Observed relationship between surface specific humidity, integrated water vapor, and longwave downward radiation at different altitudes

Abstract

[1] Atmospheric water vapor and surface humidity strongly influence the radiation budget at the Earth's surface. Water vapor not only absorbs solar radiation in the atmosphere, but as the most important greenhouse gas it also largely absorbs terrestrial longwave radiation and emits part of it back to the surface. Using surface observations, like longwave downward radiation (LDR), surface specific humidity (q) and GPS derived integrated water vapor (IWV), we investigated the relation between q and IWV and show how water vapor influences LDR. Radiation data from the Alpine Surface Radiation Budget (ASRB) network, surface humidity from MeteoSwiss and GPS IWV from the STARTWAVE database are used in this analysis. Measurements were taken at four different sites in Switzerland at elevations between 388 and 3584 m above sea level and for the period 2001 to 2005. On monthly means the analysis shows a strong linear relation between IWV and q for all-sky as well as for cloud-free situations. The slope of the IWV-q linear regression line decreases with increasing altitude of the station. This is explained by the faster decrease of IWV than of q with height. Both q and IWV are strongly related with LDR measured at the Earth's surface. LDR can be parameterized with a power function, depending only on humidity. The estimation of LDR with IWV has an uncertainty of less than 5% on monthly means. At lower altitudes with higher humidity, the sensitivity of LDR to changes in q and IWV is smaller because of saturation of longwave absorption in the atmospheric window.Citation: Ruckstuhl, C., R. Philipona, J. Morland, and A. Ohmura (2007), Observed relationship between surface specific humidity, integrated water vapor, and longwave downward radiation at different altitudes,

Using a global climate model to evaluate the influences of water vapor, snow cover and atmospheric aerosol on warming in the Tibetan Plateau during the twenty-first century

“…Thus, changes in specific humidity (q) have a greater effect on DLR in cold seasons and at higher elevations where the initial q is smaller. Ruckstuhl et al (2007) present observational evidence from the Alps to support this conclusion. They suggest an enhancement in the absorption of outgoing longwave radiation in the atmospheric window (8-13 lm) at higher elevations where the average atmospheric vapor concentrations are lower.…”

“…The power law relationships described in the figure suggest that significantly large changes occur in DLR owing to changes in q when the latter is less than 2.5 g/kg. The model's power law relationship, obtained using all the data points shown in the figure, is comparable to the power law relationship of Ruckstuhl et al (2007) for all sky conditions in the Swiss Alps. The model's power law does better in capturing the relationship between DLR and q at higher elevations (2,500-5,300 m), whereas the power law from Ruckstuhl et al (2007) does better at 0-2,500 m. The model shows a slightly higher sensitivity of DLR to q as compared to the sensitivity from the power law relationship of Ruckstuhl et al (2007) when q is higher than 0.7 g/kg which is true at all elevations and seasons in the plateau ( Table 2).…”

“…Considering similar sensitivities of DLR to q during both spring and fall, the water vapor influences on DLR during fall are not significant enough to cause the larger observed warming of the surface in fall relative to spring. We find that the observed increases in the maximum temperature at all elevations during fall are much larger than during spring between 1961 and 2000; the minimum Table 2 Comparison of sensitivities (k) of DLR to q between relationships obtained from the model and Ruckstuhl et al (2007) ) between 1961 and 2000 Elevation 4,000-5,300 m 2,500-4,000 m 0-2,500 m ; left axis) and q (g/kg; right axis) for each decade between 1960s and 2090s at the three elevation regions in the Tibetan Plateau temperature increases are similar for both seasons (Rangwala et al 2009). This phenomenon could be associated with changes in snow cover in the region.…”

“…However, there does not appear to be any apparent elevation dependence to this phenomenon in their analysis. Rangwala et al (2009) calculated seasonal increases in DLR over the Tibetan Plateau resulting from the observed increases in q for the 1961-2000 period based on the sensitivity obtained from the power law relationship of Ruckstuhl et al (2007) shown in Fig. 6.…”

The Variability of Refractivity in the Atmospheric Boundary Layer of a Tropical Island Volcano Measured by Ground-Based Interferometric Radar

“…6) gives about PWV = 19 mm. Ruckstuhl et al (2007) demonstrated a set of linear relationships between PWV and specific humidity for a range of environments. Using their data from Payerne, Switzerland at about 500 m a.s.l.…”