Combining lidar and radar measurements to derive cirrus cloud effective radii: In situ comparison and simplistic model results

Ice cloud bulk radiative properties derived from cirrus cloud observations were presented. A cirrus cloud was observed by the NOAA Doppler lidar and radar which provided estimates of the ice water path and the effective radius of the size distribution. These measurements were used to parameterize the ice cloud optical properties. The observed cirrus cloud was represented by three distinct periods, characterizing an early tenuous stage by r_e=30 μm and iwp=5 g m^-2, a mid-life convective stage with r_e = 100 μm and iwp=5 g m^-2, and a mature steady phase with r_e=150 μm and iwp=80 g m^-2. The reflectivity, transmissivity, and emissivity calculated for the cirrus cloud, observed over a mid-latitude continental site, were presented and show reasonable results. The thin, tenuous cirrus cloud exhibited the strongest shortwave transmittance and weakest IR emissivity, which indicates that this mid-latitude, shallow cirrus cloud had very little effect on the (upward and downward) IR radiation flux at the surface. This would not be true for other locations. For example, tenuous cirrus are important in the tropics and polar regions where the amount of solar radiation received at the surface has a greater impact on the net flux balance. In the mid-life stages of a cirrus cloud, the possibility of several radiative effects exist. In general, however, a moderately opaque cloud is a less effective reflector and transmitter in the shortwave region due to the increase in particle sizes. As would be expected, the IR emissivity increases substantially thereby changing the net radiative flux and warming the column. This overly simplistic assessment of cloud radiative effects punctuates the need for observations and awareness of all the various factors that must be considered when determining the radiative influence of clouds. A more thorough extension of this study is currently in progress. Using one month of cloud observations obtained in Kansas, combinations of r_e and iwp can be tabulated to address a more realistic and quantitative cloud analysis. For example, computed fluxes can be compared to the observed short and longwave fluxes in order to assess the accuracy of retrieving iwp and r_e from remote sensors, and in turn, determine how these quantities perform in the models to predict the cirrus optical properties and their radiative effects.