Rainfall rate

Fig. 18-6. Typical values of the washout coefficient as a function of rainfall rate and particle diameter. Source After Engelmann (13).

Uniform mixing in the vertical to 1000 m and uniform concentrations across each puff as it expands with the square root of travel time are assumed. A 0.01 h transformation rate from SO2 to sulfate and 0.029 and 0.007 h" dry deposition rates for SO2 and sulfate, respectively, are used. Wet deposition is dependent on the rainfall rate determined from the surface obser% ation network every 6 h, with the rate assumed to be uniform over each 6-h period. Concentrations for each cell are determined by averaging the concentrations of each time step for the cell, and deposition is determined by totaling all depositions over the period. 

Rainfall Aimual 39.6 inches. The design rainfall rates for a 5 year return period are the following Time of Concentration = 15min. Intensity = 4.0 Vhr Time of Concentration = 1 hour Intensity = 2.0"/hr... 

The overall rainfall rate and amoimt depend on these microphysical processes and even more greatly on the initial amount of water vapor present, and on the vertical motions that transport water upward, cool the air, and cause supersaturation to occur in the first place. Thus the delivery of water to the Earth s surface as one step in the hydrologic cycle is controlled by both microphysical and meteorologic processes. The global average precipitation amounts to about 75 cm/yr or 750 L/(m yr). 

A reactive contaminant may be adsorbed on the soil surface prior to rainfall then, following rainfall that canses erosion, the soil is transported by rnnoff water in the form of suspended particles redistribnted on the land snrface. In general, the settling velocity distribntion dnring runoff indicates that the finer particles are resettled initially (Proffit et al. 1991), although the details of the settling process are affected by different environmental factors, such as soil type and rainfall rate. 

The weekly sampling period resulted In a variety of meterolog-Ical conditions for each sample and, therefore, precluded any resolution of samples by unique wind direction or representative rainfall rate. Therefore, It was not possible to directly evaluate these meteorological Influences on the composition of our precipitation samples. 

Let us assume for the moment that all raindrops have the same diameter D. The rainfall rate R, the rate at which the depth of water in a rain gauge of constant cross section increases with time, is the product of the total volume of water in a unit volume of air and the terminal velocity Vt(D) of a raindrop ... 

In principle the rainfall rate is determined by two quantities, the amount of water in the air and the rate at which it is falling to the ground. But these two quantities are correlated to some extent Vt increases with increasing D as does, for a given N, the amount of water. Yet this correlation has not been established precisely. So it is perhaps best to assume that determining rainfall rates requires two independent measurements. 

Up to this point we have assumed for simplicity that all raindrops are the same size. A more realistic assumption is that raindrop diameters are distributed according to a continuous function N(D), in which instance the radar reflectivity factor and the rainfall rate are... 

The first tests of this proposed method have been encouraging. On the basis of comparisons between rainfall rates measured with the differential reflectivity technique and with a network of rain gauges, Seliga et al. (1981) concluded that these first measurements of rainfall using the ZDR technique support the theoretical expectations... that rainfall rate measurements with radar can be made with good accuracy. So it may yet be possible to accurately measure rainfall with radar—provided that measurements are made with two orthogonally polarized beams. This exemplifies one of the principal themes of this book scattered polarized radiation contains information that may be put to good use. 

Seliga, T. A., V. N. Bringi, and H. H. Al-Khatib, 1981. A preliminary study of comparative measurements of rainfall rate using the differential reflectivity technique and a raingauge network,./ Appl. Meteorol., 20. 1362- 1368. 

C1 activities measured near Richland resulted from some other cause than a low rainfall rate, such as a higher rain formation altitude. 

When a cloud forms, water vapor condenses on aerosol particles present in the atmosphere. Regardless of the size of a cloud droplet, at least one aerosol particle is required for its formation. A major portion of the radionuclides present in the initial raindrop is caused by this aerosol particle. Although increases in the drop size owing to coalescence would tend to keep the radionuclide concentration constant, increases in size owing to the condensation of moisture on drops already present would tend to decrease the radionuclide concentrations. Therefore, one would expect a maximum in radionuclide concentrations in small drops. Raindrops tend to be larger in heavier rains. Therefore, the decrease in the 38C1 and 39C1 activities with an increase in rainfall rate could be caused by an increase in the drop size. 

If the above explanation is correct, the rainwater disintegration rates of other airborne radionuclides should also decrease with increasing rainfall rate. However, the atmospheric activities of the longer lived radionuclides change with time, making it difficult to compare the activities in different rainwater samples. The 7Be activity in rainwater ap-... 

Of course, this is the same equation we had before. The missing datum is the rainfall rate in depth per unit time or p in our notation. In the northeastern United States, this value is about 80 cm/year in other words, if you stand out in a field for a year, and the rain did not drain away, the water level would be almost up to your waist. Once you have the flux, getting the flow is just the flux multiplied by the area. 

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