Cloud-assisted Retrieval of Upper Tropospheric and Lower Stratospheric (UTLS) Water Vapor From Nadir View Satellite Measurements

Water vapor is the strongest greenhouse gas in the atmosphere, yet poorly measured in its trend and variability, especially in the UTLS (upper troposphere and lower stratosphere) region. Although small in amount, stratospheric water vapor can be highly variable as it is directly influenced by injection from convection process [1], challenging the satellite detection. Researchers have found inconsistency among products from different satellite missions [2]. This brings large uncertainty to stratospheric water vapor decadal variation and its contribution to climate feedback.

The focus of this study is to apply an innovated method of sounding UTLS water vapor profiles above opaque cloud layers, using nadir view satellite measurements, with a specific interest in the small-scale water vapor variability. The retrieval algorithm is performed based on a widely used Optimal Estimation method [3] that combines satellite observation and historical data to give an optimal estimation of the atmospheric state.

We first examine the feasibility of this method with realistic instrument parameters, followed by a series of case studies. Comparisons between the retrieval from the Atmospheric Infrared Sounder (AIRS, https://airs.jpl.nasa.gov/) and collocated aircraft measurements show that the retrieval can detect the elevated water vapor concentration caused by convective moistening. Using this approach in synergy with cloud properties retrieved by CloudSat, we further investigate water vapor distribution above tropical cyclones, aiming at understanding convectively injected water vapor.

[1] Sun, Y., and Y. Huang, 2015: An examination of convective moistening of the lower stratosphere using satellite data. Earth and Space Science, 2 (7), 320–330.

[2] Schwartz, M.J., Read, W.G., Santee, M.L., Livesey, N.J., Froidevaux, L., Lambert, A. and Manney, G.L., 2013: Convectively injected water vapor in the North American summer lowermost stratosphere. Geophysical Research Letters, 40(10), pp.2316-2321.

[3] Rodgers, C. D., 2000: Inverse methods for atmospheric sounding: theory and practice.