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

Abstract

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.

Boundary Layer Clouds and Convection over Subtropical Oceans in our Current and in a Warmer Climate

“…Such variability in humidity may be set on larger scales, such as dry air intrusions from mid-latitudes or moist and dry layers set by nearby deep convection [63,74], but the importance of the moisture profile above the boundary layer for radiative effects within the boundary layer remains unclear and requires further study. Recent work assesses the ability of space-borne observing systems at mapping water vapor profiles and discusses new technologies that may provide better observations of lower tropospheric water vapor that can help unravel moistureradiation interactions [75][76][77].…”

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

“…In parallel, the ground-based and airborne lidars and aircraft in situ measurements are vital for looking at detailed micro-/macrophysical processes associated with water vapor and its interaction with clouds. In the near term, airborne process studies such as the European NARVAL and EUREC 4 A campaigns (Kiemle et al 2017;Bony et al 2017), will pave the way for next-generation space-based water vapor measurements required to overcome existing measurement deficiencies.…”

“…They have demonstrated measurement capability in the UTLS region, as well as in cirrus clouds (Kiemle et al 2008;Groß et al 2014). Recently, airborne water vapor lidar observations focused on tropical shallow convective environments have been reported (Kiemle et al 2017), while earlier work has demonstrated the value of airborne lidar to characterize the variability of humidity and of latent heat fluxes within the convective boundary layer over land (Kiemle et al 2011).…”

The vertical structure and spatial variability of lower-tropospheric water vapor and clouds in the trades

“…The differential absorption lidar WALES is installed pointing downwards on the HALO aircraft, measuring water vapor profiles throughout the tropical troposphere with three on-line laser wavelength positions in the near infrared situated on three water vapor absorption lines of cascading strength Kiemle et al, 2017;Gutleben et al, 2019). The weakest line, specially selected for the tropics, permits accurate profiling of very moist layers below the inversion that tops the cloud layer in the trades, while the stronger two lines provide reliable data of the moisture jump across the inversion and the dry regions above.…”

“…1b). This subset still contains small gaps mainly due to clouds, which we fill with the saturation value by assuming saturation wherever the HSRL backscatter coefficient is > 10×10 −6 m −1 sr −1 , which to sufficient approximation defines a water cloud (Kiemle et al, 2017). We deviate from this threshold only in two cases where the clouds are particularly small (on 12 August 2016 we use 5 × 10 −6 m −1 sr −1 to compensate for the signal dilution) or large (on 24 August 2016 we use 15 × 10 −6 m −1 sr −1 ).…”