| For relieving drought, easing the shortage of water resources, protecting national food security, Hebei carried out aircraft artificial precipitation enhancement in annual spring and autumn. At the same time the cloud physics data was detected by the airborne Particle Measuring systems (PMS) in order to study the structure of the cloud system and the formation mechanism of precipitation, improve the scientificalness, and pertinence and availability of artificial precipitation. In this paper, based on the conventional sounding data, non-conventional observation and detection data of airborne PMS system, and using a combination of methods of synoptic systems and cloud physics, we make an analysis of the macro-structure of stratiform cloud in spring and autumn in Hebei, the microphysical characteristics of precipitable stratiform clouds, and discusses the conditions of the cloud system that can be seeded, reveal the physical mechanism of precipitation formation through the observation and analysis of the typical case and numerical simulation, enrich the understanding of the physical characteristics of the cloud system in Hebei, The main results are as follows.1. The macrophysics characteristics of stratiform cloud of spring and autumn in Hebei province. There are several categories of weather systems, including the low trough (inverted trough), cold front, vortex, cyclones, shear line and return flow lead to precipitation in spring and autumn, Hebei Province. The low trough and cold front are the major influence systems. Different weather systems in spring and autumn give the different contribution of precipitation. Inhomogeneity internal structure in a wide range stratiform cloud is corresponding to the uneven distribution of ground precipitation. Most of the water vapor is concentrated in the mid and low layers. The water vapor under 500hPa makes up 93%～98% of the whole layer. Most of stratiform cloud precipitation systems are thermal stable types, and exist potential instability regions and weak vertical velocity.2. The microphysics characteristic of precipitable stratiform clouds in spring and autumn, Hebei Province. Statisticsed the microphysics characteristic of precipitable stratiform clouds in spring and autumn during 2005-2009, Hebei Province, cloud droplet concentration range from 14 to 205 cm-3, average diameter range from 7 to 18μm. Observed by King Probe, average LWC is 0.26 g/m3, the max value is 1.636 g/m3, average ice crystal concentration is 27.7/L. The value of number concentration, average diameter and LWC is higher in spring than those in autumn. During the pre and post of cold front passing over, the cloud microphysics appears significant different. Pre-cold front passing, the observed cloud number concentration is relatively high, and both the droplet diameter and cloud water content are low in the early stage of cloud system development. During the mature stage, the cloud concentration decreases obviously, the diameter and the number of ice particles increase. When the cloud front have passed over, cloud concentration will decrease continually in the rear of cloud system; and the number of ice particles and diameter of ice particles will decrease, but cloud droplet diameter and cloud water content will increase continually, both size and diameter of ice crystal will decrease. When the pre and post of cold front passing over, the cloud microphysics has significant variation, while the cloud spectrum will not change much in the As cloud and Sc cloud.3. The seedability condition of cloud system of spring and autumn in Hebei province. There is multi-layer structure in the precipitable stratiform cloud system during spring and autumn in Hebei province, the distribution of cloud is corresponding with "catalysis-supply cloud" structure. The clouds structures are in favor of precipitation are Asop cloud with Ns or Sc in low layer which make up 55.7%. The average thickness of super-cooled layer is 1414 m which is suitable for artificially seeding. Observations show that supercooled LWC is quite rich,87% of the sorties is greater than 0.1 g/m3 and potential rainfall. The average is 0.16 g/m3 from King, the spring slightly higher than the autumn. The supercooled water content of cloud is very different, with the different stages or parts of development of the cloud system, and different precipitation cloud type. The maximum cloud droplets concentrations are between 21 and 216cm-3. Ice concentration is low in precipitation clouds, more than 55% ice concentration lower 27.7/L, with a large catalytic ability of artificial seeding. The seeding layer temperature is between -2 and -8℃.4. The physical mechanism of precipitation formation. The typical cold front precipitation process happened from 4 to 5 in October,2008 is chosen to observe and numerical simulation analyze. The results show that the cold front is in accord with the "catalysis-supply" mechanism. The ice crystal is in the high layer of cloud, the snow is in the mid-layer of cloud, and graupel, supercooled cloud water is in the next layer of cloud, and the liquid water layer in the water region of cloud is in the low lay of cloud. The stratocumulus cloud under the front is water supply cloud, and there are cloud droplets, rain droplets, and snow and graupel felling from the upper layer after melting. The collision and coalescence of the liquid droplets make the main contribution to the form of rain droplets. The stratocumulus cloud over the front is catalysis cloud, consisting of ice crystal, snow, graupel and ice-water mixed clouds of supercooled cloud water. The increase of ice crystal and snow include not only sublimation and aggregation process, but also the riming and cluming process of snow crystals. The ice crystals in cirrostratus cloud affect the natural catalysis. The cold-cloud process and warm-cloud process coexist in precipitation system. The precipitation be started mainly by cold-cloud process. The cloud structure, the transformation between a variety of wet material and the formation mechanism of rain are significantly different in precipitation cloud system simulated. |
