Dynamics of Heavy Rain Spells over India during 2005

High-impact mesoscale weather events, occurring in different parts of India in all seasons, lead to major weather and climate related disasters. In view of this, an attempt has been made in the present study to understand the dynamics of atmospheric circulation over the Indian region (50˚E-100˚E and EQ-30˚N) during super active monsoon period 21st-30th June and 22nd-31st July 2005 using NCEP/NCAR reanalyzed daily winds (u and v), temperature (T) from surface to 200 hPa to compute the divergence (D), vorticity , vertical velocity ,static stability and heat source (QH). (p) profiles clearly indicate the unstable zone between 850 to 650 hPa. The necessary condition of barotropic instability and condition of baroclinic instability are also satisfied in the layer 850- 650 hPa for all the active monsoon days. Further, zonal (u) and meridional (v) winds, are examined in the zonal (x-p) and meridional (y-p) planes. It is noticed that there exist low level cyclonic circulations in the boundary layer over major portion of the country and abnormal increase in the meridional wind in the lower and middle troposphere. Significant increase in the cyclonic vorticity upto 300 hPa and the upward motion throughout in the troposphere with maximum value around 700 to 500 hPa and positive heat source explain the possibility of highly convective and unstable mid-tropospheric zone. The study indicates that there are few mesoscale systems embedded in the synoptic scale system that are already present in the large-scale monsoon circulation

Observed twin gyres and their interannual variability in the equatorial Indian Ocean using Topex/Poseidon altimetry

Recent numerical models have simulated the equatorial Indian Ocean twin gyres. However, there is no strong observational evidence for the existence of these gyres. Satellite technology and filter techniques are explored in this study to provide observational evidence for the existence of these gyres. The westward-propagating Rossby waves in the equatorial Indian Ocean, filtered from the Topex/Poseidon sea surface height anomalies (SSHA) showed the twin gyre structure which coincides with the model-simulated gyres. The present study further addresses the interannual variability of these gyres, especially during the Indian Ocean Dipole (IOD) years. The anomalous wind stress curl is found to drag the annual Rossby waves during the IOD years in the region 78° E-88° E. The downwelling favourable wind stress curl deepens the thermocline and increases SSHA and sea surface temperature

Variability in the Indian Ocean circulation and salinity and its impact on SST anomalies during dipole events

The GFDL Modular Ocean Model (MOM4) has been used to understand the variability of the Indian Ocean circulation and salinity during Indian Ocean Dipole events. The model simulations are compared with HadISST, SODA and ECCO data sets. During the positive dipole years, the climatological cyclonic circulation in the Bay of Bengal weakens or is replaced by an anticyclonic circulation. The interannual variability in the Wyrtki Jet and Bay of Bengal circulation has significant influence on fresh water transport between the equatorial Indian Ocean and Bay of Bengal. The salinity anomalies in the equatorial Indian Ocean are significant during the positive dipole years. The salinity anomalies are positive in the southeastern equatorial Indian Ocean and negative in the central equatorial Indian Ocean. The advection of low salinity water from the eastern equatorial Indian Ocean and Bay of Bengal is attributed to the salinity anomalies in the central equatorial Indian Ocean. The salinity variability in the equatorial Indian Ocean influences the surface and subsurface temperatures by forming or eroding the barrier layer

Impact of cloud parameterization on the numerical simulation of a super cyclone

This study examines the role of parameterization of convection and explicit moisture processes on the simulated track, intensity and inner core structure of Orissa super cyclone (1999) in Bay of Bengal (north Indian Ocean). Sensitivity experiments are carried out to examine the impact of cumulus parameterization schemes (CPS) using MM5 model (Version 3.7) in a two-way nested domain (D1 and D2) configuration at horizontal resolutions (45-15 km). Three different cumulus parameterization schemes, namely Grell (Gr), Betts-Miller (BM) and updated Kain Fritsch (KF2), are tested. It is noted that track and intensity both are very sensitive to CPS and comparatively, KF2 predicts them reasonably well. Particularly, the rapid intensification phase of the super cyclone is best simulated by KF2 compared to other CPS. To examine the effect of the cumulus parameterization scheme at high resolution (5 km), the three-domain configuration (45-15-5 km resolution) is utilized. Based on initial results, KF2 scheme is used for both the domains (D1 and D2). Two experiments are conducted: one in which KF2 is used as CPS and another in which no CPS is used in the third domain. The intensity is well predicted when no CPS is used in the innermost domain. The sensitivity experiments are also carried out to examine the impact from microphysics parameterization schemes (MPS). Four cloud microphysics parameterization schemes, namely mixed phase (MP), Goddard microphysics with Graupel (GG), Reisner Graupel (RG) and Schultz (Sc), are tested in these experiments. It is noted that the tropical cyclone tracks and intensity variation have considerable sensitivity to the varying cloud microphysical parameterization schemes. The MPS of MP and Sc could very well capture the rapid intensification phase. The final intensity is well predicted by MP, which is overestimated by Sc. The MPS of GG and RG underestimates the intensity

North Indian Ocean warming and sea level rise in an OGCM

The variability in the long-term temperature and sea level over the north Indian Ocean during the period 1958–2000 has been investigated using an Ocean General Circulation Model, Modular Ocean Model version 4. The model simulated fields are compared with the sea level observations from tide-gauges, Topex/Poseidon (T/P) satellite, in situ temperature profile observations from WHOI moored buoy and sea surface temperature (SST) observations from DS1, DS3 and DS4 moored buoys. It is seen that the long (6–8 years) warming episodes in the SST over the north Indian Ocean are followed by short episodes (2–3 years) of cooling. The model temperature and sea level anomaly over the north Indian Ocean show an increasing trend in the study period. The model thermocline heat content per unit area shows a linear increasing trend (from 1958–2000) at the rate of 0.0018 × 1011 J/m2 per year for north Indian Ocean. North Indian Ocean sea level anomaly (thermosteric component) also shows a linear increasing trend of 0.31 mm/year during 1958–2000

Westward propagating twin gyres in the equatorial Indian Ocean

A reduced-gravity (11/2-layer) model forced by daily climatological winds simulates twin, anticyclonic gyres, which propagate westward on either side of the equator. The gyres form at the beginning of both the Southwest Monsoon and the Northeast monsoon in the equatorial eastern Indian Ocean, and subsequently propagate across the basin. Their existence is supported by velocity observations taken during WOCE in 1995 and by TOPEX/Poseidon sea-level observations during 1993. They are also present in the ECCO model/data product. They form at the front of a Rossby-wave packet generated by the reflection of the equatorial jet (EJ) from the eastern boundary of the basin. They are likely either Rossby solitons or result from the nonlinear interaction between the EJ and the Rossby-wave front

Indian Ocean dipole mode events in a simple mixed layer ocean model

A precise knowledge of sea surface temperature (SST) is very essential for climate and oceanographic studies. In this paper a simple two dimensional mixed layer ocean model and its numerical code have been developed and used to simulate the SST fields over the north Indian Ocean (20°S-25°N and 35°E-115°Î) for a period of 10 years (1992-2001). The model simulated the SST variability reasonably well. The simple model could simulate the observed dipole of 1997 and 1994 very well, especially the eastern cooling. The model study showed that the interannual SST variability in the western equatorial Indian Ocean is not only due to the variability in the surface heat fluxes, but also due to the variability in wind and sea surface height (SSH). The OLR anomaly also shows positive (negative) anomaly over the negative (positive) anomalous SST region. The variability in the latent heat flux is found to be greatly influencing the SST variability in the eastern equatorial Indian Ocean

Patchy layered structure of tropical troposphere as seen by Indian MST radar

The MST radar observations at Gadanki (13.47° N, 79.18° E) show, almost every day throughout the year, stratified layers of intense reflectivity near the tropopause level (17 km) and also at a couple of levels between 4 km and 10 km. Highest individual reflectivity values occur near 17 km, but they occur for a short while. The region between 11 km and 15 km shows the lowest values of reflectivity alongwith vertical downward motion almost on all days of the year. High values of reflectivity are attributed to the existence of visible or sub-visible clouds; the layered structure of clouds is attributed to inertio-gravity waves with vertical wavelength of 2-3 km. It is suggested that each high reflectivity layer consists mainly of thin sheets and patches of visible and sub-visible cloud material. Hydrometeors inside the cloud material go up and down due to gravity, precipitation-loading, Brunt-Vaisala oscillations, and Kelvin-Helmholtz waves. In these small-scale motions, thin air sheets and patches get formed with sharp temperature and humidity discontinuities through contact cooling, melting, evaporation, condensation and freezing. Also, melting and freezing at low temperatures generate electrical charges in these thin sheets and patches. These thin sheets and patches have vertical dimensions ranging from a few centimetres to several metres and horizontal dimensions of the order of 1km. These thin sheets and patches have corresponding vertical and horizontal discontinuities and sharp gradients in refractive index for the MST radar beam. These show up as regions of high values of reflectivity