On the removal of the effect of horizontal fluxes in two-aircraft measurements of cloud absorption

Cloud absorption inferred from the difference between the net fluxes measur ed by stacked aircraft below and above clouds is strongly affected by the u ncertainties due to cloud horizontal inhomogeneity. The simplest way to get rid of these uncertainties is to perform grand averages over flight legs; if flight legs are long enough, grand averaging may lead to a reliable esti mate of cloud absorption. However, the amount of information on 'true' clou d absorption returned from such an expensive measurement program will be ve ry limited-often one number per flight leg. This paper contains a discussion on how to enhance the harvest of true abso rption data using two related methods: (a) subtraction and (b) conditional sampling. Both methods assume that, simultaneously with broadband measureme nts, some narrow non-absorbing-band net flux measurements are also availabl e. Both methods are related to Ackerman-Cox type corrections, where subtrac ting fluxes in a transparent spectral band from those in an absorbing band partially removes the radiative effects of horizontal inhomogeneity and all ows the recovery of spatially resolved cloud absorption. The output of the two methods is different: while the subtraction method provides a contiguou s record of recovered cloud absorption, the conditional sampling method yie lds a discrete set of data points where the vertical net flux divergence re liably estimates true cloud absorption.