Vertical profile retrievals with warm-rain microphysics using the ground-based microwave radiometer operated by the Hong Kong Observatory

• Vertical profile retrieval algorithm with absorption from rain is developed.
• Warm rain microphysics is incorporated.
• Rain water content is estimated based on radar reflectivity.
• Average error of retrieved temperature ≤ 1 °C for the bottom 2000 m of profile

We present the formulation of a variational-method-based retrieval algorithm for a ground-based microwave radiometer. Absorption by air, water vapor, cloud liquid water and rain water are incorporated into the formulation. The absorption of microwave radiation by air is calculated using a line-by-line method, while that by liquid water is calculated using an empirical formula that models the complex refractive index of liquid water. The root-mean-squared error of the retrieved temperature was no more than 1 °C in the lowest 2000 m of the profiles based on the 1-year verification data. The absorption of microwave radiation by rain water is important for improving the accuracy of the retrieval profiles during rainy conditions.