Search Results

You are looking at 1 - 9 of 9 items for

  • Author or Editor: A. Deepak x
  • Refine by Access: All Content x
Clear All Modify Search
M. A. Box and A. Deepak

Abstract

The retrieval of aerosol size distributions from solar aureole measurements, in the past, have been made tractable by assuming the single-scattering approximation, since the inclusion of multiple scattering by solving the complete radiative transfer equation would make such retrievals prohibitively expensive. Recently, ways have been sought to improve the accuracy of such retrievals without increasing the computational effort afforded by the single-scattering approximation. This has led to the development of a perturbation approximation, which makes allowance for molecular multiple scattering; aerosol multiple-scattering events are largely ignored. In this paper, the derivation of an empirical expression to account for this molecular multiple scattering is discussed, and the accuracy of this approximation is illustrated by some examples of the direct problem for the solar aureole.

Full access
Deepak A. Cherian and K. H. Brink

Abstract

Isolated monopolar eddies in the ocean tend to move westward. Those shed by western boundary currents may then interact with the continental margin. This simple picture is complicated by the presence of other flow features, but satellite observations show that many western boundary continental shelves experience cross-shelfbreak exchange flows forced by mesoscale eddies translating near the shelf break. Here we extend our previous study of eddy interaction with a flat shelf to that with a sloping shelf. Using a set of primitive equation numerical simulations, we address the vertical structure of the onshore and offshore flows forced by the eddy, the origin of the exported shelf water, and the extent to which eddy water can penetrate onto the shelf. The simulations reveal an asymmetry in the vertical structure of cross-shelfbreak flows: the offshore flow is generally barotropic, whereas the onshore flow is always baroclinic. The exported shelf water is sourced from downstream of the eddy in the coastal-trapped wave direction and is supplied by a barotropic alongshore jet. This “supply jet” has a Rhines-like cross-shore length scale proportional to (eddy velocity scale/shelf topographic beta)1/2 measured from the shelf break. Eddy water is forced onto the shelf and is present up to a distance of one internal Rossby deformation radius, defined using shelf properties, from the shelf break. We rationalize these horizontal and vertical scales, connect them to existing observations, and extend our previous parameterization of eddy-forced offshore shelf-water flux to account for nonzero shelf slopes.

Full access
Deepak A. Cherian and K. H. Brink

Abstract

At continental margins, energetic deep-ocean eddies can transport shelf water offshore in filaments that wrap around the eddy. One example is that of Gulf Stream warm-core rings interacting with the Mid-Atlantic Bight shelf. The rate at which shelf water is exported in these filaments is a major unknown in regional budgets of volume, heat, and salt. This unknown transport is constrained using a series of idealized primitive equation numerical experiments wherein a surface-intensified anticyclonic eddy interacts with idealized shelf–slope topography. There is no shelfbreak front in these experiments, and shelf water is tracked using a passive tracer. When anticyclones interact with shelf–slope topography, they suffer apparent intrusions of shelf–slope water, resulting in a subsurface maximum in offshore transport. The simulations help construct an approximate model for the filament of exported water that originates inshore of any given isobath. This model is then used to derive an expression for the total volume of shelf–slope water transported by the eddy across that isobath. The transport scales with water depth, radius, and azimuthal velocity scale of the eddy. The resulting expression can be used with satellite-derived eddy properties to estimate approximate real-world transports ignoring the presence of a shelfbreak front. The expression assumes that the eddy’s edge is at the shelf break, a condition not always satisfied by real eddies.

Full access
Thomas W. N. Haine and Deepak A. Cherian

The dynamics of the rotating shallow-water (RSW) system include geostrophic f low and inertial oscillation. These classes of motion are ubiquitous in the ocean and atmosphere. They are often surprising to people at first because intuition about rotating f luids is uncommon, especially the counterintuitive effects of the Coriolis force. The gyroscope, or toy top, is a simple device whose dynamics are also surprising. It seems to defy gravity by not falling over, as long as it spins fast enough. The links and similarities between rotating rigid bodies, like gyroscopes, and rotating fluids are rarely considered or emphasized. In fact, the dynamics of the RSW system and the gyroscope are related in specific ways and they exhibit analogous motions. As such, gyroscopes provide important pedagogical opportunities for instruction, comparison, contrast, and demonstration. Gyroscopic precession is analogous to geostrophic flow and nutation is analogous to inertial oscillation. The geostrophic adjustment process in rotating fluids can be illustrated using a gyroscope that undergoes transient adjustment to steady precession from rest. The controlling role of the Rossby number on RSW dynamics is reflected in a corresponding nondimensional number for the gyroscope. The gyroscope can thus be used to illustrate RSW dynamics by providing a tangible system that behaves like rotating fluids do, such as the large-scale ocean and atmosphere. These relationships are explored for their potential use in educational settings to highlight the instruction, comparison, contrast, and demonstration of important fluid dynamics principles.

Full access
Gail P. Box, Michael A. Box, and Adarsh Deepak

Abstract

This paper describes the retrieval of aerosol size distributions from the measurements of multispectral solar extinction data, by the use of two retrieval methods—a fast approximate method and the numerical nonlinear least-squares (NLLS) method. Comparison of eight sets of results obtained by the two methods, in general, show a good agreement to within 15%. In addition, size distribution results retrieved by the approximate method show that the method is particularly sensitive to the wavelengths used, and differences in the wavelengths used for making observations and for generating the look-up tables can lead to appreciable errors in retrieved results. Thus, one cannot assume a universal data base for use with radiometers with different wavelengths; generation of a separate data base for each set of wavelengths is recommended.

Full access
Deepak K. Rajopadhyaya, Peter T. May, and Robert A. Vincent

Abstract

Recently, the authors observed significant echoes from precipitating ice particles above the freezing level in the stratiform region of tropical squall lines with a 50-MHz wind-profiling radar. A technique is described that allows ice particle size distributions to be obtained from the 50-MHz wind-profiling radar spectra. A model ice echo is developed in which it is assumed the number density of ice particles is exponentially distributed. A composite spectrum of dendrites, plates, columns, and bullets is assumed, and good fits to the observed spectra are obtained. To test the reliability and stability of the retrieval technique, simulated data with realistic statistical properties were generated and the shape of the model size distribution varied. Only two types of ice particles are considered. It is shown that the population spectra are recovered well for a wide range of exponential slopes. Relative precision of about 10% is obtained when the clear-air spectral width is 0.1 m s−1, and is about 30% when the spectral width is 0.3 m s−1. However, when the spectral width is large, such as 0.5 m s−1, the relative error can exceed 100%. Spectral widths of about 0.3 m s−1 are typically observed in the trailing stratiform region of tropical squall lines.

Full access
Deepak K. Rajopadhyaya, Peter T. May, and Robert A. Vincent

Abstract

A technique is described that allows estimates of the raindrop size distribution to be obtained from the Doppler spectra measured by wind-profiling radars. The method makes no a priori assumptions regarding the shape of the drop size distributions. To test the accuracy of the technique, artificial data with realistic statistical properties have been generated and the shape of the model drop size distribution varied, The analysis technique obtains an accuracy of around 10% in the drop size range between 1 and 4 mm for data consistent with typical 50-MHz observations averaged over 5–10 min. There are limitations outside this range and the physical reasons for these are discussed. Simulations with multiple-peaked distributions show that the technique can also well resolve complicated distributions

Full access
G. S. Kent, U. O. Farrukh, P. H. Wang, and A. Deepak

Abstract

The SAGE-I and SAM-II satellite sensors were designed to measure, with global coverage, the 1 μm extinction produced by the stratospheric aerosol. In the absence of high altitude clouds, similar measurements may be made for the free tropospheric aerosol. Median extinction values at middle and high latitudes in the Northern Hemisphere, for altitudes between 5 and 10 km, are found to be one-half to one order of magnitude greater than values at corresponding latitudes in the Southern Hemisphere. In addition, a seasonal increase by a factor of 1.5–2 was observed in both hemispheres, in 1979–80, in local spring and summer. Following major volcanic eruptions, a long-lived enhancement of the aerosol extinction is observed for altitudes above 5 km.

Full access
Emily Shroyer, Amit Tandon, Debasis Sengupta, Harindra J. S. Fernando, Andrew J. Lucas, J. Thomas Farrar, Rajib Chattopadhyay, Simon de Szoeke, Maria Flatau, Adam Rydbeck, Hemantha Wijesekera, Michael McPhaden, Hyodae Seo, Aneesh Subramanian, R Venkatesan, Jossia Joseph, S. Ramsundaram, Arnold L. Gordon, Shannon M. Bohman, Jaynise Pérez, Iury T. Simoes-Sousa, Steven R. Jayne, Robert E. Todd, G. S. Bhat, Matthias Lankhorst, Tamara Schlosser, Katherine Adams, S. U. P Jinadasa, Manikandan Mathur, M. Mohapatra, E. Pattabhi Rama Rao, A. K. Sahai, Rashmi Sharma, Craig Lee, Luc Rainville, Deepak Cherian, Kerstin Cullen, Luca R. Centurioni, Verena Hormann, Jennifer MacKinnon, Uwe Send, Arachaporn Anutaliya, Amy Waterhouse, Garrett S. Black, Jeremy A. Dehart, Kaitlyn M. Woods, Edward Creegan, Gad Levy, Lakshmi H. Kantha, and Bulusu Subrahmanyam

Abstract

In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air–sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the United States, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air–sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ∼20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10–12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ∼20 to 50 m), cooling SST (by ∼1°C), and warming/drying of the lower to midtroposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air–sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.

Full access