The tropical lapse rate quandary

Radiosondes themselves have significant problems and were also not designed for detection of small climate changes. These problems have been well documented anecdotally, and have been dutifully acknowledged by those who have published trends in radiosonde temperatures. The cautions urged by these researchers in interpreting the results have not always been taken on board by others however.

Few if any sites have used exactly the same technology for the entire length of their record, and large artifacts have been identified in association with changes from one manufacturer to another or design upgrades by the same manufacturer. Artifacts have even been caused by changing software and bug fixes, balloon technology, and tether lengths. Alas, many changes over time have not been recorded, and consistent corrections have proven elusive even for recorded changes. While all commonly used radiosondes have nominal temperature accuracy of 0.1 or 0.2 K, these accuracies are verified only in highly idealized laboratory conditions. Much larger errors are known to be possible in the real world. The most egregious example is when the temperature sensor becomes coated with ice in a rain cloud, in which case upper tropospheric temperatures can be as much as 20 C too warm. This particular scenario is fairly easy to spot and such soundings can be removed, but one can see the potential problems if many, less obvious errors are present or if the sensor had only a little bit of ice on it! Another potential problem is pressure readings; if these are off, the reported temperature will have been measured at the wrong level.

The Sherwood et al. study in Science Express concerns one particular type of long-recognized radiosonde error, that caused by the sun shining on the “thermistor” (basically, a cheap thermometer easily read by an electric circuit). This problem has been documented, notably by Luers and Eskridge (1995,1998), but correcting for it in the past has proven difficult and previously its magnitude was poorly known except under controlled conditions. The most popular radiosonde manufacturer worldwide today is the Vaisala corporation, whose strategy for coping with solar heating is to concede that it will happen and try to correct for it: the thermistor is mounted on a “boom” that sticks into the air flow where the sun can shine on it, but the heating error is estimated from the measured ascent rate and solar zenith angle and subtracted from the reported temperature. The magnitude of this correction can be several degrees, has varied with changing designs, and may not always have been properly applied in the past especially if time of day, station location, or instrument version were incorrectly coded. The US radiosonde, until recently made exclusively by the VIZ corporation and now under contract to two separate manufacturers, has followed the strategy of trying to insulate the thermistor from solar effects by ducting it inside a white plastic and cardboard housing. However, this strategy is unlikely to completely prevent solar heating. The first US radiosonde designs, which had less effective shielding and lacked the white coating subsequently applied to the sensor to limit is solar absorption, showed obvious signs of solar heating error. Many other radiosonde designs exist; larger countries historically designed and built their own sondes, but some countries have abandoned their national sondes and started buying from (usually) Vaisala.

The Sherwood et al. study is the first to try and quantify the solar-heating error over time. We recognized that the true difference between daytime and nighttime temperatures through the troposphere and lower stratosphere should, on average, be rather small, and moreover should have changed very little over the last few decades. We also recognized that this difference could be observed quite accurately by examining consecutive daytime and nighttime observations. Nighttime observations at many stations are much more rare than daytime ones, so this strategy means throwing out most of the daytime data; this is one reason why previous, less focused investigations did not detect this particular problem. This data-treatment technique revealed that, as you go back farther in time, the daytime observations become progressively warmer compared to nighttime observations. This is a clear indication that, back in the 1960’s and 1970’s especially, the sun shining on the instruments was making readings too high. This problem disappeared by the late 1990’s.

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