Raindrop size distribution

Finally, the rainfall rate po (mm h 1) is related to the raindrop size distribution by... 

While routine measurements of po are available, knowledge of reliable raindrop size distributions remains a problem because of their variability from event to event as well as during the same rainstorm. 

The scavenging coefficient A(dp) given by (20.50) describes the rate of removal of particles of diameter dp by rain with a raindrop size distribution N(Dp ). If one assumes that all raindrops have the same diameter Dp, and a number concentration Afo, then (20.50) simplifies to... 

To calculate the scavenging coefficient A, we need to specify a raindrop size distribution n(Dp). A frequently used raindrop size distribution, as we saw in Chapter 15, is that of Marshall and Palmer (1948), (15.112),... 

TABLE 20.2 Scavenging Coefficients A(h for Below-Cloud Irreversible Scavenging of HNO3 Based on a Marshall-Palmer Raindrop Size Distribution... 

Marshal JS, Palmer WM (1948) The distribution of rain-drops with size. J Atmos Sci 5 165-166 Mircea M, Stefan S, Fuzzi S (2000) Precipitation scavenging coefficient influence of measured aerosol and raindrop size distributions. Atmos Environ 34 5169-5174 Monte MJS, Santos LMNBF, Fulem M, Fonseca IMS, Sousa CAD (2006) New static apparatus and vapor pressure of reference materials naphthalene, benzoic acid, benzophenone, and ferrocene. J Chem Eng Data 51 757-766... 

The dependence of washout on raindrop diameter results from the fact that as diameter decreases, the surface to volume ratio increases and surface adsorption becomes proportionately more important. Note also that a rainfall will have a distribution of raindrop sizes, therefore Eq. 13 must be integrated... 

The scavenging coefficient for this simplified scenario is shown in Figure 20.7 for Po = 1 mmh 1 for drops of diameters Dp = 0.2 and 2 mm. This figure indicates the sensitivity of the scavenging coefficient to sizes of both aerosols and raindrops, suggesting the need for realistic size distributions for both aerosol and drops in order to obtain useful estimates for ambient scavenging rates. 

Using expressions obtained for the collision efficiency for E(Dp, dp) in(20.53)and (20.54), one can estimate the scavenging coefficient, and the scavenging rate for a rain event. The calculation requires knowledge of the size distributions of the raindrops and the below-cloud aerosols. 

T. B. Low, R. List Collision, coalescence and breakup of raindrops. Part I Experimentally established coalescence efficiencies and fragment size distributions in breakup, J. Atmos. Sci. 39, 1591-1606 (1982). 

Raindrop impact on the soil surface results in particle detachment and splash. Although most of the soil splashed during a storm event is not transported from the field, detached soil particles are trapped in water-filled depressions and clog surface pores. This reduces infiltration and causes greater surface runoff and erosion 74). The erosivity of rainfall depends on its intensity, duration and energy size distribution and terminal velocities of raindrops slope direction and steepness wind speed and direction and surface roughness 74). 

Without an appreciation for the possible spread of sizes in real particulate systems the values of a in Fig. 11.6 are merely those of an adjustable parameter. We therefore give distribution widths for some natural and artificial aerosols and hydrosols in Table 11.1 we excluded from this list broad distributions, such as raindrops, to which the notion of a width is not really applicable. 

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... 

Most aerosols are polydisperse when formed, some more than others. For example, an examination of sawdust would reveal particles of various sizes, as would that of any material formed by attrition. Since raindrops could grow by condensation or by a series of collisions with other drops, they would also be expected to be polydisperse. In fact, monodisperse aerosols are very rare in nature, and when they do appear, generally they do not last very long. Some high-altitude clouds are monodisperse, as are some materials formed by condensation. Sometimes it is satisfactory to represent all the particle sizes by only a single size. Other times more information is needed about the distribution of all particle sizes. Of course, a simple plot of particle frequency versus size gives a picture of the sizes present in the aerosol, but this may not be enough for a complete quantitative analysis. 

---