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Eric A. Smith

Abstract

An investigation of the Arabian heat low is carried out based on observations from various satellites, an experimental aircraft and a surface energy budget monitoring station. The observations suggest that during the spring period the Arabian heat low is nearly radiatively neutral and lacks the properties of an energy sink characteristic of conventional desert heat lows. Satellite derived top-of-atmosphere radiation budget analyses illustrate the high contrast properties of the radiative exchange fields over the southern Arabian Peninsula with respect to its surroundings. However, an examination of a four-month time series of daily averaged net radiative exchange over the Arabian Empty Quarter, derived from Nimbus-7 Earth Radiation Budget (ERB) measurements, indicates that the heat low region is in slight relative excess.

Combining these results with estimates of the surface energy budget inside the Arabian Empty Quarter (described in Part I), and previously estimated tropospheric radiative heating rate profiles, provide a closed set of flux terms used to evaluate the energy exchange process within the heat low region. A synthesis of these results indicates that the heat low is a total energy source region. A conceptual structure of the heat low is offered based on a three-layer stratification of the heating mechanisms. The possible role of the Arabian heat low in controlling thermodynamic conditions and forcing baroclinicity in the western Arabian Sea is discussed. It is concluded that the surplus energy properties of the heat low may serve as an important mechanism in controlling moisture transport into the southwest monsoon rainfall regions.

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Eric A. Smith

Abstract

An investigation of the structure and likely role of the Arabian heat low is presented in two parts. In the first paper the surface energy budget of the Arabian heat low is examined. The investigation focuses on a site within the interior of the Saudi Arabian Empty Quarter during June 1981. Automated surface stations are used to collect continuous measurements of radiative fluxes, state parameters, and the subsurface thermal profiles. These data are synthesized in order to estimate the radiation properties of the desert surface within the vortex of the Arabian heat low and to obtain an estimate of sensible heat exchange that would characterize the lower boundary of the heat low during the spring/summer transition season coinciding with the onset period of the Southwest Summer Monsoon.

Results of the analysis demonstrate how radiative exchange both controls the mean properties of the desert surface and responds to perturbations in the heat low environment. The foremost characteristic of surface energy exchange is the well-balanced diurnal regularity. It is shown how the radiation budget of the surface is modulated by basic difference in the shortwave (VIS) and new-infrared (NIR) solar spectrum. More than 2:1 differences are noted in the NIR and VIS surface albedos. Diurnal averages of the surface and parameters illustrate significant day-night differences associated with the diurnal pulsation of the heat low vortex. Day-night differences in surface temperature are extreme; close to 50°C. It is shown that the diurnal amplitude of surface skin temperature is poorly correlated with the bulk Richardson number, suggesting that surface heat exchange is largely controlled by direct radiative exchange through a modulating optical path rather than by heat diffusion. It is shown how the phase lag in subsurface heating imparts a skew in the diurnal sensible heat cycle. The amplitude of the sensible heating cycle is 220 W m−2 peaking approximately 40 minutes past local noon. In a daily averaged sense, subsurface heat storage is approximately zero—thus a first order approximation for the mean heat low at that time scale equates sensible heating to the negative value of net radiation. Finally it is shown how the surface energy budget responds to an intermittent intensification of the heat low that perturbs boundary layer moisture. In Part II, the results of this investigation are incorporated with other data sources in order to examine the bulk tropospheric heat exchange process within the overall heat low system.

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Stanley G. Benjamin
,
Stephen S. Weygandt
,
John M. Brown
,
Ming Hu
,
Curtis R. Alexander
,
Tatiana G. Smirnova
,
Joseph B. Olson
,
Eric P. James
,
David C. Dowell
,
Georg A. Grell
,
Haidao Lin
,
Steven E. Peckham
,
Tracy Lorraine Smith
,
William R. Moninger
,
Jaymes S. Kenyon
, and
Geoffrey S. Manikin

Abstract

The Rapid Refresh (RAP), an hourly updated assimilation and model forecast system, replaced the Rapid Update Cycle (RUC) as an operational regional analysis and forecast system among the suite of models at the NOAA/National Centers for Environmental Prediction (NCEP) in 2012. The need for an effective hourly updated assimilation and modeling system for the United States for situational awareness and related decision-making has continued to increase for various applications including aviation (and transportation in general), severe weather, and energy. The RAP is distinct from the previous RUC in three primary aspects: a larger geographical domain (covering North America), use of the community-based Advanced Research version of the Weather Research and Forecasting (WRF) Model (ARW) replacing the RUC forecast model, and use of the Gridpoint Statistical Interpolation analysis system (GSI) instead of the RUC three-dimensional variational data assimilation (3DVar). As part of the RAP development, modifications have been made to the community ARW model (especially in model physics) and GSI assimilation systems, some based on previous model and assimilation design innovations developed initially with the RUC. Upper-air comparison is included for forecast verification against both rawinsondes and aircraft reports, the latter allowing hourly verification. In general, the RAP produces superior forecasts to those from the RUC, and its skill has continued to increase from 2012 up to RAP version 3 as of 2015. In addition, the RAP can improve on persistence forecasts for the 1–3-h forecast range for surface, upper-air, and ceiling forecasts.

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