Search

The main goal of this paper is to validate the total water vapour column (TWVC) measured by the Global Ozone Monitoring Experiment-2 (GOME-2) satellite sensor and generated using the GOME Data Processor (GDP) retrieval algorithm developed by the German Aerospace Centre (DLR). For this purpose, spatially and temporally collocated TWVC data from highly accurate sounding measurements for the period January 2009-May 2014 at six sites are used. These balloon-borne data are provided by the GCOS Reference Upper-Air Network (GRUAN). The correlation between GOME-2 and sounding TWVC data is reasonably good (determination coefficient, R2, of 0.89) when all available radiosondes (1400) are employed in the intercomparison. When cloud-free cases (544) are selected by means of the satellite cloud fraction (CF<5 %), the correlation exhibits a remarkable improvement (R2 ∼0.95). Nevertheless, the analysis of the relative differences between GOME-2 and GRUAN data shows a mean absolute bias error (weighted with the combined uncertainty derived from the estimated errors of both data sets) of 15% for all-sky conditions (9% for cloud-free cases). These results evidence a notable bias in the satellite TWVC data against the reference balloon-borne measurements, partially related to the cloudy conditions during the satellite overpass. The detailed analysis of the influence of cloud properties - CF, cloud top albedo (CTA) and cloud top pressure (CTP) - on the satellitesounding differences reveals, as expected, a large effect of clouds in the GOME-2 TWVC data. For instance, the relative differences exhibit a large negative dependence on CTA, varying from -6 to -23% when CTA rises from 0.3 to 0.8. Furthermore, the satellite-sounding TWVC differences show a strong dependence on the satellite solar zenith angle (SZA) for values above 50°. Hence the smallest relative differences found in this satellite-sounding comparison are achieved for those cloud-free cases with satellite SZA below 50°. Finally, the relative differences also show a negative dependence on the reference TWVC values, e.g. changing from C10% (TWVC below 10 mm) to -10% (TWVC above 40 mm) when cloud-free conditions with SZA below 50° are selected. Overall, relative differences within +10% with respect to reference sounding data for a large range of TWVC values can be considered as a good result for satellite retrievals. [ABSTRACT FROM AUTHOR]

Previous research has found that conventional radiosondes containing a traditional pressure sensor can be subject to a pressure bias, particularly in the stratosphere. This study examines this pressure bias, and the resulting altitude misestimation, and considers its impact on temperature, ozone, and water vapor profiles, using data obtained between December 2003 and January 2010 during the Soundings of Ozone and Water in the Equatorial Region (SOWER) campaigns. The observation package consisted of a radiosonde (Vaisala RS80), ozone and water vapor sondes, and a global positioning system (GPS) sensor. More than 30 soundings are used in this study. As GPS height data are thought to be highly accurate, they can be used to calculate pressure. The RS80 pressure bias in the tropical stratosphere was estimated to be -0.4 ± 0.2 hPa (1σ) between 20 and 30 km. As this pressure bias is negative throughout the stratosphere, it leads to altitude misestimation when heights are calculated, as this is usually achieved using the hydrostatics equation. We estimated the error in geometric height to be 42 ± 24, 110 ± 39, and 240 ± 90m (1σ) at 20, 25, and 30 km, respectively. Because of the altitude misestimation, we saw some differences in observation parameters having a vertical gradient. For the temperature profiles, the differences were approximately -0.2 ± 0.2, -0.2 ± 0.4, and -0.3 ± 0.8 K (1σ) at 20, 25, and 30 km, respectively. For the ozone profiles, there was a maximum of ozone partial pressure at around 27 km. Therefore, the differences do not monotonically increase with increasing altitude, and they are estimated to be -1.9 ± 1.6, -0.7 ± 1.0, and 3.1 ± 2.2% (1σ) at 20, 25, and 30 km, respectively. For the water vapor profiles, as there are minima and maxima associated with the stratospheric tape recorder signal, the differences are affected by the phase of the tape recorder. If we align water vapor profiles using a water vapor minimum, the differences are estimated to be -2.7 ± 8.1% at 0.5 km and 1.5± 1.0% (1σ) at 4 km above the water vapor minimum around the cold point tropopause. These biases in the meteorological soundings obtained using the RS80 may have generated an artificial trend in the meteorological records when radiosondes were changed from the RS80, which had no GPS sensor, to the new ones with a GPS sensor. Therefore, it is important to take these biases into account in climate change studies. [ABSTRACT FROM AUTHOR]

The article presents a study on the effect of nitric acid on frost point hyrgrometer performance. It offers information on chilled mirror hygrometers (CMH) which are used to measure water vapour in the troposphere and lower stratosphere from balloon-borne sondes. It discusses how the study was conducted wherein the potential interference of gas-phase nitric acid with frost point temperature measurements were examined. It also details the results of the study.

The accuracy of all types of Vaisala radiosondes and two types of Snow White chilled-mirror hygrosondes was assessed in an intensive in situ comparison with reference hygrometers. Fourteen nighttime reference comparisons were performed to determine a working reference for the radiosonde comparisons. These showed that the night version of the Snow White agreed best with the references [i.e., the NOAA frost-point hygrometer (FPH) and University of Colorado cryogenic frost-point hygrometer (CFH)], but that the daytime version had severe problems with contamination in the humid upper troposphere. Since the RS92 performance was superior to the other radiosondes and to the day version of the Snow White, it was selected to be the working reference. According to the reference comparison, the RS92 has no bias in the mid- and lower troposphere, with deviations <±5% in relative humidity (RH). In the upper troposphere, the RS92 has a ∼5% RH wet bias, which is partly due to the RS92 time lag error and the termination of the heating cycle. It was shown that the time lag effects relating to Vaisala radiosondes can be corrected. Because these were nighttime comparisons, they can be considered to be free from solar radiation effects. Neither the radiosondes nor the Snow White succeeded in reproducing reference class hygrometer profiles in the stratosphere. According to the 29 radiosonde intercomparisons, the RS92 and the modified RS90 (FN) had the best mutual agreement and no bias. The disagreement is largest (<±10% RH) at low temperatures (T ≪ -30°C), where the FN underestimated (overestimated) in high (low) ambient RH. In comparison with the RS92, the RS90 had a semilinearly increasing wet bias with decreasing temperature, where the bias was ∼10% RH at -60°C. The RS80-A suffers from a large temperature-dependent dry bias in high RH conditions, being over 30% RH at -60°C and ∼5% RH near 0°C. The RS80-A dry bias can be almost totally removed with the correction algorithm by Leiterer et al., which was chosen as the best available. The other approach tested tends to overcorrect in high RH conditions when T < -50°C. For T > -30°C it is ineffective and does not correct the RS80-A dry bias in high ambient RH. [ABSTRACT FROM AUTHOR]

Studies of global climate rely critically on accurate water vapor measurements. In this paper, a comparison of the NOAA/Climate Monitoring and Diagnostics Laboratory (CMDL) frost-point hygrometer and the Fluorescent Advanced Stratospheric Hygrometer for Balloon (FLASH-B) Lyman-alpha hygrometer is reported. Both instruments were part of a small balloon payload that was launched multiple times at Sodankylä, Finland. The comparison shows agreement well within the instrumental uncertainties between both sensors in the Arctic stratospheric vortex. The mean deviation between both instruments in the range between 15 and 25 km is -2.4% ± 3.1% (one standard deviation). The comparison identified some instrumental issues, such as a low mirror-temperature calibration correction for the NOAA/CMDL frost-point hygrometer as well as a time lag. It was found that the FLASH-B hygrometer measures water vapor reliably above 7 km in the polar atmosphere. Comparisons in the upper troposphere are affected by the gain change of the NOAA/CMDL hygrometer, causing a lag and a wet bias in the tropospheric low gain setting under the dry conditions in the upper troposphere. [ABSTRACT FROM AUTHOR]

The comparison of simultaneous humidity measurements by the Vaisala RS92 radiosonde and by the Cryogenic Frostpoint Hygrometer (CFH) launched at Alajuela, Costa Rica, during July 2005 reveals a large solar radiation dry bias of the Vaisala RS92 humidity sensor and a minor temperature-dependent calibration error. For soundings launched at solar zenith angles between 10° and 30°, the average dry bias is on the order of 9% at the surface and increases to 50% at 15 km. A simple pressure- and temperature-dependent correction based on the comparison with the CFH can reduce this error to less than 7% at all altitudes up to 15.2 km, which is 700 m below the tropical tropopause. The correction does not depend on relative humidity, but is able to reproduce the relative humidity distribution observed by the CFH. [ABSTRACT FROM AUTHOR]