Effect evaluation of an operational precipitation enhancement in cold clouds by aircraft

In the latest 60 years, artificial precipitation enhancement has been widely carried out around the world. However, the effect of artificial precipitation enhancement has not been evaluated comprehensively due to the lack of necessary information. Traditionally, statistic methods are used to evaluate the efficiency of artificial precipitation by comparing precipitation at the target area and the control area. Yet such methods have great uncertainties. Generally, there is no good solution to give convincing and scientific evaluation for an operational artificial precipitation enhancement. On 19 March 2017, an operational precipitation enhancement in supercooled stratiform clouds by aircraft was carried out in Shaanxi province. In this operation, 750 g silver iodide (AgI) was seeded along a 125 km seeding line, which resulted in a clear cloud track corresponding to the cloud seeding line that could be observed by satellite and radar. The corresponding increases in rain intensity, raindrop number concentration and raindrop diameter were also simultaneously observed by raindrop spectrometers on the ground. The above observations show the linkage between cloud seeding and enhanced precipitation from the perspective of microphysical properties. Aiming to solve the problem of effect evaluation on rain enhancement, a new method is proposed based on comprehensive analyses of the observational data in this operation. In this method, the enhanced radar echoes caused by cloud seeding are separated from the continuous radar echoes to determine the background echoes (natural precipitation echoes). A fitting relationship is established between rain enhancement echoes and rain intensity (Z-I) on the ground to quantitatively study the effect of artificial rain enhancement. The results show that: (1) The effective time of precipitation enhancement is about 4 h, and the area influenced is 5448 km² for this operation. The total precipitation and total enhanced precipitation are 1.518×10⁶ m³ and 8.04×10⁵ m³ respectively, and the rate of precipitation enhancement is 53% in the area influenced. (2) Total precipitation, enhanced precipitation and natural precipitation increased first and then decreased with time, the peak value of total precipitation occurred simultaneously with that of the enhanced precipitation and earlier than that of natural precipitation. After cloud seeding, the enhanced precipitation reached its maximum of 4.9×10⁴ m³ at 146 min (04:47 UTC), and the area of radar echo reached its maximum of 1711 km² at 174 min (05:15 UTC). The peak of the former occurred later than that of the latter. (3) The influenced area of precipitation enhancement is located in the down reaches of the moving seeding line in a form of strip while total precipitation in this area is larger at its center and smaller at its edge, which is consistent with the spatial distribution of enhanced precipitation. (4) This operation changes the temporal and spatial distribution of precipitation and promotes the formation of precipitation.