Radiometric Retrieval of LWP and COD in Radiation Fog Using Dual-Height Observations, and the Observability of Reff

A two-station radiometer configuration, vertically separated by 185 m, allows the retrieval of fog liquid water path (LWP) and cloud optical depth of fog (CODf) within an optimal estimation framework coupled with the Fu–Liou radiative transfer model. LWP is the only reliably retrieved parameter, whereas the effective radius (Reff) remains prior-dominated, because both broadband fluxes respond mainly to the LWP of the layer. A proof-of-concept evaluation against 28 tethered-balloon profiles from a three-day campaign gives a correlation of 0.71 and an RMSE of 5.4 gm−2 for LWP. The retrieval loses reliability for LWP >30 gm−2 where the radiative signal saturates. The long-term observations (2020–2025) comprise 224 fog episodes (561 h), with autumn dominating (99 episodes, 295 h). Seasonal median LWP ranges from 19.1 to 23.5 gm−2 (19.4–26.9 gm−2 with a fourfold weaker prior) and the mean CODf stays near 3.1 ± 0.8, so the radiative impact of fog in the Strzyżów Valley is controlled by the frequency of fog rather than by the CODf of individual events.