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In this article, the interannual variability of certain dynamic and thermodynamic characteristics of various sectors in the
Asian summer monsoon domain was examined during the onset phase over the south Indian peninsula (Kerala Coast). Daily average
(0000 and 1200 UTC) reanalysis data sets of the National Centre for Environmental Prediction/National Centre for Atmospheric
Research (NCEP/NCAR) for the period 1948–1999 were used. Based on 52 years onset date of the Indian summer monsoon, we categorized
the pre-onset, onset, and post-onset periods (each an average of 5 days) to investigate the interannual variability of significant
budget terms over the Arabian Sea, Bay of Bengal, and the Indian peninsula. A higher difference was noticed in low-level kinetic
energy (850 hPa) and the vertically integrated generation of kinetic energy over the Arabian Sea from the pre-onset, onset,
and post-onset periods. Also, significant changes were noticed in the net tropospheric moisture and diabatic heating over
the Arabian Sea and Indian peninsula from the pre-onset to the post-onset period. It appears that attaining the magnitude
of 40 m2 s−2 and then a sharp rise in kinetic energy at 850 hPa is an appropriate time to declare the onset of the summer monsoon over
India. In addition to a sufficient level of net tropospheric moisture (40 mm), a minimum strength of low-level flow is needed
to trigger convective activity over the Arabian Sea and the Bay of Bengal. An attempt was also made to develop a location-specific
prediction of onset dates of the summer monsoon over India based on energetics and basic meteorological parameters using multivariate
statistical techniques. The regression technique was developed with the data of May and June for 42 years (1948–1989) and
validated with 10 years NCEP reanalysis from 1990 to 1999. It was found that the predicted onset dates from the regression
model are fairly in agreement with the observed onset dates obtained from the Indian Meteorology Department. 相似文献
3.
P. N. Mahajan R. M. Khaladkar S. G. Narkhedkar Sathy Nair P. C. Joshi P. K. Pal 《Journal of Earth System Science》2004,113(2):223-233
In this paper, daily variations of satellite-derived geophysical parameters such as integrated water vapour (IWV), cloud liquid
water content (CLW), sea surface temperature (SST) and sea surface wind speed (SSW) have been studied for a case of monsoon
depression that formed over the Bay of Bengal during 19th-24th August 2000. For this purpose, IRS P4 MSMR satellite data have
been utilized over the domain equator — 25‡N and 40‡-100‡E. An integrated approach of satellite data obtained from IRS-P4,
METEOSAT-5 and INSAT was made for getting a signal for the development of monsoon depression over the Indian region. Variations
in deep convective activity obtained through visible, infrared and OLR data at 06 UTC was thoroughly analyzed for the complete
life cycle of monsoon depression. Geophysical parameters obtained through IRS-P4 satellite data were compared with vorticity,
convergence and divergence at 850 and 200 hPa levels generated through cloud motion vectors (CMVs) and water vapour wind vectors
(WVWVs) obtained from METEOSAT-5 satellite. This comparison was made for finding proper consistency of geophysical parameters
with dynamical aspects of major convective activity of the depression.
From the results of this study it is revealed that there was strengthening of sea surface winds to the south of low-pressure
area prior to the formation of depression. This indicated the possibility of increase in cyclonic vorticity in the lower troposphere.
Hence, wind field at 850 hPa with satellite input of CMVs in objective analysis of wind field using optimum interpolation
(OI) scheme was computed. Maximum cyclonic vorticity field at 850 hPa was obtained in the region of depression just one day
before its formation. Similarly, with the same procedure maximum anticyclonic vorticity was observed at 200 hPa with WVWVs
input. Consistent convergence and divergence at 850 and 200 hPa was noticed with respect to these vorticities. In association
with these developments, we could get lowest values of OLR (120 W/m2 ) associated with major convective activity that was consistent with the maximum values of integrated water vapour (6-8gm/cm2) and cloud liquid water content (50-60 mg/cm2 ) persisting particularly in the southwest sector of the monsoon depression. 相似文献
4.
V. Krishnakumar R. N. Keshavamurty S. V. Kasture 《Journal of Earth System Science》1992,101(2):123-152
The objective of this study was two-fold: the first to investigate the role of moist convection and nongeostrophic effects
on the growth of the monsoon depressions using a linearized multi-level moist primitive equation (PE) model and quasi-geostrophic
(QG) model with only vertical shear. The second was to study the nonlinear evolution, growth, movement and detailed energetics
of the monsoon depressions using a nonlinear moist global spectral model.
Our linear studies using both models revealed lower as well as upper tropospheric growing modes. For the lower tropospheric
modes the shorter scales were found to grow faster. While the PE model showed faster growth rate for shorter scales, as compared
to longer scales, the QG model showed less tendency for scale selection. The shorter scales in PE model had phase speeds ranging
from 4 to — 1 ms−1 and in QG model from 8 to — 4 ms−1. The nongeostrophic effects were found to be, in general, important.
One of the lower tropospheric modes with wavelength 2500 km was found to have many features similar to the observed monsoon
depression of the Bay of Bengal.
In the upper troposphere the PE model showed much faster growth rates compared to the QG model. Also the fastest growing mode
with a doubling time of 2.5 days had a scale of 6000 km. This was shorter than the scale predicted in the QG model. This mode
had many characteristics similar to the observed features of the monsoon upper tropospheric easterly waves.
Using a nonlinear global spectral model, we simulated the monsoon depression around 21°N starting from an antisymmetric heating
distribution (with respect to the equator) and with a specific vertical structure with and without basic flows. The model
was integrated for a period of five days incorporating a simple form of cumulus heating. The simulated model disturbance showed
a pronounced growth and a westward movement in the presence of cumulus heating. The detailed energetics calculations revealed
that the baroclinic energy exchange is the primary energy exchange process and cumulus heating is the driving force for the
generation of available potential energy. 相似文献
5.
In this study, we elucidate the temporal characteristics of the onset and withdrawal of the Indian southwest monsoon, making
use of the model integration and daily analyses of the National Centre for Medium Range Weather Forecasting, India. The onset
of the Indian southwest monsoon over the Bay of Bengal is discernable by a gradual increase in the adiabatic generation of
kinetic energy, while over the Arabian Sea it is first noticeable by a steep and abrupt increase of generation. The horizontal
transport of heat indicates a convergence regime over the Bay of Bengal prior to onset, while over the Arabian Sea a convergence
regime is indicated by a change from the divergence to the convergence regime. The withdrawal of the southwest monsoon is
characterized by the horizontal transport of heat and moisture that evince a transition from the convergence to divergence
regime; similarly, diabatic heating noticed during the active period changes to cooling. The withdrawal over the Arabian Sea
is characterized by the divergence regime of the horizontal transport of moisture. This change precedes even the circulation
changes over northwest India, which may be regarded as a precursor. The withdrawal is further supported by a monotonic decrease
in the net tropospheric moisture over the Arabian Sea, followed by a similar change at land locations. 相似文献
6.
S D Kotal S K Bhattacharya S K Roy Bhowmik P K Kundu 《Journal of Earth System Science》2014,123(7):1619-1635
The tropical cyclone Viyaru maintained a unique quasi-uniform intensity during its life span. Despite being in contact with sea surface for >120 hr travelling about 2150 km, the cyclonic storm (CS) intensity, once attained, did not intensify further, hitherto not exhibited by any other system over the Bay of Bengal. On the contrary, the cyclone Phailin over the Bay of Bengal intensified into very severe cyclonic storm (VSCS) within about 48 hr from its formation as depression. The system also experienced rapid intensification phase (intensity increased by 30 kts or more during subsequent 24 hours) during its life time and maximum intensity reached up to 115 kts. In this paper, a comparative study is carried out to explore the evolution of the various thermodynamical parameters and possible reasons for such converse features of the two cyclones. Analysis of thermodynamical parameters shows that the development of the lower tropospheric and upper tropospheric potential vorticity (PV) was low and quasi-static during the lifecycle of the cyclone Viyaru. For the cyclone Phailin, there was continuous development of the lower tropospheric and upper tropospheric PV, which attained a very high value during its lifecycle. Also there was poor and fluctuating diabatic heating in the middle and upper troposphere and cooling in the lower troposphere for Viyaru. On the contrary, the diabatic heating was positive from lower to upper troposphere with continuous development and increase up to 6°C in the upper troposphere. The analyses of cross sections of diabatic heating, PV, and the 1000–500 hPa geopotential metre (gpm) thickness contours indicate that the cyclone Viyaru was vertically tilted (westward) and lacked axisymmetry in its structure and converse features (axisymmetric and vertical) that occurred for the cyclone Phailin. In addition, there was a penetration of dry air in the middle troposphere of Viyaru, whereas high moisture existed in the middle troposphere of Phailin. The vertical wind shear (5–10 ms?1) near the core of the storm region between 850 and 200 hPa was favourable for both the systems but was higher in the northern region of the cyclone Viyaru. The divergent development of these thermodynamic features conspired to produce converse characteristic of the two cyclones. 相似文献
7.
In this paper the characteristics of surface radiative fluxes and cloud-radiative forcing are reviewed with a focus on the Arctic. Three aspects are addressed, including (i) changes in radiation flux over the global surface; (ii) characteristics of surface fluxes in the Arctic; and (iii) characteristics of cloud-radiative forcing in the Arctic. The clouds not only significantly reduce the peak summer radiative heating of the surface but also reduce the wintertime radiative cooling at the surface in higher latitudes. The downward longwave fluxes dominates the incident radiative fluxes in the Arctic during most of the year. Incoming shortwave fluxes are negligible during late fall, winter and early spring, and even during the midsummer the incoming shortwave fluxes are only slightly greater than the downward longwave fluxes. The total net surface radiative flux is negative for most of the year and only positive during midsummer in the Arctic. The global net cloud-radiative forcing is negative, but the cloud-radiative forcing is positive in the Arctic, showing a warming effect, except for a short period in mid-summer. Positive cloud-radiative forcing in the Arctic is attributed to the presence of snow and ice with high albedo and the absence of solar radiation during the polar night. 相似文献
8.
The planet's radiation budget includes practically all energy exchange between the Sun, the Earth, and space, and so is a fundamental factor of climate. The terms of this budget, observable only from space, are determined from sampled direct measurements of the solar and terrestrial radiation fields. On the contrary, however, it should be remembered that energy exchange between the Earth's surface and its atmosphere involves not only radiative but also non-radiative energy fluxes. Nevertheless, only observations from space can provide satisfactory global coverage of the different energy fluxes that determine climate at the Earth's surface, by way of indirect retrievals of radiative fluxes at the surface and at different heights in the atmosphere. We describe the methods, applied to measurements made with a variety of instruments on board different artificial satellites, that have led to our present knowledge of the Earth's radiation budget (ERB) at the “top of the atmosphere”: global annual mean values of the ERB terms, its annual cycle, its geographical structure, and its variations. We know that solar irradiance, averaged over the globe and the year, varies by only 0.1% with the solar activity cycle; we also know that planetary (Bond) albedo is close to 0.3, that the global annual mean emission of thermal infrared radiation to space is close to 240 Wm?2, and that these terms exhibit a weak but well determined annual cycle. We also know that cloud cover plays a major role in the radiation budget, both in the “shortwave” domain (global SW “cloud radiative forcing” –50 Wm?2) and in the “longwave” domain (+20 Wm?2), thus a net forcing of –30 Wm?2. Successive satellite missions give consistent results for the shape, the phase, and the amplitude of the annual cycle of the planetary radiation balance. However, the different estimates of its annual mean absolute value remain uncertain, not differing significantly from zero, although generally excessively positive. We also rapidly review the methods used to determine the surface radiation budget as well as that of the atmosphere. For the planetary (TOA) radiation budget, we examine to what extent interannual variations and interdecadal trends have been or could be detected. We conclude with a review of projects under way. We also discuss priorities for future efforts, considering in particular, on the one hand (Ringer, 1997), the need to better quantify the factors that govern climate sensitivity to modifications of the atmosphere's radiative properties, on the other hand, the importance of monitoring the evolution of the present disequilibrium situation. 相似文献
9.
The second campaign of the Arabian Sea Monsoon Experiment (ARMEX-II) was conducted in two phases viz., March–April and May–June
2003. In the present work, the buoy and ocean research vessel data collected during the second phase of ARMEX-II have been
analysed to bring out the characteristic features of monsoon onset. The results have shown that the thermodynamical features
such as build up of lower tropospheric instability and increased height of zero degree isotherm occurred about a week before
the monsoon onset over Kerala and adjoining southeast Arabian Sea. There was a sharp fall in the temperature difference between
850 and 500 hPa, and the height of zero degree isotherm about 2–3 days before the monsoon onset. The flux of sensible heat
was positive (sea to air) over south Arabian Sea during the onset phase. Over the Bay of Bengal higher negative (air to sea)
values of sensible flux prevailed before the monsoon onset which became less negative with the advance of monsoon over that
region.
The pre-onset period was characterized by large sea surface temperature (SST) gradient over the Arabian Sea with rapid decrease
towards north of the warm pool region. The buoy observations have shown that SST remained close to 30.5°C in the warm pool
region during the pre-onset period in 2003 but only 2–3 degrees away (north of this region) SSTs were as low as 28.5–29°C.
An interesting aspect of sea level pressure (SLP) variability over the Indian seas during the onset phase of summer monsoon
2003 was undoubtedly, the highest SLP in the warm pool region inspite of very high SSTs. 相似文献
10.
The satellite-derived moisture fields during different phases of two normal and poor monsoon years have been studied. Spectral
analysis was performed in different zones of the monsoon region to study the nature and modes of intraseasonal fluctuations
of lower layer moisture fields.
Seasonal mean fields of water vapour at low and middle layers show a dry anomaly over the Arabian subcontinent and a wet anomaly
over the Bay of Bengal during good monsoon years, while the anomalies show an opposite trend during the poor monsoon years.
The zonal and meridional propagation of low-frequency oscillations of moisture fields has also been examined. The southward
movement of low-frequency oscillations seems to be suppressed in good monsoon years as compared to the poor monsoon years,
whereas the northward movement of the same shows no particular difference. Fluctuations in the 30–50 day range are found shifted
to longer time-period side in the poor monsoon years. 相似文献
11.
陇中黄土高原冬季地表辐射和能量平衡特征 总被引：10，自引：0，他引：10
利用定西试验基地2003年12月至2004年2月获取的地气相互作用观测资料,分析了陇中黄土高原冬季典型天气（晴天、阴天及雪天）和平均状况下的地表辐射能量平衡和土壤温度等微气象特征的日变化规律。结果表明,在典型天气间微气象特征有较大差异,但冬季的平均特征与晴天比较接近,云和降雪的扰动影响有限。冬季地表能量平衡以感热输送为主,土壤热通量为辅,且普遍存在不平衡现象。冬季初的2／3时段净辐射及土壤热通量日总量基本为负值,之后的1／3时段为正值,冬季地面加热场为热源。冬季土壤温度在地表及20 cm土壤层存在日变化,30 cm及以下土壤温度日变化很小。冬季地表日平均反射率基本在0.20～0.25之间变化,平均值为0.22。 相似文献
12.
A. Chandrasekar 《Journal of Earth System Science》2002,111(4):437-451
A linear model of the response of a stratified atmosphere to isolated heat sources in spherical coordinates is used to study
the maintenance of the mean position of the mid tropospheric ridge and its displacement. It is well known that the performance
of the southwest Indian monsoon is related to the latitudinal position of the April 500 hPa ridge along 75‡E. It was demonstrated
that an anomalous cooling associated with the increased snow cover in Eurasia can result in moderate southward displacement
of the mid-tropospheric ridge. The results of this study indicate that the vertically integrated cooling rate (strength of
heat sink) has more effect on the southward displacement of the ridge when the sink is closer to the ridge. 相似文献
13.
S. M. Bawiskar M. D. Chipade D. K. Paul S. S. Singh 《Journal of Earth System Science》1995,104(4):613-634
Kinetic energy exchange equations (Saltzman 1957) in wave number domain are partitioned into standing, transient and standing-transient
components following Murakami (1978, 1981). These components are computed for the 1991 summer monsoon using dailyu andv grid point data at 2.5° latitude-longitude interval between the equator and 40°N at 200 hPa and 850 hPa levels for the period
June through August. The data are obtained from NCMRWF, New Delhi.
The study shows that at 200 hPa wave number 1 over Region 3 (30°N to 40°N), wave number 2 over Region 2 (15°N to 30°N) and
wave number 3 over Region 1 (equator to 15°N) dominate the spectrum of transport of momentum and wave to zonal mean flow interaction.
Wave number 1 over Region 1 and Region 3 and wave number 2 over Region 2 are the major sources of kinetic energy to other
waves via wave-to-wave interaction. At 850 hPa wave number 1 over Region 3 has maximum contribution in the spectrum of transport
of momentum and kinetic energy and more than 90% of its contribution is from the standing component. This indicates that standing
wave number 1 over Region 3 plays a very important role in the dynamics of monsoon circulation of the lower troposphere.
The study further shows that although the circulation patterns at 200 hPa and 850 hPa levels are opposite in character, a
number of energy processes exhibit a similar character at these levels. For example, (i) transport of momentum by most of
the waves is northward, (ii) small scale eddies intensify northward, (iii) eddies are sources of kinetic energy to zonal mean
flow over Region 1 and (iv) standing eddies are sources of kinetic energy to transient eddies. Besides the above similarities
some contrasting energy processes are also observed. Over Region 2 and Region 3 standing and transient eddies are sources
of kinetic energy to zonal mean flow at 200 hPa, while at 850 hPa the direction of exchange of kinetic energy is opposite
i.e. zonal mean flow is a source of kinetic energy to standing as well as transient eddies. L(n) interaction indicates that at 200 hPa waves over R2 maintain waves over R1, while at 850 hPa waves over R1 maintain waves
over R2.
It has been found that the north-south gradient of zonal mean of zonal wind is the deciding factor of wave to zonal mean flow
interaction. 相似文献
14.
15.
In this study, the Florida State University Global Spectral Model (FSUGSM), in association with a high-resolution nested regional
spectral model (FSUNRSM), is used for short-range weather forecasts over the Indian domain. Three-day forecasts for each day
of August 1998 were performed using different versions of the FSUGSM and FSUNRSM and were compared with the observed fields
(analysis) obtained from the European Center for Medium Range Weather Forecasts (ECMWF). The impact of physical initialization
(a procedure that assimilates observed rain rates into the model atmosphere through a set of reverse algorithms) on rainfall
forecasts was examined in detail. A very high nowcasting skill for precipitation is obtained through the use of high-resolution
physical initialization applied at the regional model level. Higher skills in wind and precipitation forecasts over the Indian
summer monsoon region are achieved using this version of the regional model with physical initialization.
A relatively new concept, called the ‘multimodel/multianalysis superensemble’ is described in this paper and is applied for
the wind and precipitation forecasts over the Indian subcontinent. Large improvement in forecast skills of wind at 850 hPa
level over the Indian subcontinent is shown possible through the use of the multimodel superensemble. The multianalysis superensemble
approach that uses the latest satellite data from the Tropical Rainfall Measuring Mission (TRMM) and the Defense Meteorological
Satellite Program (DMSP) has shown significant improvement in the skills of precipitation forecasts over the Indian monsoon
region. 相似文献
16.
青藏高原近地层及北侧气压系统的季节性振荡变化 总被引：3，自引：1，他引：2
基于ERA-Interim逐日4次600 hPa位势高度再分析资料, 以及青藏高原和周边地区75个气象站日平均温度、降水和相对湿度资料, 对高原近地层及北侧气压系统的季节性振荡变化进行了分析. 结果表明: 高原近地层及北侧气压系统强度在围绕中心点顺时针运动时不断加强, 逆时针运动时不断减弱. 两气压系统呈明显的跷跷板式变化, 在600 hPa上表现为高度场空间结构沿经向上的调整; 低高压差负值的开始和结束时间与高原季风起讫时间吻合. 高原夏季降水的起讫不仅与高原及北侧气压系统结构密切相关, 而且与高原东南或南部水汽输送条件息息相关. 相似文献
17.
P. L. S. Rao U. C. Mohanty P. V. S. Raju Gopal Iyengar 《Journal of Earth System Science》2003,112(1):95-111
In this study, we present the mean seasonal features of the Indian summer monsoon circulation in the National Centre for Medium
Range Weather Forecasting (NCMRWF) global data assimilation and forecast system. The large-scale budgets of heat and moisture
are examined in the analyzed and model atmosphere. The daily operational analyses and forecasts (day 1 through day 5) produced
for the summer seasons comprising June, July and August of 1995 and 1993 have been considered for the purpose. The principal
aim of the study is two-fold. Primarily, to comprehend the influence of the systematic errors over the Indian summer monsoon,
secondarily, to analyze the performance of the model in capturing the interseasonal variability.
The heat and moisture balances show reduction in the influx of heat and moisture in the model forecasts compared to the analyzed
atmosphere over the monsoon domain. Consequently, the diabatic heating also indicates reducing trend with increase in the
forecast period. In effect, the strength of Indian summer monsoon, which essentially depends on these parameters, weakens
considerably in the model forecasts. Despite producing feeble monsoon circulation, the model captures interseasonal variability
realistically. Although, 1995 and 1993 are fairly normal monsoon seasons, the former received more rainfall compared to the
latter in certain pockets of the monsoon domain. This is clearly indicated by the analyzed and model atmosphere in terms of
energetics. 相似文献
18.
From the temperature and moisture retrievals from satellites, two types of indices were derived: one indicating suppression
of convection and the other indicating organized deep convection. Sea surface skin temperature and equivalent potential temperatures
up to 500 mbar level of the atmosphere, derived from TIROS-N satellite products, are the basis of the two indices. The maps
of these indices for various phases of 1979 monsoon are compared with percentage cloudiness, a product also available from
TIROS-N satellite observations. Despite the various limitations of satellite soundings, it is shown that these satellite-derived
indices can be used to indicate the strengths of atmospheric convection and inversion over the oceans. 相似文献
19.
陇中黄土高原夏季陆面辐射和热量特征研究 总被引：4，自引：1，他引：3
利用兰州大学半干旱气候与环境观测站(SACOL)的观测资料,分析了陇中黄土高原夏季陆面辐射和热量收支的特征.通过研究不同典型天气条件对陆面过程微气象特征的影响,发现地表反射率在晴天会出现早晨偏大的不对称结构;晴天与多云天气相比不平衡量较大,而阴天时的阵性降水会使局地能量收支出现不平衡.利用最小二乘法(OLS)线性回归得到的夏季平均不闭合度是19.6%.在半干旱区云和降水对辐射和能量收支的影响不容忽视,达到约25%的削弱程度,比极端干旱的敦煌荒漠区要大,又进一步证明了半干旱区夏季的平均气候特征与云量较多的多云天气(5≤Mean total cloud amount<8)接近.另外,7月日平均波恩比最大是4.1,平均是1.95,比极端干旱区的敦煌波恩比小1个数量级,说明榆中所处的黄土高原半干旱区比敦煌所处的极端干旱区在气候上要湿润很多. 相似文献