Challenges in Doppler Measurements From Space
In spaceborne Doppler radars, due to the high platform motion, it is important to keep the pointing of the radar antenna as close as possible to the geodetic nadir or at least allow perturbations/offsets only in the cross-track direction. This is a challenging task and in cases the radar boresight will be in general not orthogonal to the instantaneous velocity vector of the platform. In these conditions, the platform motion velocity will introduce:
A Doppler bias constant along a radar profile: this bias can amount to several m/s, and only a portion of it can be removed using the a priori knowledge of the pointing and navigation coordinates of the radar;
A Doppler broadening constant along a profile in presence of homogeneous fields of reflectivity: this broadening contributes to the total spectral width in rms sense, it and ‘whitens’ the Doppler spectrum, and therefore reduces the Doppler accuracy;
A Doppler bias plus broadening, variable along the profile, in presence of non-homogeneities inside each volume of resolution: this is the general case that introduces both a broadening and and bias, the bias term is proportional to the along-track gradient of measured backscattered power within the volume of resolution and requires the most complex approaches to be corrected.
Our research focuses on the development of “tools” (Instrument and Forward Doppler Models) that represent our state-of-the-art understanding in radar hardware and software and in the interaction of the E/M wave with the hydrometeors and the surface. These tools will allow us to study using realistic scenarios, either through the use of existing data set (e.g., ARM and CloudSat observations) or high-resolution model scenes (e.g. Cloud Resolving Models), the performance of different spaceborne radar configurations.
We are also actively involved in the development of algorithms for the characterization of convective motion, estimation of sedimentation velocity of ice particles in cirrus clouds and the quantification of drizzle fluxes in stratocumulus.
European Space Agency: EarthCARE Mission – CASPER study
A method of retrieving the ice cloud microphysical parameters designed for the EarthCARE 94-GHz Doppler radar was developed. It uses combined observations of radar reflectivity and Doppler velocity supplemented by temperature. The retrieval method provides the ice water content estimates, total ice water path in the atmospheric column sampled by cloud radar, and the estimates of the effective ice particle radius.
European Space Agency: EarthCARE Mission – DAME
This study is focused on the development of a forward and retrieval model of vertical motion for the evaluation of observations of space-borne Doppler cloud and precipitation radars at various frequencies, including Ku-, Ka- and W-band. Furthermore, algorithms for the characterization of convective motion, estimation of sedimentation velocity of ice particles in cirrus clouds and the quantification of drizzle fluxes in stratocumulus shall be developed specifically for W-band Doppler radars. Quality control methods and thorough error analysis algorithms for the retrieval parameters shall be developed.
Canadian Space Agency: Snowsat Mission Concept
The unique characteristics of snow particles are not well documented with existing space-based platforms. The international community recognizes the importance of precipitation in climate studies and that Canada has the needed scientific and technological expertise to address the snowfall aspect. Requirements refinement, technical studies and development of international collaborations are proposed for the development of the Snowfall Mission Concept.
European Space Agency: EarthCARE Mission – VARSY study – VARSY
The fundamental objective of the EarthCARE mission is to improve the understanding of cloud physics and cloud-aerosol-radiation interactions so as to portray them reliably in climate and numerical weather prediction models. Clouds and aerosol two-dimensional structures can be retrieved from CPR and ATLID observations. Estimates of their three-dimensional structure could be derived by adding across-flight track observations from the MSI. The particular strength of the EarthCARE mission is the simultaneous availability of three highly synergistic cloud and aerosol observing instruments. The optimal utilization of these three observation types is expected to be achieved through a one-dimensional variational retrieval scheme, as suggested by the CASPER study. The objective of the VARSY activity is to develop a 1D-VAR RS to derive cloud and aerosol properties, as well as vertical motion parameters from ATLID, CPR and MSI measurements, taking in particular advantage of the ATLID HSRL technique and the CPR Doppler capability.