Correcting the MSU middle tropospheric temperature for diurnal drifts

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Abstract
Channel 2 of the 9 Microwave Sounding Units (MSUs) flown on NOAA polar orbiting platforms provides a 23- year time series of middle-tropospheric temperature. These measurements may be of sufficient quality for climate studies if intersatellite calibration offsets and drifts can be accurately characterized and removed. One of the most important and difficult to characterize sources of long-term drift in the data is due to the evolution of the local observing time due to slow changes in the orbital parameters of each NOAA platform, which can alias diurnal temperature changes into the long-term time series. To account for this effect, we have constructed monthly diurnal climatologies of MSU Channel 2 brightness temperature using the hourly output of a general circulation model as input for a microwave radiative transfer model. We report the results of this calculation, and validate the result by comparing with MSU observations. I. INTRODUCTION Satellite measurements of the Earth's microwave emission are a crucial element in the development of an accurate system for long-term monitoring of atmospheric temperature, providing global spatial and temporal coverage at much higher densities than attainable with in situ observations. The Microwave Sounding Units (MSU) operating on NOAA polar-orbiting platforms have been the principal sources of satellite temperature profiles to date, with measurements of microwave radiance in four channels ranging from 50.3 to 57.95 GHz on the lower shoulder of the Oxygen absorption band. These four channels measure the atmospheric temperature in four thick layers spanning the surface through the stratosphere. The measurements extend over more than two decades, beginning in January 1979 and continuing through the present. The application of the MSU time series by Christy and Spencer (1,2,3,4) to studies of climate change has played a high-profile and controversial role in the debate over the presence and magnitude of anthropogenic warming signals. In an effort to validate these results, we are performing an end-to-end independent analysis of the middle tropospheric data from MSU Channel 2. An important component of this analysis is to account for long term drifts in the measurements that arise from drifts in local measurement time that can alias the local diurnal cycle into the long term record. Spencer and Christy (4) accounted for these drifts by noting systematic differences between measurements made as the instrument scanned across the satellite sub-track, and thus made measurements at different local time. This method has the drawback that due to sampling noise, zonal averages must be used to determine the slope of the diurnal cycle at a given measurement time accurately enough to perform the correction. This results in inaccuracies when the diurnal correction is used to produce a gridded map of decadal trends, since different locations within the same zonal band can have very different diurnal cycles. In this work, we use a new method based on a general circulation model, the NCAR community climate model (CCM3) (5) to calculate a five year climatology of local diurnal anomalies in the brightness temperature for each of the 6 cross-track view angles measured by the MSU instrument We performed a special analysis run of the CCM3 where the results for 5 years (1979-1984) were output on an hourly time scale.