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Eric A. Smith
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Eric A. Smith
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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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Eric A. Smith

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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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Song Yang and Eric A. Smith
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Eric A. Smith and Lei Shi

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The role of the Tibetan Plateau on the behavior of the surface longwave radiation budget is examined, and the behavior of the vertical profile of longwave cooling over the plateau, including its diurnal variation, is quantified. The investigation has been conducted with the aid of datasets obtained during the 1986 Tibetan Plateau Meteorological Experiment (TIPMEX-86). A medium spectral-resolution infrared radiative transfer model using a simple modification for applications in idealized complex (valley) terrain is developed for the study. This study focuses on the clear-sky case where the surface effects are most significant.

The TIPMEX-86 data, obtained during the spring-summer transition into the East Asian monsoon season, are used to help validate the surface longwave radiation budget at two sites of varying elevation: Lasa (3650 m) and Naqu (4500 m). Based on the degree to which skin-temperature boundary conditions control the magnitude of infrared cooling, we define the concept of relative longwave heating and explain its influence on the vertical infrared cooling-rate profile. Relative longwave radiative heating at the higher-elevation Naqu site is found to be twice as large as that corresponding to the lower-elevation Lasa site located within a valley. Besides reducing the infrared cooling rates, it is shown that relative longwave heating extends the period of the day over which the plateau acts as a direct heat source to the atmosphere. Computational results from the infrared model help substantiate observational analyses that indicate surface longwave net radiation at the high-elevation site, on clear days, exceeds 300 W m−2; this is an order of magnitude greater than typical of sea-level oceanic conditions. As a result of the unique meteorological and surface conditions, total infrared flux convergence occurs within the deep planetary boundary layer (i.e., infrared heating of the cloud-free lower atmosphere) at the high-elevation site during the afternoon. An important characteristic of the daytime longwave heating process of the lower layers is how it turns off like a switch at approximately 1800 MST, transforming almost immediately to maximum cooling of the lower layers.

Atmospheric longwave cooling is significantly influenced by variations in the biophysical composition of the surface and the associated thermal diurnal cycle. It is estimated that natural variations of surface emissivity could modulate longwave cooling by up to 40%. The largest impact would occur at a time when the surface temperature is high and the relative longwave radiative heating of the lower atmosphere by the surface reaches its maximum value.

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Mitchell Weiss and Eric A. Smith

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A quantitative investigation of the relationship between satellite-derived cloud-top temperature parameters and the detection of intense convective rainfall is described. The area of study is that of the Cooperative Convective Precipitation Experiment (CCOPE), which was held near Miles City, Montana during the summer of 1981. Cloud-top temperatures, derived from the GOES-West operational satellite, were used to calculate a variety of parameters for objectively quantifying the convective intensity of a storm. A dense network of rainfall provided verification of surface rainfall. The cloud-top temperature field and surface rainfall data were processed into equally sized grid domains in order to best depict the individual samples of instantaneous precipitation.

The technique of statistical discriminant analysis was used to determine which combinations of cloud-top temperature parameters best classify rain versus no-rain occurrence using three different rain-rate cutoffs: 1, 4, and 10 mm h−1. Time lags within the 30 min rainfall verification were tested to determine the optimum time delay associated with rainfall reaching the ground.

A total of six storm cases were used to develop and test the statistical models. Discrimination of rain events was found to be most accurate when using a 10 mm h−1 rain-rate cutoff. Use parameters designated as coldest cloud-top temperature, the spatial mean of coldest cloud-top temperature, and change over time of mean coldest cloud-top temperature were found to be the best classifiers of rainfall in this study. Combining both a 10-min time lag (in terms of surface verification) with a 10 mm h−1 rain-rate threshold resulted in classifying over 60% of all rain and no-rain cases correctly.

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Song Yang and Eric A. Smith

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Time–space distributions of mean monthly latent heating estimated from Special Sensor Microwave/Imager (SSM/I) passive microwave satellite measurements using the Florida State University precipitation profile retrieval algorithm over ocean regions are investigated for the 1992 annual cycle. The space domain is considered in both horizontal and vertical coordinates, with vertical retrieval made possible by the profiling design of the rain algorithm and the underlying relationship between the vertical derivatives of equivalent liquid water mass fluxes and latent heat release.

Comparisons of the retrieved mean monthly rainfall and rain frequency to climatological datasets and atoll rain gauge measurements indicate reasonable agreement except at latitudes above 40° where the satellite values are low biased relative to the climatologies. The horizontal distributions of mean monthly latent heating show that the locations of maximum heating lie in the vicinity and along the axes of well-documented large-scale convergence areas, particularly the intertropical convergence zone (ITCZ) and its transient offshoots, the South Pacific convergence zone (SPCZ), the tropical monsoon systems, and the middle-latitude storm tracks. The vertical distributions show that maximum heating rates of 5°C day−1 are located near the 5-km height level with positive heating extending to the top of the troposphere in the Tropics. Convection shifts associated with the 1992 El Niño–Southern Oscillation (ENSO) episode are well represented in the latent heating field. The seasonal variations of the ITCZ, SPCZ, and monsoon systems are clearly evident. The intraseasonal oscillation of latent heating during the northward propagation of the summer Indian monsoon is also a well-defined feature. Finally, the evolution of the Walker circulation is clearly depicted for both active and inactive ENSO conditions throughout 1992.

Emphasis is given to comparison and contrast of the SSM/I-derived heating fields to results given in the published literature. Many of the stationary and transient features appearing in the retrievals are consistent with previous studies concerning cloudiness, convection, and rainfall over low latitudes, with the exceptions stemming from specific features of the 1992 ENSO event. Therefore, the study provides a framework for using SSM/I measurements as a means to estimate the four-dimensional structure of latent heating over the tropical–subtropical oceans. Since the details of these structures are of considerable importance to the earth’s weather–climate system both in terms of forcing and response, and by virtue of the design of a rain profiling algorithm, these results are presented as a necessary first step in seeking to use satellite measurements to obtain the most important component of the diabatic heating field.

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Song Yang and Eric A. Smith

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This study addresses the retrieval of tropical open-ocean latent heating using Special Sensor Microwave Imager (SSM/I) satellite measurements. The analysis is carried out for the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) intensive observation period in the western Pacific, much of it focused on the study area of the third WCRP–GPCP Algorithm Intercomparison Project (AIP-3) situated over the TOGA COARE Inner Flux Array (IFA). The retrieval algorithm is a profile-type physical inversion scheme based on the use of multispectral passive microwave (PMW) measurements. It estimates vertically distributed rain rate and latent heating by first retrieving mixing ratio profiles of liquid and frozen hydrometeors and then calculating rain fallout rates and vertical derivatives of the liquid–ice mass fluxes. Various modifications to the existing algorithm are discussed, including a combined visible–infrared–PMW–radar screening scheme for distinguishing among “clear,” “cloud without rain,” and “cloud with rain pixels” to better delineate vertical heating structure. Validation of retrieved rain rates over the AIP-3 study area indicates acceptable accuracy/precision uncertainty levels in terms of intensity, distribution, and time variation.

A procedure is developed for improving the initially retrieved heating profiles based on calibration to shipboard radar measurements. The modified algorithm and calibration scheme were applied to the IFA for estimating vertical profiles of latent heating. An optimum high-quality sounding period (1–17 February 1993) was selected for large-scale diagnostic calculations of apparent heating (Q 1) and moistening (Q 2) to analyze heat-moisture budgets of convective and stratiform cloud systems. Comparison and sensitivity tests indicate that the retrieved latent heating and Q 1/Q 2 calculations are representative. Moisture budget analyses over the IFA were carried out to study the detailed heating structures of clouds, particularly the cumulus scale heating process. This was accomplished by using residuals between the SSM/I-retrieved latent heating and the large scale Q 2 diagnostics. Results show that estimates of daily eddy vertical moisture flux divergence contain sizable uncertainties, however, by averaging over extended periods and vertically integrating to obtain surface latent heat flux transfer, close agreement to independently derived surface evaporation rates is found. This suggests that by combining the SSM/I retrievals with large-scale sounding data, it is possible to shed light on the role of cumulus convection on diabatic heating.

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Song Yang and Eric A. Smith

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This study investigates the variability of convective and stratiform rainfall from 8 yr (1998–2005) of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and TRMM Microwave Imager (TMI) measurements, focusing on seasonal diurnal variability. The main scientific goals are 1) to understand the climatological variability of these two dominant forms of precipitation across the four cardinal seasons and over continents and oceans separately and 2) to understand how differences in convective and stratiform rainfall variations ultimately determine how the diurnal variability of the total rainfall is modulated into multiple modes.

There are distinct day–night differences for both convective and stratiform rainfall. Oceanic (continental) convective rainfall is up to 25% (50%) greater during nighttime (daytime) than daytime (nighttime). The seasonal variability of convective rainfall’s day–night difference is relatively small, while stratiform rainfall exhibits very apparent day–night variations with seasonal variability. There are consistent late evening diurnal peaks without obvious seasonal variations over ocean for convective, stratiform, and total rainfall. Over continents, convective and total rainfall exhibit consistent dominant afternoon peaks with little seasonal variations—but with late evening secondary peaks exhibiting seasonal variations. Stratiform rainfall over continents exhibits a consistent strong late evening peak and a weak afternoon peak, with the afternoon mode undergoing seasonal variability. Thus, the diurnal characteristics of stratiform rainfall appear to control the afternoon secondary maximum of oceanic rainfall and the late evening secondary peak of continental rainfall. Even at the seasonal–regional scale spatially or the interannual global scale temporally, the secondary mode can become very pronounced, but only on an intermittent basis. Overall, the results presented here demonstrate the importance of partitioning the total rainfall into convective and stratiform components and suggest that diurnal modes largely arise from distinct diurnal stratiform variations modulating convective variations.

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