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Last modified
7/28/2009 2:40:09 PM
Creation date
4/23/2008 1:57:37 PM
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Weather Modification
Title
Microphysical Observations Over the Atlas Mountains in Morocco
Date
12/23/1988
Weather Modification - Doc Type
Report
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<br />f <br /> <br />simulations were specified from a one dimensional steady state cloud model run 011 the <br />Dec. 8. 1985 Beni-mellal sounding shown in figure 4a. The liquid water content and <br />updraft determined from this run are shown in figure 8. Maximum liquid water was 2 <br />gm-3, and the maximum updraft speed was 5.9 mis, which is in reasonable agreement <br />with the aircraft observations. Cloud top from the model was as -20 C. Since the air mass <br />that the observed clouds developed in was maritime. we assume that the cloud drop size <br />distribution is relatively broad near cloud base, with droplets up to 37.5 microns present <br />(the FSSP cloud droplet probe was not working on the UND aircraft during this storm). <br />Two model runs were made, one with an initial cloud droplet radius of 25 microns, and <br />another with a 37.5 micron radius particle. The cloud drop distribution with which these <br />two particles grow from is assumed to have a modified maritime concentration of 300 01- <br />3. The growth panicles were initiated near cloud base. and allowed to grow for 1:5 <br />minutes in the updraft and liquid water profile of the cloud model. Figure 9 shows a plot <br />of panicle diameter, terminal velocity and liquid water content versus time for the <br />panicle starting out at 37.5 microns radius. The results for the 25 micron particle are <br />similar. At -8 C (9 minutes), the particle has grown into a graupel particle, and has <br />reached a size of 400 microns, and by -11 C (13 minutes) the particle is nearly 1 <br />millimeter in diameter. Initially the growth panicles grow through collision and <br />coalescence with other cloud droplets. As the particles encounter temperatures below <br />zero degrees Celsius the drop freezes, and riming commences. The initial growth of the <br />particle is slow due to its relatively small size and the low liquid water contents present. <br />As the particle grows larger, its growth rate increases rapidly as it encounters larger <br />quantities of liquid water. and at the same time increases its cross-sectional area and <br />terminal velocity. The good agreement between the model results and the aircraft <br />observations suggests that the observed drops can be explained by the above described <br />mechanism. The most important parameters that allow for such growth are the high <br />values of liquid water content, and the moderate updraft velocities, allowing the growing <br />
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