Cloud structure and crystal growth in nimbostratus

Cloud structure and crystal growth in two nimbostratus were examined using in situ and airborne radar observations. In both cases, structure throughout the cloud depth was modulated by generating cells at about 8-km altitude. Large-scale horizontal homogeneity at altitudes below the generating cells was due to the rapid movement of the generating cells relative to the main cloud mass. In addition, significant horizontal variability was evident on the scale of few hundred meters, principally in the radar reflectivity data but also readily detectable in the particle data. The melting layer was clearly defined in the radar images. Thin dry layers just above the melting layer were also observed in both cases. In agreement with earlier studies, particle spectra in these clouds are adequately described by exponential relationships. There is a strong correlation between the slope (lambda) and intercept (N-o) parameters of an exponential size distribution; this relationship is well characterized by a power law with constants varying from case to case. Radar reflectivity is negatively correlated with the slope parameter lambda. It is concluded from the observations that the vapor supply made available by large-scale lifting was taken up by depositional growth of the ice crystals. Aggregation of the crystals led to the final shaping of the precipitation size spectra. (C) 2002 Elsevier Science B.V. All rights reserved.