Guest Weblog By Professor Ben Herman of the University of Arizona
In a recent post at the website The Air Vent titled
the problem of correcting MSU (Micro Wave Sounding Unit) brightness temperature data for differences in the diurnal temperature variations between land and ocean was brought up. This is indeed an issue of considerable importance if the resulting data is to be used for determining temperature trends over time for climate change purposes. The reason is quite obvious. In a stable, non-changing thermal regime, due to orbital drift, the satellite will pass over any given latitude at different times of the day, thus causing a trend in the resulting temperatures, positive for crossovers starting at, say 7:00A:M to about 3:00P:M, and negative thereafter till the following morning. Thus, to obtain the true trend (zero trend in the previous illustration) it is important to correct for these diurnal variations.
In a recent paper
Randall R. M., B. M. Herman (2008), Using limited time period trends as a means to determine attribution of discrepancies in microwave sounding unit–derived tropospheric temperature time series, J. Geophys. Res., 113, D05105, doi:10.1029/2007JD008864.
it was shown quite conclusively that the diurnal variations were indeed due to the different time of day that the observations at any given location were taken. This was recognized by the 2 teams (RSS and UAH) that were producing temperatures from the measured brightness data, but nevertheless, differences existed between these 2 data sets.
In the Randall and Herman 2008, (the second author also being the author of this weblog), we examined these differences to see if we could locate their cause or causes. We noted that these difference were greater over land than over water, and greater in the LT (Lower Troposphere) channel than in the MT ( Middle Troposphere channel). Since the diurnal variations of temperature are greater over land than over water and in the lower troposphere as compared to the upper troposphere, this was strong evidence that the discrepancies were somehow related to diurnal variations as a result of orbital changes.
We also noted that the corrected data from RSS still showed a large oscillation, closely related to the time of passage over a given latitude. Upon examining the correction schemes of the two groups (UAH and RSS) we noted that the latter was using corrections based upon climate model calculations, while UAH was using slant path observations from the satellite. Since the slant path footprint is at a different time than the vertically viewed footprint, it gives a measure of the temperature at a different time. This can then be used as the correction factor. These corrections (from UAH) were shown to result in little or no oscillations that could be correlated with time of passage . It was our conclusion then, that the UAH data set was applying a better diurnal correction to the data sets. There are other corrections that also are being applied to this data which we will not discuss here.