Aerosol Retrievals from Individual Avhrr Channels: Ii. Quality Control, Probability Distribution Functions, Information Content, and Consistency Checks of Retrievals

This second part of a two-part study evaluates retrievals of aerosol optical depths, 1 and 2, in AVHRR channels 1 and 2 centered at 1=0.63 and 2=0.83 μm, and an effective Angstrom exponent, , derived therefrom as =-ln( 1/ 2)/ln( 1/ 2). The retrievals are made with the 6S radiative transfer model from four NOAA14/AVHRR datasets, collected between February 1998 May 1999 in the latitudinal belt of 525 S. A series of quality control (QC) checks applied to the retrievals to identify outliers are described. These remove a total of ~1% of points, which presumably originate from channel mis-registration, residual cloud in AVHRR cloud screened pixels, and substantial deviations from the assumptions used in the retrieval model (e.g. bright coastal and high altitude inland waters). First, from examining histograms of the derived parameters it is found that and are accurately fit by log-normal and normal probability distribution functions (PDF), respectively. Second, the scattergrams “ 1 vs 2“ are analyzed to see if they form a coherent pattern. They do indeed converge at the origin, as expected, but frequently are outside of the expected domain in 12 space, defined by two straight lines corresponding to =0 and =2. This results in a low bias in , which tends to fill in an interval of [-1,1] rather than [0,2]. Third, scattergrams of “ vs “ are used to empirically confirm a previously drawn theoretical conclusion that errors in are inversely proportional to . More in depth quantitative analyses suggest that the AVHRR derived Angstrom exponent becomes progressively more meaningful when >0.2. Geographical trends are studied to demonstrate that the selected ocean area is reasonably uniform to justify application of consistency checks to reveal angular trends in the retrievals. These checks show that in most cases, the artifacts in the retrieved and are statistically insignificant. On average, our analysis suggests that the retrieved 1, 2, and show a high degree of selfand inter-consistency, with the exception of a troublesome May 1999 dataset. The most prominent problem noticed so far is the inconsistency between 1 and 2, persistent from one dataset to another, which calls for fine tuning some (non-aerosol model related) elements of the retrieval algorithm. These adjustments will be discussed elsewhere.