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1.
Orography profoundly influences seasonal rainfall amount in several places in south Asia by affecting rain intensity and duration. One of the fundamental questions concerning orographic rainfall is nature of the associated precipitating clouds in the absence of synoptic forcing. It is believed that these clouds are not very deep, however, there is not much information in the literature on their vertical structure. The present study explores the vertical structure of precipitating clouds associated with orographic features in south Asia using data collected with the precipitation radar on board the Tropical Rainfall Measuring Mission satellite. Two types of precipitating clouds have been defined based on cloud echo top height, namely, shallow echo-top cloud and medium echo-top cloud. In both, radar reflectivity factor is at least 30 dBZ at 1.5 km altitude, and tops of shallow and medium echo-top clouds lie below 4.5 km and between 4.5 and 8 km, respectively. The Western Ghats contains the highest fraction of the shallow echo-top clouds followed by the adjacent eastern Arabian Sea, while the Khasi Hills in Meghalaya and Cardamom Mountains in Cambodia contain the least fraction of them. Average vertical profiles of shallow echo-top clouds are similar in different mountainous areas while regional differences are observed in the medium echo-top clouds. Below 3 km, precipitation liquid water content in medium echo-top clouds is the highest over the Western Ghats and the eastern Arabian Sea. The average precipitation liquid water content increases by \(0.16\,\hbox { gm m}^{-3}\) for shallow echo-top clouds between 3 and 1.5 km altitude, while the corresponding increase for medium echo-top clouds is in 0.05–0.08 \(\hbox { gm m}^{-3}\) range. 相似文献
2.
Monthly mean anomaly fields of various parameters like sea surface temperature, air temperature, wind stress, effective radiation
at the surface, heat gain over the ocean and the total heat loss between a good and bad monsoon composite and the evaporation
rates over the Arabian Sea and southern hemisphere have been studied over the tropical Indian Ocean. The mean rates of evaporation
on a seasonal scale over the Arabian Sea during a good and bad monsoon composites were equal (about 2·48 × 10 10 tons/day). The evaporation rates over the southern hemisphere were greater during all the months. The mean evaporation rates
over the southern hemisphere on a seasonal scale for the good and bad monsoon composites were 4·4 × 10 10 and 4·6 × 10 10 tons/day respectively. The maximum evaporation rates over the southern hemisphere were observed in August. The anomalies
of wind stress, effective radiation at the surface and the heat gain over the ocean also exhibit large variations in August,
as compared to other monsoon months. 相似文献
3.
Orissa is one of the most flood prone states of India. The floods in Orissa mostly occur during monsoon season due to very
heavy rainfall caused by synoptic scale monsoon disturbances. Hence a study is undertaken to find out the characteristic features
of very heavy rainfall (24 hours rainfall ≥125 mm) over Orissa during summer monsoon season (June–September) by analysing
20 years (1980–1999) daily rainfall data of different stations in Orissa. The principal objective of this study is to find
out the role of synoptic scale monsoon disturbances in spatial and temporal variability of very heavy rainfall over Orissa.
Most of the very heavy rainfall events occur in July and August. The region, extending from central part of coastal Orissa
in the southeast towards Sambalpur district in the northwest, experiences higher frequency and higher intensity of very heavy
rainfall with less interannual variability. It is due to the fact that most of the causative synoptic disturbances like low
pressure systems (LPS) develop over northwest (NW) Bay of Bengal with minimum interannual variation and the monsoon trough
extends in west-northwesterly direction from the centre of the system. The very heavy rainfall occurs more frequently with
less interannual variability on the western side of Eastern Ghat during all the months and the season except September. It
occurs more frequently with less interannual variability on the eastern side of Eastern Ghat during September. The NW Bay
followed by Gangetic West Bengal/Orissa is the most favourable region of LPS to cause very heavy rainfall over different parts
of Orissa except eastern side of Eastern Ghat. The NW Bay and west central (WC) Bay are equally favourable regions of LPS
to cause very heavy rainfall over eastern side of Eastern Ghat. The frequency of very heavy rainfall does not show any significant
trend in recent years over Orissa except some places in north-east Orissa which exhibit significant rising trend in all the
monsoon months and the season as a whole. 相似文献
4.
Summer monsoon clouds over East Asia observed by the meteorological satellite Himawari and ESSA were analyzed in order to shed light on the dynamic climatology of the area during the Baiu season. The mean cloudiness over Monsoon Asia undergoes little change and is nearly 50%, but the monsoon cloud belt varies in time and space in accordance with the seasonal shift of the strongest westerly flow axis at the 500 mb level. The summer monsoon clouds form a belt in which a major amount of water vapor is transported, extending from South China to the Bering Sea. Monsoon rains occur in Japan when the belt of monsoon cloud is over Japan and ends when the cloud belt moves away. 相似文献
5.
The selective removal of trace metals by suspended matter in high turbidity zones plays a major role in the fluvial transport of terrigenous metals to the marine environment. The seasonal longitudinal variability of trace elements (Cu, Zn, Cd, Ni, Pb, Fe, and Mn) in Cochin estuary, a tropical positive estuary, was studied and the results were compared with the prevailing situation in other subtropical waterways. The hydrodynamical features showed increasing turbidity downstream with increasing salinities during both the seasons. In contrast with the temperate estuaries where the development of turbidity maxima causes the removal of metals, the estuaries of tropics modify the fluvial transport of metals by the way of redistribution between the dissolved and particulate fractions in the intermediate salinities. In Cochin estuary, the distributional features of trace metals are primarily influenced by the variations in salinities and river discharges. Consequently, this gives rise to two different types of distributional patterns: (1) during premonsoon, the estuarine reactivity is more pronounced and hence, mid-estuarine solubilization of the particulate metal appears to play a prominent role in controlling the fluxes of trace metals studied and (2) but during monsoon, the hydrological conditions influence the downstream transport of the metals more by physical dilution than chemical reactivity. 相似文献
6.
Performance of four mesoscale models namely, the MM5, ETA, RSM and WRF, run at NCMRWF for short range weather forecasting has been examined during monsoon-2006. Evaluation is carried out based upon comparisons between observations and day-1 and day-3 forecasts of wind, temperature, specific humidity, geopotential height, rainfall, systematic errors, root mean square errors and specific events like the monsoon depressions.It is very difficult to address the question of which model performs best over the Indian region? An honest answer is ‘none’. Perhaps an ensemble approach would be the best. However, if we must make a final verdict, it can be stated that in general, (i) the WRF is able to produce best All India rainfall prediction compared to observations in the day-1 forecast and, the MM5 is able to produce best All India rainfall forecasts in day-3, but ETA and RSM are able to depict the best distribution of rainfall maxima along the west coast of India, (ii) the MM5 is able to produce least RMSE of wind and geopotential fields at most of the time, and (iii) the RSM is able to produce least errors in the day-1 forecasts of the tracks, while the ETA model produces least errors in the day-3 forecasts. 相似文献
7.
In this observational/diagnostic study, we illustrate the time history of some important parameters of the surface energy
balance during the life cycle of a single monsoon season. This chronology of the surface energy balance portrays the differential
equilibrium state from the preonset phase to the withdrawal phase. This includes an analysis of the time history of base variables
such as soil moisture, ground temperature, cloud cover, precipitation and humidity. This is followed by an analysis of the
components of the surface energy balance where we note subtle changes in the overall balances as we proceed from one epoch
of the monsoon to the next. Of interest here is the transition sequence: preonset, onset, break, revival, break, revival and
withdrawal during the year 2001. Computations are all illustrated for a box over central India where the coastal effects were
small, data coverage was not sparse and where the semi-arid land mass changes drastically to a lush green area. This region
exhibited large changes in the components of surface energy balance. The principal results pertain to what balances the difference
among the incoming short wave radiation (at the earth’s surface) and the long wave radiation exhibited by the ground. That
difference is balanced by a dominant sensible heat flux and the reflected short wave radiation in the preonset stage. A sudden
change in the Bowen ratio going from>1 to <1 is noted soon after the onset of monsoon. Thereafter the latent heat flux from
the land surface takes an important role and the sensible heat flux acquires a diminishing role. We also examine the subtle
changes that occur in the components of surface energy balance between the break and the active phases. The break phases are
seen to be quite different from the preonset phases. This study is aimed to illustrate the major importance of moisture and
clouds in the radiative transfer computations that are central to the surface energy balance during each epoch. These sensitivities
(of moisture and clouds) have major consequences for weather and climate forecasts 相似文献
8.
Monex-79 and ISMEX-73 data have been analysed to study the sub-seasonal scale fluctuation of near equatorial oceanic intertropical convergence zone ( ITCZ) over the North Indian ocean during the summer monsoon of 1979 and 1973. The oceanic ITCZ is characterised by a narrow shear zone between the equatorial westerlies and the tropical easterlies, associated with organised convective clouds. Synoptic analysis presented in this paper shows the steady northward propagation of the oceanic ITCZ from its near equatorial position (5–10°N) to the continental position (20–25°N) during the onset and mid-season revivals of monsoon after breaks. The northward propagation is initiated by the strengthening of the equatorial westerlies which result in the intensification of the shear zone and the embedded disturbances. The establishment of the northward propagating mode near normal monsoon trough position over the continent characterises the active phase of monsoon. As the monsoon cycles from active to weak/break phase, the monsoon trough (continental ITCZ) dissipates near the foothills of the Himalayas and the oceanic ITCZ gets emphasised once again near the equatorial region. The major phase changes in the ITCZ occur at an interval of about 30–50 days which dominantly control the intra-seasonal fluctuation of the Indian summer monsoon. The paper also discusses the characteristic features of the oceanic ITCZ during different phases of the monsoon. 相似文献
9.
In this study, an attempt has been made to bring out the observational aspects of vertical wind shear in thunderstorms over
Minicoy. Case studies of thunderstorm events have been examined to find out the effect of vertical wind shear and instability
on strength and longevity of thunderstorms. Role of vertical wind shear in thunderstorms and its mechanism has been explored
in this study. Results reveal that for prolonged thunderstorms high and low instability along with moderate to high vertical
wind shear (moderate: 0.003 S −1 ≤ vertical wind shear ≤ 0.005 S −1 and high: > 0.005 S −1) play a significant role in longevity and strength of thunderstorms. The mechanism of vertical wind shear in thunderstorms
was investigated in a few cases of thunderstorm events where the duration of thunderstorm was covered by the radiosonde/rawin
ascent observation taken at Minicoy. Empirical model has been developed to classify thunderstorm type and to determine the
strength and longevity of thunderstorms. Model validation has been carried out for selected cases. Model could classify thunderstorm
type for most of the cases of thunderstorm events over island and coastal stations. 相似文献
10.
Global analyses of zonal wind field and thermal field structure at standard pressure levels of 200,150 and 100 mb have been carried out in India during July 1979—a poor monsoon year and July 1975—a good monsoon year. More than 250 stations in the belt 60°N and 60°S were selected. Contrasting features of the zonal wind field structure and thermal field are brought out, and it is shown that monsoon activity is reflected in the upper level and is controlled by planetary scale. 相似文献
11.
Kerala, a southern state of India, experienced a severe flooding due to multi-day extreme rain events during July and August months of 2018. This disaster was one of the worst floods to hit the state and resulted in heavy losses of lives and property. Natural Disaster Management Authority of India reported that 483 people lost their lives and more than 50 lakhs population were affected severely. This short communication focuses on examining this flood event using satellite remote sensing. It is reported that Kerala received an excess of about 56% rainfall during July and August from multi-day extreme rainfall episodes. Few regions of Kerala received the rainfall in the range of 270–300 mm on August 14 and 15. Hourly rainfall events in the excess of 25 mm have also been reported during heavy rainy days. The present study reports that multi-day heavy rainy events during July and August brought an accumulated rainfall of about 1600 mm, which resulted in extreme flooding over Kerala. 相似文献
12.
The measurement of surface energy balance over a land surface in an open area in Bangalore is reported. Measurements of all
variables needed to calculate the surface energy balance on time scales longer than a week are made. Components of radiative
fluxes are measured while sensible and latent heat fluxes are based on the bulk method using measurements made at two levels
on a micrometeorological tower of 10m height. The bulk flux formulation is verified by comparing its fluxes with direct fluxes
using sonic anemometer data sampled at 10Hz. Soil temperature is measured at 4 depths. Data have been continuously collected
for over 6 months covering pre-monsoon and monsoon periods during the year 2006. The study first addresses the issue of getting
the fluxes accurately. It is shown that water vapour measurements are the most crucial. A bias of 0.25% in relative humidity,
which is well above the normal accuracy assumed by the manufacturers but achievable in the field using a combination of laboratory
calibration and field intercomparisons, results in about 20W m −2 change in the latent heat flux on the seasonal time scale. When seen on the seasonal time scale, the net longwave radiation
is the largest energy loss term at the experimental site. The seasonal variation in the energy sink term is small compared
to that in the energy source term. 相似文献
13.
During MONTBLEX 1990, various observational platforms were operated at Kharagpur and the nearby Kalaikunda Air Base. Using the data from all the platforms, one can draw the following conclusions. The temperature and wind data obtained from various sensors have overall compatibility. Sodar wind data indicate the presence of a low level jet at around 300 m above ground. The inversion height may be evaluated from the vertical profile of the sodar back-scatter echo intensity. The sub-synoptic or synoptic scale convergence modulates the inversion height and the presence of cloud-base within the inversion height in turn modulates the sensible heat and momentum fluxes. 相似文献
14.
Having recognized that it is the tropospheric temperature (TT) gradient rather than the land–ocean surface temperature gradient that drives the Indian monsoon, a new mechanism of El Niño/Southern Oscillation (ENSO) monsoon teleconnection has been unveiled in which the ENSO influences the Indian monsoon by modifying the TT gradient over the region. Here we show that equatorial Pacific coralline oxygen isotopes reflect TT gradient variability over the Indian monsoon region and are strongly correlated to monsoon precipitation as well as to the length of the rainy season. Using these relationships we have been able to reconstruct past Indian monsoon rainfall variability of the first half of the 20th century in agreement with the instrumental record. Additionally, an older coral oxygen isotope record has been used to reconstruct seasonally resolved summer monsoon rainfall variability of the latter half of the 17th century, indicating that the average annual rainfall during this period was similar to that during the 20th century. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
15.
A “hot blob”, distinct hot region, is identified over Vidarbha in the south-central parts of the Indian subcontinent during the pre-monsoon season from the analysis of gridded surface air maximum temperature data from India Meteorological Department for the period 1951–2019. Spatial distribution and frequencies of temperatures?>?40 °C and?>?42 °C establish the hot blob over Vidarbha region. A similar analysis of simulated maximum temperatures from the NEX-GDDP substantiates the revelation of the “hot blob” over Vidarbha. Further, analysis of the wind circulation at 850 hPa over South Asia region indicates that the “COL” region between the two seasonal high-pressure systems over the Indian Ocean seas, Bay of Bengal and Arabian Sea promotes accumulation of heat over Vidarbha. Further, horizontal temperature convergence complimented by strong local heating of the black soil aids and abets the sustenance of the “hot blob”. This “hot blob” region is observed to be hotter as well as having higher frequencies of hot days than the north-west desert Rajasthan region and assumes importance as its modulation causes heatwaves over the south-east coastal regions. This study establishes the presence of the hottest region over Vidarbha in south-central parts, paradoxically hotter than the desert north-west region of India. 相似文献
16.
In situ measurements of near-surface ozone ( \(\hbox {O}_{3})\), carbon monoxide (CO), and methane ( \(\hbox {CH}_{4})\) were carried out over the Bay of Bengal (BoB) as a part of the Continental Tropical Convergence Zone (CTCZ) campaign during the summer monsoon season of 2009. \(\hbox {O}_{3}\), CO and \(\hbox {CH}_{4}\) mixing ratios varied in the ranges of 8–54 ppbv, 50–200 ppbv and 1.57–2.15 ppmv, respectively during 16 July–17 August 2009. The spatial distribution of mean tropospheric \(\hbox {O}_{3}\) from satellite retrievals is found to be similar to that in surface \(\hbox {O}_{3}\) observations, with higher levels over coastal and northern BoB as compared to central BoB. The comparison of in situ measurements with the Monitoring Atmospheric Composition & Climate (MACC) global reanalysis shows that MACC simulations reproduce the observations with small mean biases of 1.6 ppbv, –2.6 ppbv and 0.07 ppmv for \(\hbox {O}_{3}\), CO and \(\hbox {CH}_{4}\), respectively. The analysis of diurnal variation of \(\hbox {O}_{3}\) based on observations and the simulations from Weather Research and Forecasting coupled with Chemistry (WRF-Chem) at a stationary point over the BoB did not show a net photochemical build up during daytime. Satellite retrievals show limitations in capturing \(\hbox {CH}_{4}\) variations as measured by in situ sample analysis highlighting the need of more shipborne in situ measurements of trace gases over this region during monsoon. 相似文献
17.
Sea-breeze-initiated convection and precipitation have been investigated along the east coast of India during the Indian southwest
monsoon season. Sea-breeze circulation was observed on approximately 70–80% of days during the summer months (June–August)
along the Chennai coast. Average sea-breeze wind speeds are greater at rural locations than in the urban region of Chennai.
Sea-breeze circulation was shown to be the dominant mechanism initiating rainfall during the Indian southwest monsoon season.
Approximately 80% of the total rainfall observed during the southwest monsoon over Chennai is directly related to convection
initiated by sea-breeze circulation. 相似文献
18.
District-wide drought climatology over India for the southwest monsoon season (June–September) has been examined using two
simple drought indices; Percent of Normal Precipitation (PNP) and Standardized Precipitation Index (SPI). The season drought
indices were computed using long times series (1901–2003) of southwest monsoon season rainfall data of 458 districts over
the country. Identification of all India (nation-wide) drought incidences using both PNP and SPI yielded nearly similar results.
However, the district-wide climatology based on PNP was biased by the aridity of the region. Whereas district-wide drought
climatology based on SPI was not biased by aridity. This study shows that SPI is a better drought index than PNP for the district-wide
drought monitoring over the country. SPI is also suitable for examining break and active events in the southwest monsoon rainfall
over the country. The trend analysis of district-wide season (June–September) SPI series showed significant negative trends
over several districts from Chattisgarh, Bihar, Kerala, Jharkhand, Assam and Meghalaya, Uttaranchal, east Madhya Pradesh,
Vidarbha etc., Whereas significant positive trends in the SPI series were observed over several districts from west Uttar
Pradesh, west Madhya Pradesh, South & north Interior Karnataka, Konkan and Goa, Madhya Maharashtra, Tamil Nadu, East Uttar
Pradesh, Punjab, Gujarat etc. 相似文献
19.
An increase in carbon dioxide (CO 2) concentrations in the atmosphere due to anthropogenic activities is responsible for global warming and hence in recent years, CO 2 measurement network has expanded globally. In the monsoon season (July–September) of year 2011, we carried out measurements of CO 2 and water vapour (H 2O) concentrations along with wind and air temperature over a tropical site in south-east India having rural topography. To collect these observations, the instrumentations used were the sonic anemometer for wind and temperature, and the open path H 2O/CO 2 infrared gas analyzer for CO 2 and H 2O concentrations. Using these observations, we explored the diurnal variability of CO 2 flux along with sensible and latent heat. The CO 2 flux was positive during night-time and negative during daytime and in phase with convective instability. The CO 2 flux relationships with the meteorological parameters such as wind speed, temperature and heat fluxes have been analysed. The seasonal (monsoon) half hour mean of CO 2 flux which was ?3.55 μmol m ???2 s ???1 indicated the experimental site as a CO 2 sink region (net seasonal uptake). An increase in CO 2 concentrations during weekends was not observed due to unavailability of heavy vehicular traffic. 相似文献
20.
Characteristics of trace gases (O 3, CO, CO 2, CH 4 and N 2O) and aerosols (particle size of 2.5 micron) were studied over the Arabian Sea, equatorial Indian Ocean and southwest part
of the Bay of Bengal during the monsoon transition period (October–November, 2004). Flow of pollutants is expected from south
and southeast Asia during the monsoonal transition period due to the patterns of wind flow which are different from the monsoon
period. This is the first detailed report on aerosols and trace gases during the sampled period as the earlier Bay of Bengal
Experiment (BOBMEX), Arabian Sea Monsoon Experiment (ARMEX) and Indian Ocean Experiments (INDOEX) were during monsoon seasons.
The significant observations during the transition period include: (i) low ozone concentration of the order of 5 ppbv around
the equator, (ii) high concentrations of CO 2, CH 4 and N 2O and (iii) variations in PM2.5 of 5–20μg/m 3. 相似文献
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