Retrieving Parameters of the Planetary Boundary Layer from Near- and Thermal-infrared Satellite Observations

We present our progress towards satellite-based monitoring of one or more key parameters of the well-mixed, planetary boundary layer (PBL). These may include the boundary layer height and the boundary layer moisture for clear-sky pixels. These influence the initiation and life cycle of convective clouds and storms. Remote sensing of these parameters could be beneficial to short-range forecasting, nowcasting and process studies.

In our work, we utilize observations in the near-infrared (NIR) at 0.9 µm and in the thermal-infrared (TIR) at 11 µm. In the NIR-region we exploit bands placed inside and shortly outside a water vapour (WV) absorption feature which allows us to sense the total column of WV (TCWV). In the TIR-region, we use the so-called split-window bands. Typically, these bands are positioned at 11 µm and 12.2 µm, respectively. The difference of the two channels is the split-window difference (SWD). The SWD is affected by the surface emissivity, the air temperature and the WV content in the layers above. For example, the new Flexible Combined Imager (FCI) on the geostationary satellite Meteosat Third Generation (MTG) carries these measurements, allowing the monitoring at 10 min temporal resolution and at 1 km spatial resolution.

In initial sensitivity studies, we explore the potential of the SWD for PBL remote sensing. The SWD can be linked to the thickness of the mixed layer, as well as the moisture contained in the mixed-layer. However, ambiguity exists in distinguishing between thickening and moistening processes within the PBL. Combining these observations, with their different sensitivities to WV distribution, may help resolve the ambiguity to some extent and provide more robust information on the moisture distribution in the PBL.

Our current efforts focus on investigating how to optimize the integration of these complementary measurements to enhance the retrieval of not only the TCWV but also additional information on the moisture structure in the lower levels of the atmosphere.