysis for two contrasting winter months and
two, near average, summer months over the eastern Mediterranean. The surface and 500 hPa disturbance tracks were subjectively
analyzed from two points of view. First, looking at tracks of conventionally defined cyclone centers (eddies) based on actual
pressure and height distribution and second, looking at tracks of transient cyclonic disturbances (TRADs), defined as centers
of negative deviations from the time mean. The second type of analysis demonstrated a considerable increase in the number
of detectable tracks. Over the Mediterranean and vicinity the ratio between the number of surface TRAD tracks to cyclone tracks
is, about 2, whereas at 500 hPa the ratio is much higher, about 5. However, the average life span of transient disturbances
was only slightly longer than that of conventional cyclones (mainly at 500 hPa). At the surface and at 500 hPa about 50% of
the cyclone tracks coincided to a certain extent with TRAD tracks. In summer, when conventional analysis over the eastern
Mediterranean yields mostly quasi‐stationary low pressure centers associated with the Persian Gulf Trough, we detected clear
signs of transient disturbances. Some interpretations of the differences between cyclones and TRADs in terms of weather in
the eastern Mediterranean are also made.
Received January 19, 1999Revised June 23, 1999 相似文献
9.
K. Prasad 《Meteorology and Atmospheric Physics》2006,91(1-4):183-199
Summary Results of an earlier study of cyclone track prediction using a quasi-Lagrangian model (QLM) to generate track forecasts of
up to 36 hours were reported by Prasad and Rama Rao (2003). Further experiments to produce track forecasts of up to 72 hours
with an updated version of the same model have been carried out in the present study. In this case, the ability of the model
to predict recent historical cyclones in the Bay of Bengal and Arabian Sea has been assessed. Analysis of some of the structural
features of analyzed and predicted fields has been carried out. Such fields include wind distribution and vertical motion
around the cyclone centre. In addition, the merging of an idealized vortex with the large scale initial fields provided by
a global model, has been carried out for a particular case study of a May 1997 storm, which hit the Bangladesh coast. This
current study has demonstrated that the model generates a realistic structure of a tropical cyclone with an idealized vortex.
Performance evaluation has been carried out by computing the direct position errors (DPE). The results of which show that
the mean error for a 24 h forecast is about 122 km, which increases to about 256 km for a 48 h forecast and 286 km for a 72 h
forecast. These figures are comparable to similar errors in respect of tropical cyclone forecasts produced by an advanced
NWP centre, viz., the UKMO global model during the corresponding period, 1997–2000 (obtained from UKMO web site). The average
forecast errors of the UKMO model are 160 km for 24 h, 265 km for 48 h, 415 km for 72 h forecast ranges. 相似文献
10.
Summary. ?Cyclone track predictions in the Indian seas (Bay of Bengal and Arabian Sea) with a quasi-Lagrangian model (QLM) have been
attempted. QLM has a horizontal resolution of 40 km and 16 sigma levels in the vertical. It is integrated in a domain of about
4400 × 4400 km2. A new initialization procedure to provide initial fields for running the model has been designed. The initialization procedure
consists of updating the global model forecasts, used as first guess, provided by the National Center for Medium Range Weather
Forecasting (NCMRWF), New Delhi. A new version of IMD’s operational optimum interpolation scheme has been created to suit
the QLM grid structure. Lateral boundary conditions are computed from the extended forecasts of NCMRWF. The track forecasts
in each case show a reasonable skill of the forecast model in predicting the direction of movement within acceptable limits
of forecast errors, which are comparable to some of the best models operated by advanced NWP centers of the world. Even the
recurving storms are well predicted. Evolution of the vertical motion fields are also studied which reveal some interesting
features, which are described in detail in the text. The composited vertical motion fields are projected against observed
rainfall distribution, which show a good spatial correspondence.
Received August 9, 2001; revised March 12, 2002; accepted June 17, 2002
Published online: May 8, 2003 相似文献
11.
L. M. Leslie R. F. Abbey Jr. M. S. Speer T. C. L. Skinner 《Meteorology and Atmospheric Physics》2002,80(1-4):89-101
Summary The 1998/99 tropical cyclone (TC) season over northwest Australia was notable for an above average number of TCs (seven compared
to five on average) and a number of unusually intense TCs making landfall (three category 5 TCs). The active 1998/99 TC season
is attributed here to a combination of a number of broad-scale features over the south east Indian Ocean and the Australian
region, with identifiable precursors favoring tropical cyclogenesis. These precursors include: below normal MSLP, abnormally
warm ocean temperatures, above average relative humidity in the low- to mid-tropospheric levels and weak wind shears in the
genesis region under study, that is, between 10° S to 20° S and 105° E to 135° E. These favorable conditions first appeared
as early as August 1998. The appearance of favorable conditions so far ahead of the TC season indicates that they are the
likely cause of the enhanced TC activity rather than simply an effect. Although the season as a whole was an active one, strong
intra-seasonal variability was evident in that there were two named TCs in December 1998, forming within three days of each
other. Only one formed in January 1999 and none in February. By contrast, in March and April 1999, TC activity was enhanced
once again, with four named TCs, three of which attained category 5 status. The importance of the above-mentioned precursors
in favoring tropical cyclogenesis during the 1998/99 season is discussed in terms of seasonal time scales of the preceding
spring and down to synoptic and mesoscale time scales ranging from several days to 48 hours or less.
Received October 5, 2001 Revised December 28, 2001 相似文献
12.
Summary Based on the six-hourly re-analysis sea-level pressure data of the European Centre for Medium-Range Weather Forecast (ECMWF)
a cyclone statistics for the Arctic region north of 60° is elaborated for the period 1 November 1986 to 31 October 1991. For
each low pressure center on a weather map its location, central pressure and horizontal pressure gradients in E, W, N, and
S direction are determined. Furthermore, cyclone centers are followed with time to calculate trajectories, pressure tendencies,
and lifetimes.
A horizontal grid of 300 km × 300 km is used as unit area for the statistical computations. A unit area experiences about
20 cyclone passages per year (range 5–40). On the average, six cyclones occur simultaneously in the Arctic region. Lifetimes
vary from 6 h to 15 days.
The annual cyclone activity over the 5-year period is nearly the same. Cyclones are more frequent in summer (about 94 per
month) than in winter (77 per month). In general summer cyclones are weaker than winter cyclones. On the average, the minimum
central pressure during the lifetime of a cyclone is about 1000 hPa (typical range 980–1020) in summer and about 988 hPa (typical
range 940–1030) in winter.
In winter, a zone of high cyclone frequency extends from the region near Iceland over the Greenland Sea, Barents Sea, and
Kara Sea to the Laptev Sea while the interior of the Arctic shows little cyclone frequency. In summer, the region near Iceland
and the interior of the Arctic are separate centers of high cyclone frequency. Both in winter and summer very high cyclone
frequencies are observed over the northern Baffin Bay. The regional distribution of mean central pressures and maximum pressure
gradients roughly follows the distribution of cyclone frequencies except for the Baffin Bay cyclones which are generally weak.
Cyclolysis dominates cyclogenesis over largest parts of the Arctic. Regions of high cyclone frequency are also regions of
frequent cyclogenesis and frequent cyclolysis. One third of all cyclones is generated in a region with an already existing
cyclonic circulation.
Cyclones in the Fram Strait are studied in more detail because of their special impact on the ice export from the Arctic Ocean
to the Atlantic Ocean. On the average, there are 5 cyclones per month. the cyclone frequency in the Fram Strait is higher
during the winter period than during the summer period. This is in contrast to the overall Arctic frequency which is higher
in summer than in winter. Cyclogenesis predominates in winter and cyclolysis in summer in the Fram Strait. The most frequent
direction of motion is from SW to NE.
Received November, 1999 Revised June 22, 2000 相似文献
13.
Summary A newly developed non-hydrostatic model (MOLOCH), operating at a resolution of about 2 km, is run for a case of heavy precipitation
over southeastern France. The event (8–9 September 2002) was characterized by intense convective activity leading to a severe
flash flood in the region of the Gard river, south of the Massif Central. An almost stationary mesoscale convective system
(MCS), developing well in advance of an approaching cold front, discharged a huge amount of rainfall over the same area, more
than 600 mm in 24 hours.
Several simulations are performed in order to test the model set-up, evaluate the sensitivity on different initial conditions,
and analyse the case-study. The quantitative precipitation forecasts (QPF) appear to vary widely among the experiments, depending
on the initialization time chosen (00, 06, and 12 UTC, September 8). Only the run starting at 06 UTC predicts, with a satisfactory
degree of accuracy, the location where the MCS developed and its almost stationary behavior during the first stage (∼12 hours)
of the event. In all the simulations, the convective system then propagated northward over the Massif Central.
In addition to experiments starting from standard ECMWF analyses, an assimilation procedure, based on Optimal Interpolation,
is applied to the initial conditions. Surface observations of temperature, wind and relative humidity have been assimilated.
The assimilation produces an improvement in the forecasts of surface fields and leads to a better location of the initial
triggering phase.
Further experiments, performed by changing the orography in the model, allow the investigation of the role of the Massif Central
in triggering the mesoscale convective system and in controlling its evolution. 相似文献
14.
Summary Seasonal variations of gravity wave characteristics are investigated using rawinsonde data observed at Pohang observatory,
Korea (36°2′N, 129°23′E) during the one-year period of 1998. Analysis is carried out for two atmospheric layers representing
the troposphere (2–9 km) and stratosphere (17–30 km). There exist clear seasonal variations in amplitudes of temperature and
wind perturbations and wave energy in the stratosphere, with their maxima in wintertime and minima in summertime. A strong
correlation is found between the wave activity and the strength of the jet stream, but there is no clear correlation between
the wave activity and the vertical gradient of static stability. The intrinsic frequency and vertical and horizontal wavelengths
of gravity waves in the stratosphere are 2f–3f, where f is the Coriolis parameter, and 2–3 km and 300–500 km, respectively. The intrinsic phase velocity directs westward in January
and northeastward in July. The vertical flux of the stratospheric zonal momentum is mostly negative except in summertime with
a maximum magnitude in January. Topography seems to be a major source of stratospheric gravity waves in wintertime. Convection
can be a source of gravity waves in summertime, but it is required to know convective sources at nearby stations, due to their
intermittency and locations relative to floating balloons. 相似文献
15.
Summary Tropical Cyclone Drena, a relatively long lived cyclone lasting from January 2, 1997 to January 10, 1997, crossed over three well separated island groups and affected a fourth in the south western Pacific Ocean during different stages of its life cycle. Midway through its transition into an extra-tropical low, it passed over the eastern edge of Norfolk Island, an isolated island located in the western Pacific Ocean midway between New Zealand and New Caledonia, at 0615 UTC (1745 local) on January 9, 1997. The tropical cyclone exhibited markedly different characteristics during each phase of its life cycle, with thick fog being reported during the eye passage at Norfolk Island. Although routine global and Australian region numerical models were able to provide the operational forecasters with broad scale guidance as to the movement of the tropical cyclone, the level of detail available at these resolutions is insufficient to identify the characteristics important to communities in the tropical cyclone's path. The high resolution numerical model (HIRES) developed by the University of New South Wales, was run at a resolution of 25 kilometres to investigate the evolution of the tropical cyclone from a warm cored, quasi-symmetrical vortex into a highly asymmetrical mid latitude low pressure system. The model captures the observed wind and precipitation structure of the cyclone very well during this crucial transition phase. The comparison extends into the vertical with a model derived vertical cross section depicting the key features observed in a sequence of ten specially requested radiosonde flights, released at six hourly intervals, from Norfolk Island which transect the passage of the tropical cyclone over this remote island.With 10 Figures 相似文献
16.
Stabilization of thermohaline circulation by wind-driven and vertical diffusive salt transport 总被引：1，自引：0，他引：1
The stability of the thermohaline circulation is investigated using an ocean general circulation model coupled to a simple
atmospheric model. The atmospheric model is so developed that it represents the wind stress and the freshwater flux more realistically
than existing energy balance models. The coupled model can reproduce the realistic deep ocean circulation without any flux
adjustment. Effects of the wind stress and the vertical diffusion on the thermohaline circulation are studied by conducting
various experiments with the coupled model. The Ekman upwelling between 60∘N and 90∘N brings up salt to the sea surface, while the compensation flow of the Ekman transport and the wind-driven gyre circulation
between 30∘N and 60∘N carry salt horizontally to the high latitudes. By carrying out experiments where the wind stress is completely or partly
removed, it is demonstrated that either of the vertical or the horizontal salt transport prevents the halocline formation
at high latitudes and maintains the thermohaline circulation. For an experiment in which the vertical diffusivity is enhanced
at high latitudes, it is shown that the vertical diffusion at high latitudes also prevents the halocline formation and stabilizes
the thermohaline circulation. It is also shown that the value of the vertical diffusivity at high latitude affects the existence
of the multiple equilibria of the thermohaline circulation.
Received: 26 April 2000 / Accepted: 10 January 2001 相似文献
17.
M. P. Guishard E. A. Nelson J. L. Evans R. E. Hart D. G. O’Connell 《Meteorology and Atmospheric Physics》2007,97(1-4):239-253
Summary This investigation focuses on North Atlantic subtropical cyclones which tracked within 100 nautical miles (185 km) of Bermuda
from 1957 to 2005, identified through subtropical structural characteristics distinguished using Cyclone Phase Space, from
the European Centre for Medium-Range Weather Forecasts 45-year reanalyses. The study assesses the characteristics of these
hybrid storms that affect the Island, in order to aid the local forecaster. Reanalysis charts, surface analyses, local observations,
HURDAT tracks, and satellite pictures, where available, were examined. This data shows that subtropical cyclones affecting
Bermuda usually form in close proximity, to the south-southwest, over water of an average of 26 °C, under moderate vertical
wind shear, with an upper trough lying to the west-north-west. They then move in a north-northeastward direction, intensifying
quickly, but not often reaching a peak intensity of more than 26 m s−1. They generally have their beginnings along old baroclinic zones. September is the peak month of occurrence. A direct hit
by a severe subtropical cyclone, producing locally observed winds of over 26 m s−1, appears to be a rare event. However, these storms are certainly a threat to the Island, particularly due to their lack of
predictability, and conditions conducive to an incipient subtropical cyclone with potential to affect the Island should always
be closely monitored. 相似文献
18.
Summary A community mesoscale model is used to simulate and understand processes that led to the formation and intensification of
the near-equatorial typhoon Vamei that formed in the South China Sea in December, 2001. The simulated typhoon resembles the
observed in that it had a short lifetime and a small size, formed near the equator (south of 2° N), and reached category-one
intensity. The formation involved the interactions between the scales of the background cyclonic circulation (the Borneo Vortex
of order ∼100 km) and of mesoscale convective vortices (MCVs, in the order ∼10 km). Before tropical cyclone formation MCVs
formed along a convergent, horizontal shear vorticity line on the eastern edge of an exceptionally strong monsoonal northerly
wind surge.
The typhoon genesis is marked by three rapid intensification periods, which are associated with the rapid growth of potential
vorticity (PV). A vorticity budget analysis reveals that the increases in low-level vorticity during the rapid intensification
periods are attributed to enhanced horizontal vorticity fluxes into the storm core. The increase of the horizontal vorticity
flux is associated with the merging of areas of high PV associated with MCVs into the storm core as they are advected by background
cyclonic flows. The increases in PV at upper levels are associated with the evaporation of upper level stratiform precipitation
and increases of vertical potential temperature gradient below the maximum stratiform cloud layer. It appears that two key
sources of PV at upper and lower levels are crucial for the build up of high PV and a deepening of a cyclonic layer throughout
the troposphere. 相似文献
19.
D. Narayana Rao H. R. Singh J. R. Kulkarni A. Y. Chandrika S. Vijaya Bhaskara Rao 《Meteorology and Atmospheric Physics》2000,73(1-2):55-59
Summary Mesosphere-Stratosphere-Troposphere (MST) Radar wind data for the period June through September 1996 have been examined to
study vertical variation of Madden-Jullian Oscillations in wind and eddy kinetic energy (eke) in the normal monsoon season.
The domain of analysis in the vertical is from 6 to 20 km with a height resolution of 150 m. Fast-Fourier-Transformation (FFT)
has been applied to zonal (u), meridional(v) components of wind to extract the Madden-Jullian oscillations and eke. There
are three dominant modes viz., 50–70, 30–40 and 10–20 day periodicity, which contain considerable fraction of energy and show
high degree of vertical variability. The peak amplitude of 50–70 day mode in u, 30–40 mode in v and eke were observed at 16–17 km
just below the tropopause level. The peak amplitudes of 30–40 day mode in u and 50–70 day mode in v were found in the height
region of 13–16 km. To understand the origin and propagation of these waves, wave energy is calculated. The wave energy is
higher at tropospheric heights than at lower stratospheric heights indicating that the origin of these waves is in the troposphere,
and a part of the energy leaks into the stratosphere.
Received September 17, 1998/Revised September 26, 1999 相似文献
20.
Summary A mesoscale convective system (MCS) case that developed over the Yellow Sea (12–13 July 1993) is studied by using a 23-level,
30 km-mesh Penn State/NCAR mesoscale model MM5. This MCS was generated in northern China, south of the Changma front, in a
convectively unstable environment, under the influence of a short-wave trough accompanied by a marked cold vortex aloft.
The model with all model physics (refereed to as CNTL) captured the major features of this MCS. A mesoscale low-level jet
(mLLJ), with a horizontal scale of a few hundred km, developed within the MCS. Available wind data support the realism of
this mLLJ. This mLLJ not only transports convectively unstable air directly toward the MCS but is also responsible for a strong
low-level convergence in the MCS. At 200 hPa, an anticyclonic northwesterly flow with a relatively high wind speed core on
the east of MCS was simulated. This relatively high-speed flow can be regarded as a mesoscale upper level jet (mULJ), acted
as an upper outflow over the MCS. Low-level convergence on the left-front of the mLLJ and upper divergence in the right-rear
of the mULJ creates a strong upward motion (≅ 40 cm s−1) in the MCS.
Heavy precipitation up to 45 mm between 1800–2100 UTC was observed after this MCS landed on the southern Korean Peninsula.
The CNTL run captured this heavy rainfall event. A maximum rainfall of 50 mm 3 h−1 was simulated. In another experiment, with surface sensible and moisture fluxes withheld (NOSF), the 3-h simulated rainfall
was decreased to 30 mm. Less latent heat released in the NOSF led to a weaker MCS and mLLJ. The concurrent surface fluxes
sustained a high low-level moisture field over the Yellow Sea, which helped the development of the MCS and enhanced its precipitation
in this case.
Received January 8, 1999 相似文献