ZGG09: Error and Atmos Temp Trends in Obs from the MSU
Error Structure and Atmospheric Temperature Trends in Observations from the Microwave Sounding Unit
CHENG-ZHI ZOU, MEI GAO, MITCHELL D. GOLDBERG
The Microwave Sounding Unit (MSU) onboard the National Oceanic and Atmospheric Administration polar-orbiting satellites measures the atmospheric temperature from the surface to the lower stratosphere under all weather conditions, excluding precipitation. Although designed primarily for monitoring weather processes, the MSU observations have been extensively used for detecting climate trends, and calibration errors are a major source of uncertainty. To reduce this uncertainty, an intercalibration method based on the simultaneous nadir overpass (SNO) matchups for the MSU instruments on satellites NOAA-10, -11, -12, and -14 was developed. Due to orbital geometry, the SNO matchups are confined to the polar regions, where the brightness temperature range is slightly smaller than the global range. Nevertheless, the resulting calibration coefficients are applied globally to the entire life cycle of an MSU satellite. Such intercalibration reduces intersatellite biases by an order of magnitude compared to prelaunch calibration and, thus, results in well-merged time series for the MSU channels 2, 3, and 4, which respectively represent the deep layer temperature of the midtroposphere (T2), tropopause (T3), and lower stratosphere (T4).
Focusing on the global atmosphere over ocean surfaces, trends for the SNO-calibrated T2, T3, and T4 are, respectively, 0.21 +/- 0.07, 0.08 +/- 0.08, and 20.38 +/- 0.27 K decade21 from 1987 to 2006. These trends are independent of the number of limb-corrected footprints used in the dataset, and trend differences are marginal for varying bias correction techniques for merging the overlapping satellites on top of the SNO calibration.
The spatial pattern of the trends reveals the tropical midtroposphere to have warmed at a rate of 0.28 +/- 0.19 K decade21, while the Arctic atmosphere warmed 2 to 3 times faster than the global average. The troposphere and lower stratosphere, however, cooled across the southern Indian and Atlantic Oceans adjacent to the Antarctic continent. To remove the stratospheric cooling effect in T2, channel trends from T2 and T3 (T23) and T2 and T4 (T24) were combined. The trend patterns for T23 and T24 are in close agreement, suggesting internal consistencies for the trend patterns of the three channels.
We have intercalibrated the Microwave Sounding Units on the NOAA-10, -11, -12, and -14 satellites using the simultaneous nadir overpass (SNO) method. In this method the radiance differences between pairs of satellite measurements are analyzed when both satellites are viewing the same area on the earth at the same time. Such SNO matchups occur in polar regions; nevertheless, the calibration coefficients obtained from the matchups are applied globally for the entire life cycle of each satellite. After the recalibration, intersatellite radiance biases at the SNO matchups are exactly zero. Mean biases in the global oceanic atmosphere of the gridded dataset generated from the recalibrated, limb-corrected radiances are generally O(0.05–0.1 K). This is an improvement of nearly an order of magnitude over use of the prelaunch calibration. Orbital-drift-related warm target contamination has been reduced to a minimum (nearly zero for channels 2 and 4) by the SNO calibration. This leads to stable intersatellite difference time series with an averaged standard deviation of 0.04 ;0.05 K for means of the global oceanic atmosphere.
Trend differences for two independent techniques for merging the data of overlapping satellite records—the SNO 1 constant bias and SNO 1 Christy methods—are minimal: only 0.015 K decade21 for the midtroposphere (T2) and tropopause layer (T3) and 0.04 K decade for the lower stratosphere (T4). Mean trends of global oceanic atmosphere for T2, T3, and T4 during 1987–2006 are respectively 0.207 +/- 0.068, 0.083 +/- 0.081, and 20.379 +/- 0.267 K decade21 for the average of the two merging techniques.
By removing inter-satellite biases for each grid cell, regional trends have been derived from the MSU observations. Large warming trends are found for most regions of the troposphere. The tropical mid-troposphere (T24 or T23) is found to be warming at a rate of 0.28 +/- 0.19 to 0.32 +/- 0.21 K decade21 from 1987 to 2006. The Arctic troposphere is warming at 0.6 +/- 0.2 to 0.8 +/- 0.3 K decade21 for the same period, which is two to three times faster than the global average. The upper troposphere is warming much faster than estimated by previous studies. Meanwhile, a cooling area is found throughout the atmosphere across the southern Indian and Atlantic Oceans adjacent to the Antarctic continent. However, because statistical uncertainty is high for the T3 and T4 trends over the polar region, one should expect the values to be highly variable for different statistical periods.
With the characteristics shown in this study, the MSU observations for the different NOAA satellites appear to be inter-calibrated quite well, and they are expected to benefit future reanalysis in reducing spurious climate jumps and variability related to satellite transitions.
Jeff Id jokingly stated that it was time to fix the thermometers. It won’t be the first time the models were right and the satellite analysis wrong.
For context, the T2 (MSU channel 2) is a broad tropospheric channel with contributions from the surface and the stratosphere. It is used in the construction of both the LT and MT. MSU channel 2 is now defunct. AMSU channel 5 is the rough equivalent for LT and MT and replaces or augments MSU T2 when available. I don’t believe that any of the MSU satellites are still operational, having been replaced by NOAA-15 with AMSU and AQUA.
Magicjava has some good posts on this stuff.
Spencer wrote in 2006 that the near nader overlap was insufficient to justify the correction. Zou published his first SNO paper in the same year.
I saw no mention of Zou on Dr Roy Spencer’s site (google search).
IPCC AR4 WG1 discussion
h/t to Gavin's Pussycat