Analysis of Forecast Failure of a Regional Rainstorm in Qinghai

Based on the ground observation, ERA5 and other data, the regional rainstorm that occurred in northeastern Qinghai on the night of August 28, 2020 was analyzed. The results show that this precipitation occurred in the climate background of relatively high temperature, high humidity and extreme low pressure, and the precipitation process was divided into warm-area precipitation before the front and frontal precipitation, among which the warm-area precipitation was dominant, and it was a regional warm-area rainstorm. The global models, mesoscale models and forecasters as important operational reference all failed to make effective forecasts or prompts for the warm-area precipitation before the front in advance(24 or 12 h), the predicted precipitation was obviously small, and the predicted frontal precipitation by the models were obviously large. The western low-level meso-β-scale wind direction convergence system moving eastwards encountered the high-humidity area at the front of the meso-γ-scale wind speed convergence system to trigger this regional warm-area rainstorm. From the analysis of the mesoscale convergent system based on the vorticity budget equation, it is found that different terms played different roles in the process of warm-area rainstorm. The advective term dominated before the appearance of precipitation, which was favorable for the generation of mesoscale eddies. During the precipitation period, the torsion term and the convergence term were dominant. The torsion term was beneficial to the conversion of horizontal vorticity to vertical vorticity and the enhancement of precipitation intensity. Its maximum was generated 1-2 h earlier than the heavy precipitation. In the later period of precipitation, the convergence term was dominant, which was beneficial to the maintenance of precipitation. In the early stage of precipitation, the apparent heat source was located behind the apparent water vapor sink, which was conducive to the increase in the thickness of the heating column, and the precipitation intensity gradually increased. During the occurrence of heavy precipitation, the apparent heat source and the apparent water vapor sink basically coincided, and the latent heat released by condensation strengthened the upward movement, so that precipitation intensity increased. In this process, the water vapor mainly came from the southeast of the plateau(southwest airflow), followed by the plateau slope area(southeast airflow). During this regional warm-area rain that was wrongly predicted, the extreme minimum pressure, the torsional term in the vorticity budget equation and the abnormal water vapor transport have certain indications for the warm-area rainstorm.

Evaluation of Reanalysis Data Based on the Three-dimensional High-density Sounding Data of the Qinghai-Tibet Plateau

Based on the data of the third Qinghai-Tibet Plateau atmospheric science experiment from 2015 to 2017,the applicability of plateau weather systems and meteorological elements of two commonly used reanalysis data(NCEP/NCAR reanalysis data set,and ERA-Interim reanalysis data set)in the plateau was evaluated.Some conclusions are obtained as follows.Compared with EC reanalysis data,NCEP reanalysis data are more consistent with the scientific experimental data.The correlation of geopotential height is above 0.99,followed by temperature;The correlation of specific humidity is the worst.Seen from average deviation,geopotential height and temperature are both lower;for EC,the westerly and southerly winds are both weaker;for NCEP,westerly wind is weaker,while southerly wind is stronger;specific humidity is higher.From the perspective of monthly and seasonal distribution characteristics,the average deviation of geopotential height is larger in spring and summer,and that of temperature is slightly worse in late spring and early summer.In terms of wind field,EC deviation is more obvious in winter,while NCEP deviation is more obvious in late spring and early summer.Seen from spatial distribution,the deviations of geological height and temperature in the north of the plateau are smaller than those in the south of the plateau.For wind field,the westerly wind in the Qaidam Basin is weaker,and the southerly wind in the southern plateau is weaker.In vertical profile,the deviation of geopotential height at high levels is greater than that of low levels.The deviation of temperature and wind field is larger near the ground.The temperature at middle levels and the westerly wind at middle and high levels are smaller,and southerly wind is stronger for NCEP.The establishment of the three sounding stations(Gaize,Shenzha and Shiquanhe)is conducive to the discovery of plateau vortex and plateau shear line in the western plateau.The western plateau vortex and plateau shear line mostly appeared in the flood season.Most plateau weather systems were maintained within 24 h,and mainly appeared and disappeared in situ.The objective recognition rate of EC for plateau weather systems is higher than NCEP,so EC is more conducive to the diagnosis and analysis of evolution characteristics of plateau weather systems.

Analysis of Predictability of Local Rainstorm in Qinghai during August 19-20,2019

Based on conventional meteorological observation data,ECMWF model data,etc.,the causes of rainstorm in the northeast of Qinghai from the afternoon to the night of August 19,2019 were analyzed,and the causes and predictability of major forecast errors were discussed.The results show that there was high temperature,high humidity and high energy in the middle and low layers in the early period of rainstorm.As the subtropical high retreated eastwards,the high-altitude trough of the westerly belt and the vertical shear line of the plateau moved eastwards to make ground cold air move eastwards,and local short-term heavy precipitation appeared under the influence of this weather system.At the same time,the high-level dry and cold air moved into convective unstable areas.The wind speed at 700 hPa strengthened,and the low-level jet stream appeared to aggravate the instability of atmospheric stratification,which was more conducive to the generation of convection.The forecast results of precipitation based on various numerical models were significantly smaller,and the predicted western cold air was obviously weaker,which was also one of the reasons why the weak transportation of the western water vapor resulted in a small value of predicted precipitation.In the adjustment of model forecast,the proportion and amount of convective precipitation should be adjusted to be large,which has a certain indicating significance for the forecast of convective precipitation.In short-term nowcasting,the deviation of major weather systems predicted based on the model should be corrected in real time,and attention should be paid to the location of ground specific humidity and the convergence line.The falling area of heavy precipitation can be judged according to the high-value area of specific humidity,and the maximum of specific humidity appeared 1-2 h earlier than short-term heavy rainfall.