Forecast Assessment of GRAPES_GEPS Model on the 500-hPa High Pressure Ridge in Eurasia in Winter 2018

The presence of a 500-hPa geopotential height region represents a high-pressure ridge or anticyclone that will cause a rise in ground pressure, which then leads to extreme disaster weathers such as a cold wave or high-temperature heat wave. Most of the existing methods in identifying the blocking system are to study the closed blocking high, but some studies have shown that the existence of the open high-pressure ridge can also lead to the emergence of heat waves and cold events. Based on a recently developed approach that can effectively recognize both persistent blocking and ridge events, the characteristics of persistent open ridges and blocking high (maxima) of 500-hPa geopotential height (PMZ) events in Eurasia are investigated. The climatological distribution characteristics of PMZ events reveal that the winter PMZ in Eurasia mainly occurred in the Ural Mountains area. The life history of PMZ events in this area is mainly 2–3 days with a moving distance of less than 1000 km, which has the characteristic of stability. The blocking test mainly focused on the prediction techniques of blocking high. By examining the blocking situation in the Okhotsk Sea in the winter of 2018, it is concluded that the strong blocking situation can be predicted by GRAPES (Global/Regional Assimilation and Prediction Enhanced System) and ECMWF (European Centre for Medium-Range Weather Forecasts), with the ECMWF performing better than the GRAPES. However, there are obvious errors as far as the weak blocking situation is concerned. This paper focuses on the diagnosis and analysis of the maintenance time and the moving path of PMZ events. The prediction ability of GRAPES_GEPS control and the ensemble forecast for the PMZ events in the Northern Hemisphere from 19 Dec 2018 to 28 Feb 2019 are then verified. Most PMZ events in winter 2018 occurred in Eurasia in the west of Lake Baikal between 40°N and 60°N. PMZ events occurred most frequently in Jan 2019 in Eurasia compared to the months of Dec and Feb, with a total of nine events. As far as the PMZ events are concerned, the medium-term control forecast provides better results than the short-term control forecast. Moreover, the ensemble forecast is better than the control forecast, wherein more ensemble members are able to predict PMZ events close to the zero field with the forecast time effect approaching. However, the prediction ability of the control prediction model for PMZ events in winter is not very good. For the PMZ event on 25–31 Dec 2018, the GRAPES_GEPS ensemble model provides the forecast signal in the extension period. As the forecast period approaches, most members give an effective forecast. However, the maintenance time forecast is shorter and the intensity of the high-pressure ridge is weaker than those of the actual situation. At present, the numerical assessment of the medium and extended range forecast on the general circulation system is mostly on the traditional test scores, and the main weather systems that affect the weather and climate anomalies in China lack a tracking and testing technology. In this paper, the PMZ prediction ability of GRAPES_GEPS for the extended range in winter is given, which can provide a reference for the extended range forecast blocking situation in the future.