One of the most pressing climate issues identified by the IPCC's Fourth Assessment is the need for a long-term analysis of cloud properties to better understand the impact of cloud radiative forcing on various aspects of climate, especially surface temperature and its diurnal variation. To understand this radiative forcing over long time periods, it is necessary to measure global cloud properties using a consistent set of proven algorithms applied to a long-term record of consistently calibrated and quality-controlled satellite imager data. Knowing how clouds vary with climate change and how well climate models reproduce such variability through modeled feedbacks is critical to understanding how well the models can predict climate.

As part of the NOAA NCDC Climate Data Record (CDR) program, we are currently developing a Thematic CDR (TCDR) consisting of cloud amount, phase, optical depth, effective particle size, height, and temperature extending back to 1978 using data from the AVHRR instrument. The TCDR will be consistent with CERES MODIS cloud properties, though some modifications to these algorithms will be required to operate on the 5-channel and lower spatial resolution AVHRR Global Area Coverage (GAC) data. Stable and accurate visible channel calibration is ensured through matching modern AVHRR data with that of Aqua MODIS using the Nearly Simultaneous Ray-matched Technique and the Deep Convective Cloud Technique. These calibrations are then transferred back in time through the use of time-overlapping LEO and GEO data. This presentation will highlight progress to-date on this CDR effort, emphasizing cloud retrievals from the NOAA-18 AVHRR and comparisons with Aqua MODIS and CALIPSO.