The Synergy of Active and Passive Thermodynamic Profilers: A Ground-Based Example

The 2017 NASA Decadal Survey heavily emphasized the need to provide improved thermodynamic profiles in the planetary boundary layer (PBL), as improved accuracy, resolution, timeliness are needed for a wide range of operational and research applications. Passive spectrally resolved radiance measurements made in both the mid-infrared and microwave portions of the spectrum are already being made from orbit by a range of different sensors, and from these observations low vertical resolution profiles can be retrieved. One advantage of these passive observations is that they can cover significant swaths of the earth (most are made from low-earth orbit). However, primarily due to the difficulty in separating out the emission from the planet’s surface from that in the PBL, these retrievals have relatively poor quality in the PBL. Several groups are pursuing active techniques that use lidar technology to profile water vapor using either Raman scattering or differential absorption techniques. While no active water vapor profiling lidar has yet been launched, several have been demonstrated on research aircraft and thus there is good promise that these approaches can be made to work in low-earth orbit. These active profiles have improved vertical resolution and accuracy in the PBL, which is an advantage over passive radiometers, but are only able to profile along a curtain under the satellite and therefore have much poorer spatial sampling than passive spectrometers.

The optimal future spaceborne thermodynamic profiling mission may be a combination of an active lidar with a passive spectrometer. We use ground-based observations collected during the Perdigao field campaign in Portugal to demonstrate how the information content changes in both the retrieved water vapor and humidity profiles when active differential absorption lidar data are included in either a microwave radiometer or infrared spectrometer retrieval. A one-dimensional optimal estimation based retrieval is used to combine the observations in a physical retrieval, which provides both the information content results and propagates the uncertainties in the observations and the sensitivity of the forward model to generate a full error characterization of the retrieved profile. While the actual results depend strongly on the error characteristics of each instrument, the best results from a ground-based system result from the combination of the infrared spectrometer and differential absorption lidar. However, if accurate cloud properties, and in particular liquid water path, is desired, then the combination of all three instruments provides the best solution.