Passive Ground-based Remote Sensing of Radiation Fog

Abstract

Accurate boundary layer temperature and humidity profiles are crucial for successful forecasting of fog, and accurate retrievals of liquid water path are important for understanding the climatological significance of fog. Passive ground-based remote sensing systems such as microwave radiometers (MWRs) that have multiple channels between 22–31 GHz and 51–59 GHz, and infrared spectrometers such as the Atmospheric Emitted Radiance Interferometer (AERI) which measures spectrally resolved infrared radiation (3.3 to 19.2 μm) can retrieve both thermodynamic profiles and liquid water path. Both instruments are capable of long-term unattended operation and have the potential to support operational forecasting. Here we compare physical retrievals of boundary layer thermodynamic profiles and liquid water path during 13 cases of supercooled radiation fog from a MWR and an AERI collocated in central Greenland. We compare both sets of retrievals to in-situ measurements from radiosondes and surface-based temperature and humidity sensors. The retrievals based on AERI observations accurately capture shallow surface-based temperature inversions (0–10 m a.g.l) with lapse rates of up to -1.2 °C m−1, whereas the strength of the surface-based temperature inversions retrieved from MWR observations are uncorrelated with in-situ measurements. For all but one case study, the retrievals based on AERI observations detect fog onset (defined by a threshold in liquid water path) earlier than those based on MWR observations by up to 4 hours. We propose that due to the high sensitivity of the AERI instrument to near-surface temperature and small changes in liquid water path, the AERI (or an equivalent infrared spectrometer) could be a useful instrument for improving fog monitoring and nowcasting.