On the analysis of ground-based microwave radiometer data during fog conditions

The goal of this study is twofold. First, we verify the retrievals of temperature and humidity profiles from a ground-based microwave radiometer (MWR) using radiosonde soundings. Second, we assess the potential of the MWR to nowcast fog formation and dissipation in a hyper-arid environment through the analysis of its profiles. The MWR is installed and operated at Masdar Institute in Abu Dhabi, United Arab Emirates (UAE), adjacent to Abu Dhabi's International Airport. MWR observations collected from 18 February 2017 to 31 March 2019. A comparison with airport radiosonde data reveals a good agreement in particular for temperature, with the two measurements generally within 1 K from the surface to 10 km. For the specific humidity, the biases can reach 5 g kg(-1) in the PBL. Biases can be attributed to errors in the radiosonde measurements and a non-optimized retrieval algorithm for the MWR. A total of 14 fog events are identified during the study period. A strong near-surface inversion with lapse rates in the lowest 250 m of up to 30 K km(-1) is needed for fog to form. Subsidence warming and drying associated with the subtropical anticyclone can descend down to 500 m, aiding the formation of fog at night. Cloud base height retrievals from the infrared (IR) camera capture fog onset and dissipation and complement the MWR observations. A synergy of different instruments could allow for more accurate fog detection.