Investigation on the consequential features of Southwest Monsoon-2007 Onset and Super cyclone "Gonu" using Satellite, Model and Ground-based data

Onset features of the Summer monsoon-2007 were analyzed using data from five different sources, namely,the Tropical Rainfall Measuring Mission (TRMM) 3-hourly rainfall, National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) wind reanalysis data set, NOAA Outgoing Longwave Radiation (OLR), cloud imageries from the meteosat satellite,cloud base height and cloud occurrence frequency from a ground-based Vaisala Laser Ceilometer at Thiruvananthapuram. On the day of onset, 33 mm of rainfall was registered by TRMM satellite over south Kerala region with 80% cloud frequency and an averaged cloud base height of 2 km. On the next day, the formation of ‘Gonu’ super cyclone as a consequence of the convergence of monsoon onset surge in the Arabian Sea has caused the dissipation of cloud bands in the Arabian Sea and in the Bay of Bengal, except over the region of the system. This caused a lull situation for about ten days after the India Meteorological Department (IMD) declared monsoon onset. In fact, the remarkable characteristics of onset, such as deepening of westerlies and strengthening of low level jet streams were observed only after two weeks of IMD declared monsoon onset. Another unique behavior of 2007 monsoon onset was that the Arabian Sea branch of monsoon onset surge has advanced faster than the Bay of Bengal branch in the early stage

Convective thundercloud development over the western ghats mountain slope in Kerala

Studies were carried out on the data from Braemore mountain observatory (lat. 8°45'N, long. 77°5'E) using a single-lens ceilometer (LIDAR), an electric field mill and a portable automatic weather station throughout the year 2010. The simultaneous data collected using the above instruments indicate the existence of strong updrafts followed by the formation of thunderclouds, a characteristic of the mountain slopes, during the thunderstorm months. Changes in atmosphere related to condensation and formation of water droplets during updraft events on the mountain slope could be detected from the ceilometer scattering data. Results of the study point to the cause of relatively more thunderstorm activity in that zone. This seems to be due to excessive updraft, which is strongly related to lightning activity in the region

Indian summer monsoon rainfall and its relation with SST in the equatorial Atlantic and Pacific Oceans

A better understanding of the factors responsible for the variability in Indian summer monsoon rainfall (ISMR) is important in the present day climate because it is vulnerable to climate change. Here, we report the relationship between the Atlantic and Pacific sea surface temperature (SST) anomalies with ISMR. We developed a new ISMR index based on the high correlation of the SST anomaly over the oceanic regions during March–April, prior to the southwest monsoon season. The study was performed by utilizing rainfall data from the India Meteorological Department and SST anomaly derived from monthly extended reconstruction sea surface temperature data. We found that the relationship of the SST anomaly in the southwest Pacific region during the March–April period with the following ISMR is persistent in the 31-year sliding window correlation analysis. SST anomaly in the Equatorial Atlantic during the March–April period is related almost in a similar manner to the following ISMR. However, persistence of the significance level is fluctuating. This indicates that the ISMR and the SST anomalies have varying relationships. To find out the fluctuation in the relationship between ISMR and the SSTs, we subjected the parameters to harmonic analysis. We noticed that the SST anomalies have similar kind of variability in the multidecadal (32 year), decadal (10.5) and interdecadal (5.6) periodicities in both Atlantic and southwest Pacific regions. The harmonics of SST and ISMR in the multidecadal time scale is out of phase and that for decadal and interdecadal time scales are in phase

The cold tongue in the South China Sea during boreal winter and its interaction with the atmosphere

A distinct cold tongue has recently been noticed in the South China Sea during the winter monsoon, with the cold tongue temperature minimum occurring in the January or February. This cold tongue shows significant links with the Maritime Continent's rainfall during the winter period. The cold tongue and its interaction with the Maritime Continent's weather were studied using Reynolds SST data, wind fields from the NCEP-NCAR reanalysis dataset and the quikSCAT dataset. In addition, rainfall from the GOES Precipitation Index (GPI) for the periods 2000 to 2008 was also used. The propagation of the cold tongue towards the south is explained using wind dynamics and the western boundary current. During the period of strong cold tongue, the surface wind is strong and the western boundary current advects the cold tongue to the south. During the period of strong winds the zonal gradient of SST is high [0.5A degrees C (25 km)(-1)]. The cold tongue plays an important role in regulating the climate over the Maritime Continent. It creates a zonal/meridional SST gradient and this gradient ultimately leads in the formation of convection. Hence, two maximum precipitation zones are found in the Maritime Continent, with a zone of relatively lower precipitation between, which coincides with the cold tongue's regions. It was found that the precipitation zones have strong links with the intensity of the cold tongue. During stronger cold tongue periods the precipitation on either side of the cold tongue is considerably greater than during weaker cold tongue periods. The features of convection on the eastern and western sides of the cold tongue behave differently. On the eastern side convection is preceded by one day with SST gradient, while on the western side it is four days

Spatial and temporal characteristics of rain intensity in the peninsular Malaysia using TRMM rain rate

The present study is focused on the intensity distribution of rainfall in different classes and their contribution to the total seasonal rainfall. In addition, we studied the spatial and diurnal variation of the rainfall in the study areas. For the present study, we retrieved data from TRMM (Tropical Rain Measuring Mission) rain rate available in every 3 h temporal and 25 km spatial resolutions. Moreover, station rainfall data is used to validate the TRMM rain rate and found significant correlation between them (linear correlation coefficients are 0.96, 0.85, 0.75 and 0.63 for the stations Kota Bharu, Senai, Cameron highlands and KLIA, respectively). We selected four areas in the Peninsular Malaysia and they are south coastal, east coastal, west coastal and highland regions. Diurnal variation of frequency of rain occurrence is different for different locations. We noticed bimodal variation in the coastal areas in most of the seasons and unimodal variation in the highland/inland area. During the southwest monsoon period in the west coastal stations, there is no distinct diurnal variation. The distribution of different intensity classes during different seasons are explained in detail in the results

Diurnal and spatial variation of Indian summer monsoon rainfall using tropical rainfall measuring mission rain rate

This paper deals with the diurnal and spatial variation of summer monsoon (JJAS) rainfall, extremes and its frequency for a period of 12. years using the TRMM (Tropical Rainfall Measuring Mission) rain rate data set. Rainfall and frequency of rain events are more observed over the western coastal belts, northeastern regions, foothills of Himalayas and Bay of Bengal regions. Average seasonal rainfall is about 400. mm from 150. rainy hours. The rainfall amount and frequency of rain events show a peak at 1730 IST over Central India, 0230 IST over the foothills of Himalayas and 0830 IST over the north Bay of Bengal. Rainfall over the west coast is more evenly distributed throughout the day. Diurnal pattern of rain intensity is different than that of the rainfall, it is more over the Gujarat regions and peak is observed at 1730 IST and 0230 IST. The relative contribution of rainfall by each octet towards the seasonal total is different. Contribution of rainfall towards its seasonal is maximum at 1730 IST over the main land and is different over other regions. In addition to this, the diurnal pattern of high intensity (90th percentile and above) and low intensity (10th percentile and below) rainfall during the season were also analyzed to find out their spatial distribution. We found that the diurnal pattern is more prominent for the high intensity rainfall than that for the low rainfall intensity. Maximum rainfall hours show systematic southward propagation from early morning to late night

Indian Summer Monsoon Rainfall Characteristics During Contrasting Monsoon Years

The present paper presents a diagnostic study of two recent monsoon years, of which one is dry monsoon year (2009) and the other is wet monsoon year (2010). The study utilized the IMD gridded rainfall data set in addition to the Reynolds SST, NCEP-NCAR reanalysis wind and temperature products, and NOAA OLR. The study revealed that the months July and August are the most crucial months to decide whether the ISMR is wet or dry. However, during July 2009, most of the Indian subcontinent received more than 60 in the central and western coastal regions. In a wet monsoon year, about 35-45 of rainfall is contributed during June and July in most parts of India. During these years, the influence of features in the Pacific Ocean played vital role on the Indian summer monsoon rainfall. During 2009, Pacific SST was above normal in nino regions, characteristic of the El Nino structure; however, during 2010, the nino regions were clearly below normal temperature, indicating the La Nina pattern. The associated atmospheric general circulation through equatorial Walker and regional Hadley circulation modulates the tropospheric temperature, and hence the organized convective cloud bands. These cloud bands show different characteristics in northward propagation during dry and wet years of ISMR. During a dry year, the propagation speed and magnitudes are considerably higher than during a wet monsoon year

Precipitation extremes during Indian summer monsoon: role of cyclonic disturbances

The Indian summer (JJAS) shows high variability in both space and timescales. Changes in precipitation extremes play an important role on the regional scale due to their serious socio-economic consequences. This study, therefore, is mainly focused on understanding the variation of precipitation extremes during summer monsoon season in the presence of cyclonic disturbances forming over the Bay of Bengal (BOB), Arabian Sea, Land Area (LA) and Total. For this, several indices of observed precipitation extremes, in terms of frequencies, intensities and spell duration have been computed for the period 1951–2007 using daily APHRODITE data of 0.5° latitude × 0.5° longitude resolution. Correlation analysis reveals that a large part of the country exhibits positive relationship between the indices of precipitation extremes and frequency of cyclonic disturbances. Correlations with the indices of frequencies defined as seasonal count of days when rainfall exceeds 30, 20 and 10 mm show that spatial extent and strength of the positive relationship decreases with increase in threshold values. Disturbances forming over BOB play dominant role in precipitation during Indian summer monsoon