The diurnal evolution of the atmospheric boundary layer frequently observed around the Hail Creek mine is well represented by the conceptual model described by Stull Ronald B., 1988 and illustrated in Figure 1.7 of that work. Within this framework, it is reasonable to distinguish two primary boundary layer regimes that exert a strong influence on the dispersion of emissions.
Based on this framework, three distinct periods can be identified with respect to emission dispersion:
Noon to sunset: Emissions are well mixed within the Convective Mixed Layer (CML) and are advected away from the source at wind speeds characteristic of the CML.
Sunset to sunrise: Emissions become trapped within the Stable Nocturnal Boundary Layer (SNBL) and remain relatively localised. Limited transport may still occur under the influence of local low level flows, such as katabatic winds or low level jets.
Sunrise to noon: As convection resumes, emissions are remixed into the developing CML and are again transported away from the source.
Assuming this conceptual boundary layer model is applicable, one would expect the following plume evolution. During the CML regime prior to sunset, an initial plume (Plume 1) forms and is advected downwind by the CML flow. After sunset, emissions become confined within the SNBL, forming a second plume (Plume 2) that remains stationary or moves only slowly under weak local winds. Following sunrise, as the boundary layer transitions back to a convective regime, Plume 2 is remixed into the CML and transported downwind. In this final stage, the highest concentrations are expected near the plume head, reflecting methane accumulated overnight within the mine pit.
Figure 1.7 in Stull Ronald B., 1988 illustrates that transitions between boundary layer regimes do not occur precisely at sunset and sunrise. Instead, the transition from the convective to the stable regime typically begins before sunset, when net radiative cooling of the surface exceeds incoming solar radiation, and reverses sometime after sunrise as surface heating re establishes convective mixing.
The separation of the boundary layer into a CML and a SNBL is relatively common under the influence of the STR, though it can also occur, albeit less frequently, under low pressure conditions. Importantly, the meteorological conditions required for successful satellite retrieval of , particularly dry and cloud free skies, are themselves conducive to the formation of SNBL conditions. Consequently, any inversion algorithm must be capable of reliably handling plume evolution and dispersion under such regimes.