Droplet spectra broadening in cumulus clouds. part I: Broadening in adiabatic cores

Measurements of cloud droplet spectra performed with the Fast-Forward Scatt ering Spectrometer Probe during the Small Cumulus Microphysics Study (1995) are analyzed. Fifty cloud samples with narrow droplet spectra are selected . They are characterized by values of liquid water content slightly below t he adiabatic value. Each observed spectrum is then compared to a narrow adi abatic spectrum predicted at the same level with the current theory of cond ensational growth in an adiabatic cloud cell, initialized with a reference spectrum measured right above the activation level, at cloud base. Broadeni ng is characterized for each observed spectrum by the probability density f unction of condensational growth expressed as the Lagrangian integral of th e ratio of supersaturation to vertical velocity, along the droplet trajecto ries. In particular it appears that the derived density functions show high probabilities of very low and very large values of condensational growth. The large values are related to a high relative density of big droplets in the measured spectra, higher than predicted by the adiabatic model. The con tribution of the instrument to this feature is examined with a model of pro be functioning. The simulations suggest that most of those big droplets are instrumental artifacts. The remaining broadening is parameterized by a lin ear relationship between the mean value and the standard deviation of the d ensity function of condensational growth. This result will be used to exami ne the respective contributions to spectra broadening of microscale heterog eneities of the droplet concentration, in Part II, and of the mixing proces ses, in Part III of this series.