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  • Author or Editor: P. V. Joshi x
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V. Sathiyamoorthy, P. K. Pal, and P. C. Joshi

Abstract

Using the top-of-the-atmosphere radiative flux and cloud data from satellites, as well as atmospheric data from NCEP–NCAR reanalysis, this paper investigates the reason for the unusually large high-cloud amount in the Asian monsoon region during the summer monsoon season (June–September). Earlier studies attributed the large negative net cloud radiative forcing in the Asian monsoon region to the unusually large high-cloud amounts with high optical depth. Analysis during 1985–89 suggests that the unique upper-tropospheric easterly wind shear [tropical easterly jet (TEJ)], present over the Asian monsoon region during the summer monsoon season, may be responsible for the unusual increase in cloud amount. This strong wind shear sweeps the cloud tops and may be unfavorable for cloud growth beyond about 300 hPa. The spreading of cloud tops by wind may increase the high-cloud amount. A significant association is found between the high-cloud amount and the speed of the easterly jet. In addition, magnitudes of the shortwave, longwave, and net cloud radiative forcing also strongly depend upon the variations in the speed of TEJ.

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V. Rakesh, Randhir Singh, P. K. Pal, and P. C. Joshi

Abstract

Assimilation experiments have been performed with the Weather Research and Forecasting (WRF) model’s three-dimensional variational data assimilation (3DVAR) scheme to assess the impacts of NASA’s Quick Scatterometer (QuikSCAT) near-surface winds, and Special Sensor Microwave Imager (SSM/I) wind speed and total precipitable water (TPW) on the analysis and on short-range forecasts over the Indian region. The control (without satellite data) as well as WRF 3DVAR sensitivity runs (which assimilated satellite data) were made for 48 h starting daily at 0000 UTC during July 2006. The impacts of assimilating the different satellite dataset were measured in comparison to the control run, which does not assimilate any satellite data. The spatial distribution of the forecast impacts (FIs) for wind, temperature, and humidity from 1-month assimilation experiments for July 2006 demonstrated that on an average, for 24- and 48-h forecasts, the satellite data provided useful information. Among the experiments, WRF wind speed prediction was improved by QuikSCAT surface wind and SSM/I TPW assimilation, while temperature and humidity prediction was improved due to the assimilation of SSM/I TPW. The rainfall prediction has also been improved significantly due to the assimilation of SSM/I TPW, with the largest improvement seen over the west coast of India. Through an improvement of the surface wind field, the QuikSCAT data also yielded a positive impact on the precipitation, particularly for day 1 forecasts. In contrast, the assimilation of SSM/I wind speed degraded the humidity and rainfall predictions.

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P. V. N. Nair, P. V. Joshi, U. C. Mishra, and K. G. Vohra

Abstract

Computed values of equilibrium sizes and composition of uncharged and charged aqueous solution droplets of HNO3 and HCl at 25°C, relative humidifies from 5 to 101%, and solute vapor activities from 10 to 107 are presented. Threshold concentrations for heteromolecular nucleation of HNO3 and HCl at 40, 30, 20, 10, 0, and −10°C are also given as a function of the relative humidity. It is shown that atmospheric concentrations of HNO3 and HCl can participate in nucleation at temperatures below 20°C and relative humidifies above 98%. The nature of the nucleation of HNO3 and HCl in the atmosphere including ion-induced nucleation for which there is no threshold and some of the available experimental results are discussed.

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P. N. Vinayachandran, Adrian J. Matthews, K. Vijay Kumar, Alejandra Sanchez-Franks, V. Thushara, Jenson George, V. Vijith, Benjamin G. M. Webber, Bastien Y. Queste, Rajdeep Roy, Amit Sarkar, Dariusz B. Baranowski, G. S. Bhat, Nicholas P. Klingaman, Simon C. Peatman, C. Parida, Karen J. Heywood, Robert Hall, Brian King, Elizabeth C. Kent, Anoop A. Nayak, C. P. Neema, P. Amol, A. Lotliker, A. Kankonkar, D. G. Gracias, S. Vernekar, A. C. D’Souza, G. Valluvan, Shrikant M. Pargaonkar, K. Dinesh, Jack Giddings, and Manoj Joshi

Abstract

The Bay of Bengal (BoB) plays a fundamental role in controlling the weather systems that make up the South Asian summer monsoon system. In particular, the southern BoB has cooler sea surface temperatures (SST) that influence ocean–atmosphere interaction and impact the monsoon. Compared to the southeastern BoB, the southwestern BoB is cooler, more saline, receives much less rain, and is influenced by the summer monsoon current (SMC). To examine the impact of these features on the monsoon, the BoB Boundary Layer Experiment (BoBBLE) was jointly undertaken by India and the United Kingdom during June–July 2016. Physical and biogeochemical observations were made using a conductivity–temperature–depth (CTD) profiler, five ocean gliders, an Oceanscience Underway CTD (uCTD), a vertical microstructure profiler (VMP), two acoustic Doppler current profilers (ADCPs), Argo floats, drifting buoys, meteorological sensors, and upper-air radiosonde balloons. The observations were made along a zonal section at 8°N between 85.3° and 89°E with a 10-day time series at 8°N, 89°E. This paper presents the new observed features of the southern BoB from the BoBBLE field program, supported by satellite data. Key results from the BoBBLE field campaign show the Sri Lanka dome and the SMC in different stages of their seasonal evolution and two freshening events during which salinity decreased in the upper layer, leading to the formation of thick barrier layers. BoBBLE observations were taken during a suppressed phase of the intraseasonal oscillation; they captured in detail the warming of the ocean mixed layer and the preconditioning of the atmosphere to convection.

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