Fine structure of cloud droplet concentration as seen from the Fast-FSSP measurements. Part I: Method of analysis and preliminary results

A statistical analysis of a series of droplet arrival times measured by the Fast Forward-Scattering Spectrometer Probe (FSSP) during aircraft flights in cumulus clouds was conducted. The main purpose of the analysis was to de termine whether droplet concentration fluctuates at small scales on the ord er of a few centimeters or whether these fluctuations are negligible as com pared with the mean concentration. In the analysis, the series of droplet arrival times is regarded as a gener alized Poisson random process with time-dependent (or space dependent) para meters. The method developed is based on the representation of droplet conc entration in a cloud along the aircraft track as the sum of three component s: average droplet concentration in a cloud, large-scale fluctuations of dr oplet concentrations described by the Fourier series, and a small-scale non coherent fraction of concentration fluctuation characterized by the energy spectrum and the correlation function. The efficiency of the method to esti mate the amplitude and spatial characteristics of small-scale droplet conce ntration fluctuations and to calculate the profile of large-scale component s of droplet concentration along the aircraft track was carefully tested us ing model-simulated series of droplet arrival times. The method was used to analyze a measurement sample in a cumulus cloud on a 350-m segment. The properties of droplet concentration were calculated bot h over the whole cloud traverse and within the adiabatic core. The results of the calculations show the existence of pronounced small-scal e droplet concentration fluctuations in the case study. The rms of small-sc ale droplet concentration fluctuations was estimated to be about 31% of the mean values of droplet concentration both over the whole cloud and in a mo re homogeneous adiabatic core. The power spectrum shows that fluctuations w ith spatial scales within the 0.5-5-cm range contain over 80% of the energy of small-scale fluctuations.