Abraham Zangvil

Wind tunnel experiments and field investigations of eolian dust deposition on photovoltaic solar collectors

Abstract Wind tunnel simulations and field experiments were executed to investigate the deposition of atmospheric dust on solar collectors. In the wind tunnel, dust storms were simulated on scale models of a photovoltaic collector. The effect of wind velocity, wind direction, and diurnal collector rotation upon the total amount of dust settling on the collector is examined. Also investigated is the distribution of the dust over the three distinct panels (2 mirrors + 1 photovoltaic panel) of the collector. Wind direction and solar collector rotation appear to have a serious impact on dust deposition and dust deposition distribution. Wind velocity, on the other hand, has only a small effect on dust deposition distribution, provided wind speed is sufficiently high (some m · s−1 at minimum). Field experiments were executed on real-size collectors to verify the wind tunnel data. The wind tunnel results approximate the field results very closely. This demonstrates the usefulness and importance of the technique in solar collector research.

Investigation of the Large-Scale Atmospheric Moisture Field over the Midwestern United States in Relation to Summer Precipitation. Part II: Recycling of Local Evapotranspiration and Association with Soil Moisture and Crop Yields

Abstract The relative contributions of locally evapotranspired (i.e., recycled) moisture versus externally advected water vapor for the growing-season precipitation of the U.S. Corn Belt and surrounding areas (1.23 × 106 km2) are estimated in this paper. Four May–August seasons with highly contrasting precipitation and crop yields (1975, 1976, 1979, and 1988) are investigated. A simple recycling equation—developed from the traditional atmospheric moisture budget and involving regional evapotranspiration and atmospheric water vapor inflow—is applied on daily, monthly, and seasonal time scales. Several atmospheric moisture budget components {moisture flux divergence [MFD], storage change [or change in precipitable water (dPW)], and inflow [IF]} are evaluated for 24-h periods using standard finite difference and line integral methods applied to objectively analyzed U.S. and Canadian rawinsonde data (50-hPa vertical resolution, surface to 300 hPa) for 0000 and 1200 UTC. Daily area-averaged precipitation (P) t...

Estimating the Role of Local Evaporation in Precipitation for a Two-Dimensional Region

Abstract Budykos model for estimating the contributions of locally evaporated and advected moisture to regional precipitation is extended to two dimensions. It is shown that a simple extension by analogy of the one-dimensional Budykos formula to a two-dimensional region is inconsistent unless the flow in the region is parallel and uniform. The correct extension based on the two-dimensional equations of conservation of water vapor in the region leads to a generalization of Budykos formula that includes a correction factor depending on the atmospheric flow structure. A general procedure for calculating the correction factor for a given atmospheric flow field is presented. Calculations of the correction factor for specific flow structures show that the deviations of the flow from the rectilinear structure can significantly affect the degree to which the local evaporation contributes to precipitation.

Investigation of the large-scale atmospheric moisture field over the midwestern United States in relation to summer precipitation. Part I : Relationships between moisture budget components on different timescales

Abstract Atmospheric moisture budget components are evaluated for a large area (1.23 × 106 km2) in the midwestern United States for all 12-h (1200–0000, 0000–1200 UTC) and 24-h (1200–1200 UTC) periods during the contrasting summers (May–August) of 1975, 1976, 1979, and 1988. The atmospheric moisture flux divergence (MFD, separated into horizontal and vertical advection components, HA and VA) and storage change (dPW) are estimated using a standard finite-difference method applied to objectively analyzed U.S. and Canadian rawinsonde data (50-hPa vertical resolution, surface–300 hPa) for 0000 and 1200 UTC. Area-averaged precipitation (P) totals are derived from approximately 600 relatively evenly distributed (but ungridded) recording rain gauges. Evapotranspiration (E) is estimated as a residual of the moisture budget equation and compares favorably with the few existing observations, especially when totaled for periods of 1 month or longer. Relationships between the budget components are established for the...

A method for monitoring insolation in remote regions

Abstract A method is proposed for measuring the beam and diffuse components of solar radiation via the use of a set of fixed pyranometers tilted in various orientations. A detailed error analysis is performed for the two cases of three and four pyranometers, and it is shown how orientations may be found such that the resultant errors on the derived beam and diffuse components may be expected to be of a magnitude comparable to the errors associated with the pyranometers themselves. Attention is drawn to the fact that certain anisotropic models for the diffuse component may be recast—via the definition of “effective” beam and diffuse components—in such a manner that they take on the mathematical simplicity of an isotropic model.

Aerosol optical depths in a semiarid region

To monitor changes in surface conditions from space requires information on the state of the atmosphere to correct for atmospheric effects in the remotely sensed signal. The least observed and most variable atmospheric parameters are aerosols. Of particular interest are changes of surface conditions in areas where desert encroachment is of concern. An effort was initiated at one of such locations to monitor atmospheric conditions from the ground and surface conditions from space. In this paper we present results only on the ground observations of the aerosol optical depths, in particular their annual and diurnal variations, in the context of prevailing synoptic conditions.

Investigation of Large-Scale Atmospheric Moisture Budget and Land Surface Interactions over U.S. Southern Great Plains including for CLASIC (June 2007)

AbstractThe atmospheric moisture budget and surface interactions for the southern Great Plains are evaluated for contrasting May–June periods (1998, 2002, 2006, and 2007) as background for the Cloud and Land Surface Interaction Campaign (CLASIC) of (wet) 7–30 June 2007. Budget components [flux divergence (MFD), storage change (dPW), and inflow (IF/A)] are estimated from North American Regional Reanalysis data. Precipitation (P) is calculated from NCEP daily gridded data, evapotranspiration (E) is obtained as moisture budget equation residual, and the recycling ratio (PE/P) is estimated using a new equation. Regional averages are presented for months and five daily P categories. Monthly budget results show that E and E − P are strongly positively related to P; E − P generally is positive and balanced by positive MFD that results from its horizontal velocity divergence component (HD, positive) exceeding its horizontal advection component (HA, negative). An exception is 2007 (CLASIC), when E − P and MFD are ...

Characterization of sky conditions by the use of solar radiation data

Abstract A method is proposed to characterize the sky and cloud conditions (SCC) by using ground based global/diffuse irradiance data. The term SCC is defined for the purpose of this paper as the bulk effect of clouds, and other atmospheric constituents on the values of the diffuse fraction of solar radiation ( D G ), the ratio between, the global to extra terrestrial radiation (KT) and the transmissivity Tr measured at the earths surface. However, there is a difficulty in using the measured values of KT and ( D G ) for describing the SCC since these values depend on the solar zenith angle and not only on meteorological factors. This difficulty is overcome by the use of a simple semi-empirical procedure by which one can eliminate the seasonal and latitudinal variations (due to variations in the solar zenith angle) in D G and KT, or similar relations, leaving only variations due to SCC. The procedure is based on a paper by Zangvil and Aviv on the effect of latitude and season on the relation between the diffuse fraction of solar radiation and the ratio of global to extraterrestrial radiation, Solar Energy 39, 321–327 (1987) where a numerical simulation of the seasonal and latitudinal (zenith angle dependent) changes of positions of points representing different simulated SCC on a K T - D G diagram was performed. The method is demonstrated on monthly mean D G -K T relations calculated from three years of global and diffuse irradiance measured at Sede Boker, Israel. Results show that the large scatter of the original data points on a D G -K T diagram is, to a considerable extent due to a seasonal effect (variations of the solar zenith angle). After these effects are removed by the use of the proposed technique the transformed monthly data points have much less scatter and they describe the observed seasonal variations of sky and cloud conditions correctly.

On the effect of latitude and season on the relation between the diffuse fraction of solar radiation and the ratio of global to extraterrestrial radiation

Abstract Data sets of global and diffuse irradiance at Sede Boqer, Israel, for clear, hazy and overcast conditions have been used to simulate the behavior of the relation between the diffuse fraction of solar radiation and the ratio of global to extraterrestrial radiation ( D/G-K T ) as a function of latitude and season. It was found that in the summer solstice the latitude dependence is very small, and in the winter solstice very large. For the equinox the latitude dependence is closer to that of the summer. At a fixed latitude the seasonal dependence is large for high latitudes and negligible near the equator. Some consequences of these findings are discussed.

A portable evaporimeter for rapid measurement of the evaporation rate of water

Abstract A new simple portable evaporimeter is described. Some suggestions for its possible use are made. One possibility is to use it for estimation of Class-A Pan evaporation. It is also shown how to obtain good estimates of potential evapotranspiration from measurements with the new evaporimeter.