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Spring 2002

How dry they are:
Public-private team fixes sounding bias

by Bob Henson

When Richard Johnson (Colorado State University), Edward Zipser (now at the University of Utah), and colleagues took a close look at the humidity readings collected in TOGA COARE—the Tropical Ocean and Global Atmosphere’s Coupled Ocean-Atmosphere Response Experiment—they knew something was amiss. Thousands of readings taken from Vaisala radiosondes showed moisture levels in the lower atmosphere a few percentage points lower than expected from independent measurements at the surface. The resulting effect on instability indices was even more pronounced, hinting that large stretches of the western Pacific shouldn’t have been experiencing clouds and storms when they actually were.

Zipser, Johnson, and other COARE investigators suggested in 1997 that NCAR’s Atmospheric Technology Division look into this dry bias, which was becoming apparent in other studies as well. For instance, ATD visitor Françoise Guichard and colleagues showed that the radiative impacts from these errors were larger than the radiative changes predicted from a doubling of carbon dioxide.

Now, after almost five years of meticulous work the mystery of the too-dry radiosonde data has been solved. A number of interwoven factors led to the bias, according to Junhong Wang. The ATD scientist is lead author of an upcoming paper in the Journal of Atmospheric and Oceanic Technology that details the problems and provides algorithms that researchers can use to fix archived data. The corrections were produced through teamwork with engineers Ari Paukkunen and Tapani Laine at Vaisala, which makes just over half of the radiosondes used for daily global measurements

Clockwise from bottom left: Junhong Wang, Hal Cole, Katy Beierle and ATD director Dave Carlson. Cole heads the Surface and Sounding Systems Facility within ATD. Beierle oversaw corrections for more than 8,000 Vaisala soundings.(Photo by Carlye Calvin.)

Tests at a Vaisala plant found one source of error: Nonwater molecules from the radiosonde packaging gradually seeped onto the polymer-based humidity sensor, reducing its ability to absorb moisture. Even when a radiosonde is stored for only three or four months, as is typical in operational settings, this factor can induce a dry bias. In a field project, researchers may use sondes that have been sitting on the shelf for years, creating even larger errors than those found in TOGA COARE.

Another problem was the equation used to derive humidity at the ambient temperature from the humidity at the calibration temperature. This formula assumed a linear relationship, while Vaisala found that the actual response was nonlinear. A third source of error involved the basic calibration model used to derive humidity from the sensor capacitance, and yet another bias arose from the humidity sensor arm, which can heat quickly before launch on hot days and produce a misleadingly high saturation temperature.

Two generations of Vaisala sensors were analyzed (the older one has been in use since 1980), and each had a different bias profile. Based on the tests conducted at Vaisala, ATD came up with the algorithms to address these sources of bias. The Vaisala soundings collected in COARE—more than 8,000 in all—have been scrutinized and fixed one by one, says Wang, creating "one of the most-examined and highest-quality radiosonde data sets ever collected." Meanwhile, Vaisala added a new type of protective shield around the temperature humidity sensors, and it changed the desiccant in the sonde packaging from clay to an impurity-absorbing mix of charcoal and silica gel. Wang stresses the importance of careful storage and launch techniques, including a quick ground-truth observation taken before launch.

At IHOP2002, Wang and colleagues plan to examine data from high-end reference radiosondes. Each will carry three humidity sensors, including an extra-precise Swiss device that uses a chilled mirror on which moisture condenses. Deluxe sondes like these may be invaluable in detecting climate change, says Wang, since the standard global network now includes a mix of radiosondes from roughly a dozen manufacturers, each with their own biases. Wang also plans to compare sondes with other humidity sensors at IHOP. "We want to validate some of the new instruments and get their accuracy for data assimilation and modeling. Eventually we want to integrate all the data."

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Edited by Bob Henson, bhenson@ucar.edu
Prepared for the Web by Carlye Calvin
Last revised: Mon Mar 11 16:42:17 MST 2002