Estimation of Tropospheric Temperature Trends from MSU Channels 2 and 4MSU sensors measure temperature by detecting the total amount of microwave radiation emitted by a layer of the atmosphere, the altitude and thickness of which depend on the frequency (wavelength) being measured. These measurements are taken on multiple channels, each of which looks at a different frequency and therefore a different “slice” of the atmosphere. The channel that is of most interest to global warming studies is Channel 2 which looks at a layer that covers most of the troposphere—where we expect the biggest global warming fingerprint. The stratosphere above it, which is measured by Channel 4, has long been known to be cooling (for other reasons). One problem that has beset these studies is that the atmospheric layer seen by Channel 2 dips a little ways into the lower stratosphere, making it a good, but not exact look at the troposphere. As such, it includes some unrelated stratosphere cooling into its look at the troposphere, making its numbers artificially small to some extent.

This paper from a team led by Qiang Fu of the University of Washington (in my home town of Seattle) describes a method whereby data from Channel 4 is used to estimate the size of this spurious stratosphere cooling in the Channel 2 measurements and correct for them. In doing so, they removed much of the disagreement between observed troposphere warming and that predicted by climate models. The study proved controversial not only because it gave results that were unpalatable to global warming skeptics, but because the UAH team and a few other researchers expressed concerns about whether the stratospheric impact on Channel 2 data was uniform enough spatially and temporally for such a correction to be made. In the second paper Fu’s team responds to its critics in this regard, and in the third Fu and Johanson (also of the UW) present an improved version of the analysis that supports the original conclusions with more data, granularity, and independent cross-checks. The last paper is from the UAH team and presents their critic of the method. Generally speaking, the UAH concerns about what has come to be called the “Fu Method” are theoretically valid, but of relatively little impact because the stratosphere is uniform enough with respect to tropospheric variability to allow it to be used.

2000 KLM User's Guide, Section 3.4 (AMSU-B)These are the NOAA User’s Guides for NOAA’s Polar Orbiting Environmental Satellite (POES) series of spacecraft and their onboard systems, including the Microwave Sounding Unit (MSU) packages that are used to gather atmospheric temperature data. The 1998 Guide covers all satellites in the NOAA POES Series up to 1998 from TIROS-N (launched on Oct. 31, 1978 with the first generation MSU package) through NOAA-14 (launched in December of 1994). The second is for the next-generation NOAA KLM Series of spacecraft, beginning with NOAA-K (launched in December of 1994). These satellites carry the Advanced MSU, or AMSU, which has been separated into two packages with separate tasks: AMSU-A and AMSU-B. These have improved sensitivity, better noise reduction, and an expanded number of views and view angles per cross-track sweep of the detectors. All MSU based analysis products account for the differences in their temperature trend analyses. The last three links are to the subsections of the corresponding User’s Guide covering the MSU or AMSU packages used in both satellite series.