The radar discrepancy: why do cloud radars measure less stratocumulus reflection than expected?

Background During the Baltex Bridgs Campaign (BBC) in 2001 at the Cabauw site in The Netherlands, a whole suit of instruments was used to probe the water clouds. An important element of the BBC-campaign was to develop and test advanced remote sensing techniques to measure cloud properties, especially the combination of radar, lidar and microwave radiometers. One of the preliminary findings of the campaign is the radar discrepancy, which is the systematic difference (although depending on the cloud type) between radar observations of reflectivity factor Z and the aircraft/balloon predictions of the same quantity. As the radar reflectivity is one of the key elements in many sensor synergetic retrieval algorithms, it is essential to understand the phenomenon.

Hypotheses The following hypotheses were considered to explain the radar discrepancy: 1)- The radars are not well calibrated; 2)- The radar and aircraft are not collocated and therefore the aircraft predictions are not representative for the true radar reflectivity; 3)- Low-level boundary clouds are in the near-field of the antenna; 4)- The variability of internal and external cloud strucutre is too large with respect to the temporal and/or spatial sampling by the radar; 5)- The internal cloud dynamics is too small to cause incoherent scatter, and the radar reflectivity is partly caused by coherent scattering,

but careful data analysis, using different radar systems, balloon data as well as different airborne instruments, led to the conclusion that 1), 2) 3) and 4) could be excluded. Hypothesis 5) when narrowed to the effect of inhomogeneous beam filling due to the adiabatic lapse rate of the liquid water content could contribute to the radar discrepancy but only under a-typical circumstances. Research in this direction is still progressing. At the conference we will present a critical discussion of the problem as well as potential solutions, which may necessitate new strategies for radar measurements of stratocumulus.