Radar Measurements of Wind, Temperature, and Moisture in the Planetary Boundary Layer (Invited Presentation)

Historically, the use of radars has become prevalent because 1) microwaves have the unique ability of penetrating through storms, 2) scattering at microwaves is dominated by precipitation, a phenomenon otherwise hard to measure that strongly impacts human activities, and 3) measurements can be taken over large areas in 4 dimensions (x,y,z,t); as a result, it remains the best instrument for monitoring the occurrence and movement of precipitation patterns and of severe storm threats. But radars see best the result of the storm process (precipitation, storm outflows), not its cause (pressure, temperature, moisture, winds, and clouds). There is hence a need to expand the role of radars and other sensors (GPS receivers and radiometers to probe inside storms, other sensors to measure the environment around them) to monitor those critical components, and radars can provide some additional information on them, especially in the boundary layer.

Constraints on thermodynamic variables can be obtained using radar either by direct measurement or indirectly using retrievals. Radars can directly measure one component of target velocity, or more given additional radars or receivers, so long as there are detectable targets available. At close range, they can measure the refractive index of air near the surface given appropriate processing, providing a constraint on temperature and humidity. At far range, they remain blind to the lower troposphere, hence the push towards gap-filling radars.

Indirectly, using one or more radars, the time evolution of reflectivity and especially winds can be used to retrieve the forces shaping the storms, namely buoyancy and pressure gradients forces. From these, pressure and temperature gradients can be inferred, though the accuracy of such retrievals remains uncertain.

Unobtainable are average temperature and pressure profiles as well as humidity anywhere but near the surface and near a radar, except perhaps with dual-wavelength radars under very specific conditions.