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K. G. Vernekar, Brij Mohan, Sangeeta Saxena, and M. N. Patil

Abstract

A tethersonde observational program was conducted for eight days during February and March 1989 at Pune (18°32′N, 73°51′E, 559 m MSL), India, during daytime for the study of mixed-layer evolution. A surface- based nocturnal inversion of intensity 8–10 K (100 m)−1 was observed on most of the days. It takes about 2 h after sunrise for this inversion to start eroding and 4–5 h for complete erosion. Profiles of potential temperature, mixing ratio, and wind are discussed. Using Tennekes and Driedonks’ convective boundary-layer model an average sensible heat flux of 129 W m−2 is estimated during morning hours, which is comparable to the flux value obtained by an earlier study using an eddy correlation technique at a nearby site during March and April.

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David G. Dewitt, Edwin K. Schneider, and Anandu D. Vernekar

Abstract

The roles played by the large-scale motion induced by vertical diffusion of heat from the lower boundary and condensational heating due to deep convection in maintaining the precipitation zones in the Tropics of an atmospheric general circulation model (GCM) are explored. A steady linearized version of the GCM is used to diagnose the wind forced by these processes. The wind field obtained from the linear model is combined with the time-mean moisture field from the GCM in order to determine the zonally asymmetric moisture flux convergence, which is the primary factor maintaining the zonally asymmetric precipitation distribution. The role of the other diabatic heating processes is explored as is the role of the orographic forcing in maintaining the precipitation distribution.

The vertically integrated moisture flux convergence forced by vertical diffusion of heat and condensational heating are found to be in phase over the ocean and 180 degrees out of phase over the land. Over the ocean, both of these forcings contribute to moisture flux convergence in the regions of largest precipitation. The moisture flux convergence forced by the vertical diffusion of heat tends to narrow the precipitation zones in the meridional direction over the ocean. Over the land, the condensational heating leads to moisture flux convergence in the regions of large precipitation, while the vertical diffusion of heat leads to moisture flux divergence. This indicates that the motions forced by the surface temperature provide a negative feedback on the precipitation. This feedback is apparently due to the relatively cool surface temperatures present in the regions of large precipitation over land. This locally cool surface temperature leads to a low-level divergent circulation from the cool region to warmer regions. Other forcing functions are found to play a minor role in the moisture flux convergence by the time-mean flow with the exception of the orographic forcing in some regions.

The lowest model sigma-level wind field over the tropical Pacific Ocean is examined. In general both the zonal and meridional wind fields are dominated by the response to convective condensational heating. Exceptions include the meridional wind in the western Pacific and the zonal wind along the equator. In these regions, the response to low-level temperature gradients is found to be nonnegligible in comparison with the response to convective condensational heating. The role of the orographic forcing is also significant along the coasts of the tropical continents and in the western Pacific.

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P. C. S. Devara, P. Ernest Raj, B. S. Murthy, G. Pandithurai, S. Sharma, and K. G. Vernekar

Abstract

Coordinated experiments to study the nocturnal lower atmosphere were conducted on selected nights during April–August 1991 using an argon ion lidar and a Doppler sodar at the Indian Institute of Tropical Meteorology, Pune (18°32′N, 73°51′E, 559 m MSL), India. The lidar and the sodar have been operated simultaneously so as to detect the nocturnal atmospheric structure in the common air volume sampled by both the techniques. By analyzing the thermal and aerosol structures in the vertical profiles of the sodar and the lidar signal intensity, the nocturnal mixed-layer height or ground-based inversion height and the stably stratified or multiple elevated layers aloft have been determined. The top of the nocturnal ground-based inversion observed in the sodar records is taken as the height above the ground where the negative vertical gradient in aerosol concentration first reaches a maximum in the lidar records. The results of the study indicate an agreement between the lidar-derived mixing depth and the sodar-derived heights of the ground-based inversion and the low-level wind maximum.

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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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