The Impact of Liupan Mountain Terrain and Relative Humidity on Microwave Radiometer-based Temperature Inversion

Using the data of a microwave radiometer (MWR) placed at the foot of Liupan Mountain at Longde station from April 2018 to November 2019 and radiosonde data from Pingliang station, the humidity sensitivity of the retrieved temperature of an MWR (TM) is studied. The humidity sensitivity rate (HSR) and the zero-degree drift (ZDD) are proposed for the first time. On the basis of the distribution characteristics of HSR and ZDD at different altitude layers, the influence of mountain terrain on layer humidity and temperature is studied. This study shows that: (1) Under non-precipitation conditions, the wetter the air is, the larger the difference between HSR and 1.0, and the greater the value of ZDD. (2) Terrain has a great influence on the vertical distribution of layer humidity. The climbing air flow or uplift air flow substantially increases the humidity level of layers below 3 km. The air humidity reaches a maximum in the layer at 0.5 km above the mountain top on clear days and in the layer at 1.0–2.0 km above the mountain top on cloudy days. The difference between TM and the actual station temperature reaches a maximum of 2.7°C. (3) Terrain affects the vertical distribution of the layer humidity and then affects the layer temperature. The downward atmospheric radiation on cloudy days heats the lower layer and causes the average temperature of layers below 3 km to increase by 2.3°C, and the temperature increase caused by humidity sensitivity reaches 0.9°C, up to 1.7°C.