Anant Parekh

Comparison of wind data from QuikSCAT and buoys in the Indian Ocean

The performance of QuikSCAT‐derived wind vectors is evaluated using in‐situ data from moored buoys over the Indian Ocean. The results show that the mean differences for wind speed and wind direction are 0.37 ms−1 and 5.8°, root mean square deviations are 1.57 ms−1 and 44.1° and corresponding coefficients of correlation are 0.87 and 0.75, respectively. The matching between in‐situ and satellite estimates seems to be better in the North Indian Ocean than in the Equatorial Indian Ocean. The effects of sea surface temperature and air–sea temperature difference on wind residuals were also investigated. In general, QuikSCAT is found to overestimate the winds. It is speculated that low wind speed during rain‐free conditions and high wind speed, normally associated with rain, may be the reason for the less accurate estimation of the wind vector from QuikSCAT over the Indian Ocean.

A Comparative Assessment of Surface Wind Speed and Sea Surface Temperature over the Indian Ocean by TMI, MSMR, and ERA-40

Abstract A 2-yr (June 1999–June 2001) observation of ocean surface wind speed (SWS) and sea surface temperature (SST) derived from microwave radiometer measurements made by a multifrequency scanning microwave radiometer (MSMR) and the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) is compared with direct measurements by Indian Ocean buoys. Also, for the first time SWS and SST values of the same period obtained from 40-yr ECMWF Re-Analysis (ERA-40) have been evaluated with these buoy observations. The SWS and SST are shown to have standard deviations of 1.77 m s−1 and 0.60 K for TMI, 2.30 m s−1 and 2.0 K for MSMR, and 2.59 m s−1 and 0.68 K for ERA-40, respectively. Despite the fact that MSMR has a lower-frequency channel, larger values of bias and standard deviation (STD) are found compared to those of TMI. The performance of SST retrieval during the daytime is found to be better than that at nighttime. The analysis carried out for different seasons has raised an important question as to...

Estimation of Improvement in Indian Summer Monsoon Circulation by Assimilation of Satellite Retrieved Temperature Profiles in WRF Model

This study estimates the improvement in the simulation of Indian summer monsoon (ISM) circulation in the weather research and forecasting (WRF) model by assimilating temperature profiles from atmospheric infrared sounder. Two experiments are carried out from 1st May to 1st October during 2003-2011. In the first experiment control (CTRL), National Centers for Environmental Prediction final analysis forcing is used; whereas the second one (WRFAIRS) is same as CTRL but temperature profiles are assimilated. The improvements in the simulation are quantified using different statistical scores. Overall, the assimilation has improved the spatial and temporal distribution of various fields associated with ISM. Some of the major improvements are 1) elimination of asymmetric (north-south) SLP bias; 2) larger error reduction in winds; 3) reduction in the temperature biases at boundary layer and midtroposphere; 4) improvement in the vertical wind shear; 5) reduction in the water vapor mixing ratio errors by 0.3-0.6 g · Kg-1; and 6) improved simulation of monsoon circulation indices. Further improvements are noticed in dynamic and thermodynamic fields over different convective regions. This study advocates that accurate representation of the thermal structure in WRF is crucial for the simulation of realistic monsoon circulation. It may further pave way for developing/improving convective parameterization schemes for the model.

Evaluation of the impact of AIRS profiles on prediction of Indian summer monsoon using WRF variational data assimilation system

This study investigates the impact of temperature and moisture profiles from Atmospheric Infrared Sounder (AIRS) on the prediction of the Indian summer monsoon, using the variational data assimilation system annexed to the Weather Research and Forecasting model. In this study, three numerical experiments are carried out. The first is the control and includes no assimilation; in the second, named Conv, assimilation of conventional Global Telecommunication System data is performed. The third one, named ConvAIRS, is identical to the Conv except that it also includes assimilation of AIRS profiles. The initial fields of tropospheric temperature and water vapor mixing ratio showed significant improvement over the model domain. Assimilation of AIRS profiles has significant impact on predicting the seasonal mean monsoon characteristics such as tropospheric temperature, low-level moisture distribution, easterly wind shear, and precipitation. The vertical structure of the root-mean-square error is substantially affected by the assimilation of AIRS profiles, with smaller errors in temperature, humidity, and wind magnitude. The consequent improved representation of moisture convergence in the boundary layer (deep convection as well) causes an increase in precipitation forecast skill. The fact that the monsoonal circulation is better captured, thanks to an improved representation of thermal gradients, which in turn leads to more realistic moisture transport, is particularly noteworthy. Several previous data impact studies with AIRS and other sensors have focused on the short or medium range of the forecast. The demonstrated improvement in all the predicted fields associated with the Indian summer monsoon, consequent to the month long assimilation of AIRS profiles, is an innovative finding with large implications to the operational seasonal forecasting capabilities over the Indian subcontinent.

Bimodal variation of SST and related physical processes over the North Indian Ocean: Special emphasis on satellite observations

Using sea surface temperature (SST) and wind speed retrieved by the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), for the period of 1998–2003, we have studied the annual cycle of SST and confirmed the bimodal distribution of SST over the north Indian Ocean. Detailed analysis of SST revealed that the summer monsoon cooling (winter cooling) over the eastern Arabian Sea (Bay of Bengal) is more prominent than winter cooling (summer monsoon cooling). A sudden drop in surface short wave radiation by 57 W m−2 (74 W m−2) and rise in kinetic energy per unit mass by 24 J kg−1 (26 J kg−1) over the eastern Arabian Sea (Bay of Bengal) is observed in summer monsoon cooling period. The subsurface profiles of temperature and density for the spring warming and summer monsoon cooling phases are studied using the Arabian Sea Monsoon Experiment (ARMEX) data. These data indicate a shallow mixed layer during the spring warming and a deeper mixed layer during the summer monsoon cooling. Deepening of the mixed layer by 30 to 40 m with corresponding cooling of 2°C is found from warming to summer monsoon cooling in the eastern Arabian Sea. The depth of the 28°C isotherm in the eastern Arabian Sea during the spring warming is 80 m and during summer monsoon cooling it is about 60 m, while over the Bay of Bengal the 28°C isotherm is very shallow (35 m), even during the summer monsoon cooling. The time series of the isothermal layer depth and mixed layer depth during the warming phase revealed that the formation of the barrier layer in the spring warming phase and the absence of such layers during the summer cooling over the Arabian Sea. However, the barrier layer does exist over the Bay of Bengal with significant magnitude (20–25 m). The drop in the heat content with in first 50 m of the ocean from warming to the cooling phase is about 2.15 × 108 J m−2 over the Arabian Sea.

A study of air-sea interaction following the tsunami of 26 December 2004 in the eastern Indian Ocean

A time series analysis of various atmospheric as well as oceanic parameters derived from different satellites covering eastern Indian Ocean and Bay of Bengal has been carried out for the period December 2004 to February 2005. The purpose is to assess the likely perturbations in the air–sea exchanges associated with the tsunami event of 26 December 2004. Satellite derived sea level anomaly in the northern Bay of Bengal show a rise in the sea level of roughly about 5–6 cm after 26 December 2004. A significant cooling of the order of 0.5°C in a span of 5 days (21–27 December 2004) in the sea surface temperature (SST) is observed near Andaman and Nicobar Island. The formation of such anomalies is certainly associated with the rising of the sub‐surface water towards surface due to the enhanced turbulent exchange. An enhanced turbidity near Sumatra seen in the MODIS colour data in the week following 26 December 2004 is also suggestive of turbulent mixing. After an initial dip in the integrated water vapour (IWV) on 27 December 2004, an apparent growth in the water vapour loading is observed in the Tropical Rainfall Measuring Mission/Microwave Imager (TMI) data from 31 December 2004 to 11 January 2005 near Andaman and Nicobar Island. An analysis of the boundary layer parameters suggests that the increased water vapour loading in the atmosphere following the tsunami is not due to the winds and neither due to the increase in SST. One of the possible reasons could be direct injection of water into the atmosphere. The study could be used as an indicative to understand changes in a global context under sea level rise scenario.

Interannual variability of upper ocean stratification in Bay of Bengal: observational and modeling aspects

The annual cycle and interannual variability of stratification in Bay of Bengal (BoB) are studied using both observations and Global Ocean Data Assimilation System (GODAS) analysis during 2003–2012. Annual cycle of stratification and sea surface temperature (SST) evolve coherently, highlighting its role on modulating air-sea interaction over this climatologically important region. Spatial distribution of stratification shows strong seasonality in ARGO observations, whereas it is highly underestimated in GODAS with highest discrepancies during fall and spring. The annual cycle of sea surface salinity (SSS) in GODAS is out of phase with observations implying potential feedbacks. During La Niña years, SSS drop in fall and winter and are lesser than those reported during El Niño years. All these features are misrepresented in GODAS. As stratification modulates air-sea interaction over BoB especially during El Niño and La Niña years, such misrepresentation of ocean stratification may lead to unrealistic thermocline-SST coupling in the models. The mean stratification and its interannual variability in GODAS are weaker than observed even though interannual variability in freshwater flux (P-E) is higher in GODAS. Detailed analysis of GODAS with in situ observations reveals that upper ocean current shear (vertical) is overestimated in GODAS, leading to unrealistically strong mixing which is primarily responsible for the deeper penetration of surface warm and freshwater resulting weaker stratification. As GODAS is used to initialize the ocean component of the coupled forecasting system for seasonal prediction of Asian monsoon, proper representation of stratification is essential. This study advocates the need of accurate representation of upper ocean salinity in GODAS for improved stratification. We speculate that improved stratification and mixing in the BoB improve summer monsoon forecast.

Processes Associated with the Tropical Indian Ocean Subsurface Temperature Bias in a Coupled Model

AbstractSubsurface temperature biases in coupled models can seriously impair their capability in generating skillful seasonal forecasts. The National Centers for Environmental Prediction (NCEP) Climate Forecast System, version 2 (CFSv2), coupled model, which is used for seasonal forecast in several countries including India, displays warm (cold) subsurface (surface) temperature bias in the tropical Indian Ocean (TIO), with deeper than observed mixed layer and thermocline. In the model, the maximum warm bias is reported between 150- and 200-m depth. Detailed analysis reveals that the enhanced vertical mixing by strong vertical shear of horizontal currents is primarily responsible for TIO subsurface warming. Weak upper-ocean stability corroborated by surface cold and subsurface warm bias further strengthens the subsurface warm bias in the model. Excess inflow of warm subsurface water from Indonesian Throughflow to the TIO region is partially contributing to the warm bias mainly over the southern TIO region....

The role of the Indian Summer Monsoon variability on Arabian Peninsula summer climate

This study investigates the influence of the Indian Summer Monsoon (ISM) on the atmospheric circulation over the Arabian Peninsula (AP) using the European Centre for Medium Range Weather Forecasts’ twentieth century reanalysis (ERA-20C) for the period 1901–2010. After describing the summer climate of the AP using various dynamic and thermodynamic parameters, we investigate the link between extreme ISMs and atmospheric circulation over the AP on inter-annual time scale. Analysis of composites of different parameters during extreme monsoon (strong and weak) years reveals that the ISM plays an important role in the summer circulation over the AP and adjoining regions. The major noticeable changes in modulating circulation during extreme monsoons are: (1) a strengthening of lower tropospheric northerly winds, westerly winds passing through the Tokar Gap, Shamal winds, and the upper tropospheric easterly jet stream during strong ISM; (2) a northward (southward) shift of the subtropical westerly jet stream during strong (weak) monsoon years; (3) the development of strong upper level ridge above the surface thermal low during strong ISM years, which result in a baroclinic structure over the AP and adjoining regions; (4) an increase in adiabatic warming, and hence aridity, over the AP during strong monsoon years, caused by intense subsidence of the middle to upper troposphere due to zonal overturning circulation; and (5) convective instability during strong monsoon years caused by an intensification of the upward motion over the southern AP. Furthermore, during strong monsoons, the availability of excess moisture leads to atmospheric instability, which in turn triggers the formation of clouds that lead to more rainfall over the southwestern AP. Finally, the westward propagation of a Gill-type Rossby waves induced by the ISM play an important role in the variations of the AP summer climate by enhancing the warm core structure over the AP and through their interaction with the midlatitude westerlies during strong monsoons.

Sea Level Variability and Trends in the North Indian Ocean

Sea level rise (over the globe as well as over the Indian Ocean) is one of the most serious issues considering its potential threat to the low-lying coastal areas and coastal population. TOPEX/Poseidon altimeter observations suggest a sea level rise over the Arabian Sea and Bay of Bengal. The rate of sea level rise over the Arabian Sea is about 0.5–3 mm/year, whereas over the Bay of Bengal, it is 0.75–6 mm/year. Major contributors to these changes in the Indian Ocean are steric effect and short-term climate variability such as El Nino–Southern Oscillation (ENSO) and the Indian Ocean Dipole. This affirms that sea level trends over north Indian Ocean get modulated by inter-annual and decadal scale natural climate variability. The inter-annual variability is stronger than decadal variability, which in turn is stronger than the long-term sea level trend. Sea level change in the Indian Ocean is about 1.5 mm/year in the past sixty years or so, whereas the global sea level trends are a bit higher. This picture completely changed in the recent years with very significant sea level rise in the Indian Ocean and is comparable to the global trends. A future projection for the sea level rise over the Indian Ocean based on the highest emission scenario is 40–67 cm for the averaged span of 2071–2100 with the base period of 1921–1990.