Boundary layer pressure averaged wind

The pressure averaged boundary layer wind derived from ERA5 is used by the CSF algorithm in four distinct ways:

• Wind direction defines the orientation of the upwind background box.
• Wind direction is used to initialise the orientation of the downwind box.
• Wind speed enters directly into the flux calculation used to estimate emissions.
• Wind speed is applied as a quality filter: cases with pressure averaged boundary layer wind speeds below 2 ($\frac{m}{s}$) are excluded Sadavarte, et al., 2021 .

To compute the pressure averaged boundary layer wind, the following parameters are required: surface pressure, the vertical wind profile, and the height (or pressure level) of the top of the boundary layer. Surface pressure, vertical wind components, and the vertical structure of the atmosphere can all be obtained from the ERA5 reanalysis datasets.

Using TROPOMI orbit 09956 as an example, the satellite overpass above the Hail Creek mine occurred at 2019-09-15 03:49:58 UTC . The closest available ERA5 time step is 2019-09-15 04:00 UTC , which is therefore used for all subsequent calculations. The nearest ERA5 grid point to the mine location is at 21.5°S and 148.25°E, and this grid point is selected for deriving the meteorological parameters used in the analysis.

Surface pressure data were obtained from the Climate Data Store ERA5 Single Level for 2019-09-15 04UTC . At the nearest ERA5 grid point, the surface pressure is 977.34 ($\mathrm{hPa}$). The boundary layer height, also retrieved from the Climate Data Store ERA5 Single Level single level dataset at the same time, has a value of 2269.34 ($m$) at this location. Finally, the vertical wind profile was extracted from the Climate Data Store ERA5 Pressure Levels . The resulting pressure level wind data used in the analysis are summarised in Table 9 below.

Combining the surface pressure, boundary layer height, and vertical wind profile data allows calculation of the pressure averaged boundary layer wind. For this case, wind values between the 975 ($\mathrm{hPa}$) and 800 ($\mathrm{hPa}$) pressure levels were used. The 1000 ($\mathrm{hPa}$) level lies below the surface pressure, while the 775 ($\mathrm{hPa}$) level and higher are above the top of the boundary layer (approximately 2660 ($m$)) and were therefore excluded. The resulting pressure averaged wind components are:

$U=-5.695$($\frac{m}{s}$) and $V=-1.070$($\frac{m}{s}$),

corresponding to a wind speed of $5.795$($\frac{m}{s}$), which exceeds the minimum wind speed threshold required by the CSF algorithm.