Vertical profiles of the cloud thermodynamic phase of tropical deep-convective clouds (DCC) were investigated based on measurements using an imaging spectroradiometer system during the ACRIDICON-CHUVA experiment conducted in September 2014 over the Brazilian rain forest near Manaus. In particular the phase transition from liquid to ice phase is relevant for the development of precipitation and may be altered by aerosol particles. Since cloud penetrations and so in situ measurements are limited due to strong updraft and downdraft in DCCs, passive remote sensing methods based on cloud side observations were applied. The retrieval method for phase discrimination uses the spectral slope of the reflected radiances in the near-infrared spectral range. A phase index was defined which considers the different spectral slopes for ice and liquid water clouds between 1.55 µm and 1.7 µm. Measurements have shown a strong increase of the phase index for the mixed phase layer. Based on the slope of the phase index, profiles of the thermodynamic phase for several clouds were identified. Stereographic analysis of concurrent video camera data was performed to estimate the distance and height / temperature of the observed cloud element. From our data we could not find an effect of aerosol conditions on the vertical position of the mixed phase layer. In fact, the variation of the upper and lower limit of the mixed phase layer can yield values of 900 m for one single cloud scene, probably related to downdrafts and precipitation occurrence.
© 2016 Optical Society of AmericaPDF Article