Formation of the Positive Net Longwave Radiation at the Ground in the Cold Season According to Measurements and Modeling: The Role of Temperature, Clouds, and Specific Humidity

Longwave radiation plays an important role in the middle and high latitudes’ climate formation. According to measurements of the RAD–MSU (BSRN) complex at the Moscow State University Meteorological Observatory, there were recorded cases with positive daily doses of net longwave radiation (NLR) for 23 days over the period from 2021 to 2024. All these cases were connected with warm or occluded fronts in condition of snow cover. For the two periods with positive NLR (December 19–21, 2023 and December 29–31, 2023), the experiments were performed with the ICON numerical weather prediction model. The positive NLR values were mostly reproduced by the model. We analyzed the factors (temperature profile, specific humidity, and cloud cover), which could influence the generation of positive NLR values. A comparison of the difference between the calculated and measured air temperature at various altitudes with the difference between the calculated and measured NLR values has revealed their statistically significant relationship up to a height of 500 m with correlation coefficients r > 0.5 and an increase in r to 0.7 in the lower atmosphere. A significant relationship with r = 0.55 was found between the difference in the low layer cloud cover amount and the difference in NLR. A closer relationship (r = 0.66) between these characteristics is observed with a 30-minute inertia. We discuss physical mechanisms, which are responsible for the revealed patterns.