Convection in the marine atmospheric boundary layer (MABL) manifests itself into distinct coherent structures, such as kilometer-scale three-dimensional thermals. Momentum transfer from the convective MABL to the sea surface results in centimeter-scale sea surface roughening beneath and down-mean-wind of thermal downdrafts. In contrast, the sea surface beneath and down-mean-wind of thermal updrafts is less perturbed on the centimeter-scale. Microwave radar imagery of the sea surface, such as synthetic aperture radar (SAR), can reveal the existence of the convective MABL via a characteristic kilometer-scale mottled backscatter pattern which is tied to the above given sea surface roughening arguments.
The aforementioned SAR imagery can be converted to neutral wind imagery using a standard transfer function such as CMOD4. Two methods have previously been presented which employ Monin-Obukhov and mixed layer similarity theory to produce Obukhov length, convective velocity scale, buoyancy flux, and a stability corrected wind speed from the SAR-derived neutral wind imagery. These procedures are based on the variance and spectral shape of the neutral wind imagery (based on the above mentioned mottling).
The ability of SAR to reveal the existence of convection via the mottling is, among other things, a function of the MABL static stability and mean wind speed (the Richardson Number). In the presence of MABL convection, it is reasonable to expect that as the static stability and mean wind speed change, mottling variability will also change. An increase or decrease in MABL static stability will cause a corresponding decrease or increase in mottling variability, all else being equal. An increase or decrease in mean wind speed will cause a corresponding decrease or increase in mottling variability, all else being equal. It is therefore reasonable to expect that there is a static stability / mean wind speed limit in SAR’s ability to detect the presence of MABL convection. This limit is when the mottling variability approaches zero but the MABL is still convective, not neutral. In short, it is hypothesized that SAR is not a perfect convection finder. This poses a potentially serious problem when employing SAR for the above mentioned MABL techniques. The current research will present neutral wind spectra from Radarsat imagery of the sea surface in the face of varying degrees Richardson Number in order to quantify and account for this potential problem.