However, surface power densities are a function not only of rain cloud thickness, but also cloud base height. Normally large drops present in clouds may be assumed to reach the ground as precipitation. However, in dry climatic regions, virga conditions may prevail in which drops evaporate before reaching the surface. In addition, large drops may form in developing cloud cores long before precipitation is initiated. These large drops are often found near the cloud tops. In order to simulate such situations, the following cases were studied with variable cloud base height. Increasing the cloud base height to 2 km for the 5 km thick cloud decreases surface power density at the edge of the receiver to aboout 4.2 gW/cm2, but significantly increases surface power density levels at larger distances from the rectenna. In fact, the surface power density levels at distances of r = 10 km to r = 20 km from the beam center for the 5 km thick cloud with base height of 2 km are nearly identical to those power density levels produced by the 7 km thick cloud with base height at the surface. Similar results hold for the 3 km thick cloud. Increasing cloud base height from the surface to 2 and 4 km steadily decreases the power density levels at the edge of the rectenna, but strongly increases surface power density levels at greater distances from the rectenna. These results demonstrate that the vertical distribution of large drops within a cloud may be an important consideration when considering environmental impact. This may be particularly important in strongly developing clouds in which the maximum drop sizes and the largest liquid water contents are found in the upper regions of the cloud. Due to the lack of detailed microphysical results, more detailed calculations involving cloud vertical structure are beyond the scope of the present investigation. F. Drop Size Distribution Surface scattering is very small for frequencies less than about 5 GHz for light rain (using the Deirmendjian Rain L drop size distribution). For the 7 km thick rain cloud at a beam nadir angle of 0°, surface power density is only 0.2 gW/cm2 at the edge of
RkJQdWJsaXNoZXIy MTU5NjU0Mg==