Emerging Technologies and Synergies for Airborne and Space-Based Measurements of Water Vapor Profiles

In situ, airborne and satellite measurements are used to characterize the structure of water vapor in the lower tropical troposphere-below the height, z Ã ; of the triple-point isotherm, T Ã : The measurements are evaluated in light of understanding of how lowertropospheric water vapor influences clouds, convection and circulation, through both radiative and thermodynamic effects. Lower-tropospheric water vapor, which concentrates in the first few kilometers above the boundary layer, controls the radiative cooling profile of the boundary layer and lower troposphere. Elevated moist layers originating from a preferred level of convective detrainment induce a profile of radiative cooling that drives circulations which reinforce such features. A theory for this preferred level of cumulus termination is advanced, whereby the difference between T Ã and the temperature at which primary ice forms gives a 'first-mover advantage' to glaciating cumulus convection, thereby concentrating the regions of the deepest convection and leading to more clouds and moisture near the triple point. A preferred level of convective detrainment near T Ã implies relative humidity reversals below zÃ which are difficult to identify using retrievals from satellite-borne microwave and infrared sounders. Isotopologues retrievals provide a hint of such features and their ability to constrain the structure of the vertical humidity profile merits further study. Nonetheless, it will likely remain challenging to resolve dynamically

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Airborne Lidar Observations of Water Vapor Variability in Tropical Shallow Convective Environment

Kiemle

¹

,

Groß

²

,

Wirth

³

et al. 2017

Space Sciences Series of ISSI

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An airborne downward-pointing water vapor lidar provides two-dimensional, simultaneous curtains of atmospheric backscatter and humidity along the flight track with high accuracy and spatial resolution. In order to improve the knowledge on the coupling between clouds, circulation and climate in the trade wind region, the DLR (Deutsches Zentrum für Luft-und Raumfahrt) water vapor lidar was operated on board the German research aircraft HALO during the NARVAL (Next Generation Aircraft Remote Sensing for Validation Studies) field experiment in December 2013. Out of the wealth of about 30 flight hours or 25000 km of data over the Tropical Atlantic Ocean east of Barbados, three ~2 h long, representative segments from different flights were selected. Analyses of Meteosat Second Generation (MSG) images and dropsondes complement this case study. All observations indicate a high heterogeneity of the humidity in the lowest 4 km of the tropical troposphere, as well as of the depth of the cloud (1 -2 km thick) and sub-cloud layer (~1 km thick). At the trade inversion with its strong humidity jump of up to 9 g/kg in water vapor mixing ratio, the mixing ratio variance can attain 9 (g/kg)² while below it typically ranges between 1 and 3 (g/kg)². Layer depths and partial water vapor columns within the layers vary by up to a factor of 2. This affects the total tropospheric water vapor column, amounting on average to 28 kg/m², by up to 10 kg/m² or 36 %. The dominant scale of the variability is given by the extent of regions with higherthan-average humidity and lies between 300 -600 km. The variability mainly stems from the alternation between dry regions and moisture lifted by convection. Occasionally, up to 100 km large dry regions are observed. In between, convection pushes the trade inversion upward, sharpening the vertical moisture gradient that is co-located with the trade inversion. In most of the water vapor profiles this gradient is stronger than the one located at the top of the sub-cloud layer. Lidar observations, in concert with models accurately reproducing the observed variability are expected to help evaluate the role these details play for climate.

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Structure and Dynamical Influence of Water Vapor in the Lower Tropical Troposphere

Stevens

¹

,

Brogniez

²

,

Kiemle

³

et al. 2017

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In situ, airborne and satellite measurements are used to characterize the structure of water vapor in the lower tropical troposphere-below the height, z Ã ; of the triple-point isotherm, T Ã : The measurements are evaluated in light of understanding of how lowertropospheric water vapor influences clouds, convection and circulation, through both radiative and thermodynamic effects. Lower-tropospheric water vapor, which concentrates in the first few kilometers above the boundary layer, controls the radiative cooling profile of the boundary layer and lower troposphere. Elevated moist layers originating from a preferred level of convective detrainment induce a profile of radiative cooling that drives circulations which reinforce such features. A theory for this preferred level of cumulus termination is advanced, whereby the difference between T Ã and the temperature at which primary ice forms gives a 'first-mover advantage' to glaciating cumulus convection, thereby concentrating the regions of the deepest convection and leading to more clouds and moisture near the triple point. A preferred level of convective detrainment near T Ã implies relative humidity reversals below zÃ which are difficult to identify using retrievals from satellite-borne microwave and infrared sounders. Isotopologues retrievals provide a hint of such features and their ability to constrain the structure of the vertical humidity profile merits further study. Nonetheless, it will likely remain challenging to resolve dynamically -017-9420-8 important aspects of the vertical structure of water vapor from space using only passive sensors.

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Emerging Technologies and Synergies for Airborne and Space-Based Measurements of Water Vapor Profiles

Cited by 4 publications

References 108 publications

Structure and Dynamical Influence of Water Vapor in the Lower Tropical Troposphere

Structure and Dynamical Influence of Water Vapor in the Lower Tropical Troposphere

Airborne Lidar Observations of Water Vapor Variability in Tropical Shallow Convective Environment

Structure and Dynamical Influence of Water Vapor in the Lower Tropical Troposphere

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