EOL Seminar: Differential Absorption Radar: A New Method to Remotely Sense Water Vapor Profiles Within Clouds

Monday, March 20, 2017 - 15:30 to 16:30
Contact Name: 
Dr. Holger Vömel
Contact Email: 
Contact Phone: 

Matthew Lebstock
Jet Propulsion Laboratory, California Institute of Technology

Differential Absorption Radar (DAR) is a multi-frequency radar method that exploits absorption lines to remotely profile gaseous species within cloudy or precipitating volumes. It is the microwave analogue of the common Differential Absorption Lidar (DIAL) approach. Strong water vapor and oxygen absorption lines in the microwave spectrum permit sounding of both the moisture and mass fields using DAR. I will outline the theory and the history of the concept which was first put forward nearly 40 years ago. I will next describe our progress to develop the first ever water-vapor DAR, called VIPR (Vapor In-Cloud Profiling Radar). Our specific implementation exploits the strong water vapor absorption in the wings of the 183 GHz absorption line and leverages many years of experience developing Frequency Modulated Continuous Wave (FMCW) radars at very high frequencies for security applications. VIPR is only recently enabled by technological advances that have allowed us to demonstrate 0.5 W of continuous power at G-band.

Our instrument test-bench was recently used to profiled water vapor out to 1 km range during a light rain event at JPL. The spectral dependence of the attenuation was in excellent agreement with the predicted attenuation based on nearby weather stations, proving for the first time the feasibility of the concept. VIPR’s SNR will be increased by 30 dB over these preliminary tests enabling sounding out to several km with instantaneous uncertainty better than 20%. I will describe our current demonstration and validation plans on an airborne platform at the EUREC4A (Elucidating the role of cloud-circulation coupling in climate) field campaign, where we will help constrain boundary layer humidity profiles and small-scale variability in the moisture field.

The roadblocks and potential paths forward towards a spaceborne DAR will be presented. The approach works best from above clouds because the water vapor burden and line width increases towards the Earth surface allowing increased sampling from the top-down compared with bottom-up. From an airborne or satellite platform channels can be selected that target either upper-tropospheric or lower-tropospheric clouds. Our theoretical studies suggest that the water vapor concentration can be retrieved to within 1-3 gm-3 and the column integrated water vapor can be retrieved to within 1 kgm-2.

Tuesday, 20 March 2018, 3:30 PM
Refreshments 3:15 PM
NCAR-Foothills Laboratory
3450 Mitchell Lane • Bldg 2 Large Auditorium (Rm1022)
Webcast: https://ucarconnect.ucar.edu/live (Room 2: FL2-1022)