The radiometric noise, NeDT, in AMSU channel 4 ( center frequency 52.8 GHz, weighting function peaking at 850 hPa) has recently increased significantly. At launch it was 0.12 K, and it had slowly risen as the instrument aged to 0.2 K through April of 2007. Beginning in May, 2007 it began to rise more rapidly and reached a level of 0.4 K in September, 2007. Beginning in late September, the rate of increase accelerated so that by the end of October the NeDT is near 1 K. See the JPEG image below for the trend of AMSU channel NeDT from January, 2007 through September, 2007.
The AMSU channel 4 NeDT has now risen to a level that may adversely affect the accuracy of the MW-only retrieval products in the lower atmosphere. The AIRS Project is assessing its impact on both the MW-only products and the possible consequences of the loss of this channel on the standard products, which are the result of the combined MW/IR retrieval algorithm. The Project is also examining strategies to mitigate the impact of the higher NeDT or possible loss of this channel in the future.
Data users are advised to exercise caution in the use of the radiances from AMSU channel 4 at any date after July, 2007. The MW-only geophysical products should also be used with caution at any date after September, 2007.
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