The Structure of the Arabian Heat Low. Part I: Surface Energy Budget

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.