## 4 Discussion
### Short-Term Snow Forecast  
Convective weather nowcasting refers to the detection of the occurrence, development, evolution, and extinction of convective weather systems within an hour. The method of short-term precipitation forecasting is mainly based on the MWR observations. However, this method is affected by the observation frequency and cannot be continuously observed. The time scale of atmospheric thermal structure changes caused by some short-term atmospheric activities is far less than that of RAOB. For rapidly developing weather systems, the lack of radiosonde detection frequency restricts the development of precipitation forecasts. Therefore, it is particularly important to obtain atmospheric data with high frequency and accuracy. In this section, MWR detection is applied to the approaching forecast of snow weather. Some scholars use instability indices, atmospheric instability indices, and water vapour parameters to predict the precipitation process [16; 17; 32; 39; 40]. [PERSON] et al. [41] found that concurrent changes in brightness temperatures (_BT_s) at 22 GHz and 58 GHz are suitable to nowcast rain, with a prediction efficiency of approximately 90% with alarms generated approximately 25 min in advance. [PERSON] et al. [31] found that _PWV_ and _CAPE_ values may serve as a preliminary reference indicators for nowcasting. This section studies the critical conditions of short-term and imminent precipitation forecasts from brightness temperature, instability index (_CAPE_), and atmospheric precipitable water. Figure 9 presents the temporal evolution of _BT_ at the sampling channel from 22.2 GHz to 60.0 GHz for snowy day. According to the absorption spectra of water vapour and oxygen, _BT_ responses of 22-30 GHz and 50-60 GHz bands are shown. In the microwave frequency range from 22.0 GHz to 30.0 GHz, the higher the frequency, the lower the _BT_ values measured by MWR, except for 30. 0 GHz. The _BT_ values and corresponding variation trend for frequencies of 26.234 GHz, 28.000 GHz and 30.000 GHz were largely consistent, especially near the _BT_ peaks, approximately 40 K. For frequencies between 50 and 54.94 GHz, the measured _BT_ values were positively correlated with the detection frequency. The corresponding peak _BT_ was the range of 137 K to 266 K. When the detection frequency was greater than 54.94 GHz, the _BT_ values did not change significantly with increasing frequency. Due to the resonance absorption of the atmosphere, the variations in radiation _BT_ in 8 low-frequency _BT_ curves and 14 high-frequency _BT_ curves were to some  
Figure 8: Statistical characteristics of vapour density and liquid water content (from 8:00 to 20:00 monsoon) under different weather conditions. (**a**) Precipitable water vapour (_PWV_), (**b**) integrated liquid water content (_LWC_). The _t_(h) is expressed in UTC+8. extent related to the precipitation process near the ground, especially for 30.0 GHz and 51.24 GHz. Figure 10 presents the temporal evolution of _PWV_, _LWC_, _CAPE_, _DCAPE_ and \(R\) for snowy days. For a few hours before and after the occurrence of snowfall, the _PWV_ value in the atmosphere generally showed an increasing trend. When it increased from 0.47 cm at 0.65 h to the extreme value at 0.90 h, namely, 0.52 cm, snow occurred in the next 0.1 h. As snowfall continued, it was found that the precipitable water in the atmosphere decreased to 0.48 cm at 1.35 h. The variation trend of _LWC_ was almost synchronous with _PWV_ with an increasing amplitude of 0.12 mm. The _LWC_ during the no precipitation period was very low, only 0.01 mm; namely, snowfall can increase the _LWC_ by 120%. At 0.6 h before the occurrence of snowfall, _CAPE_ and _DCAPE_ had maximum values at the same time, namely, 1071.29 J/kg and 280.26 J/kg, respectively. Then, _CAPE_ and _DCAPE_ started to decline until they reached the minimum value 0.1 h before the occurrence of snowfall, with corresponding decline amplitudes of 202.28 J/kg and 180.32 J/kg.