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Initialization and the spin‐up effect are critical to model performance in the four‐dimensional variational data assimilation (4DVar) system. The incremental analysis update (IAU) technique is able to combat these problems. This study introduces the IAU technique into the 4DVar framework (IAU‐4DVar) of the operational China Meteorological Administration Global Forecast System (CMA‐GFS) and evaluates the performance of the IAU‐4DVar. In this IAU‐4DVar framework, the initial increments optimally estimated are split up into equally weighted forcing terms and integrated into the tangent model and its adjoint model. Then, the optimal estimation of the analysis increment is gradually introduced into the model integration during the assimilation window. A set of two‐month assimilation and forecast experiments were established and executed in this study, and the preliminary results show that the IAU technique can reduce the spin‐up effect of the initial analysis increment from the operational 4DVar system with digital filter initialization (DFI‐4DVar) by retaining the balance in dynamics and thermodynamics. In the optimal minimization process, the IAU‐4DVar converges faster than the DFI‐4DVar with a reduction of the computational cost of up to ca 40%. In addition, the introduction of IAU‐4DVar into the CMA‐GFS model could have a positive impact on the analysis and forecast skills relative to the operational DFI‐4DVar. [ABSTRACT FROM AUTHOR]

Tropical cyclone (TC) genesis forecasting is essential for daily operational practices during the typhoon season. The updated version of the Tropical Regional Atmosphere Model for the South China Sea (CMA-TRAMS) offers forecasters reliable numerical weather prediction (NWP) products with improved configurations and fine resolution. While traditional evaluation of typhoon forecasts has focused on track and intensity, the increasing accuracy of TC genesis forecasts calls for more comprehensive evaluation methods to assess the reliability of these predictions. This study aims to evaluate the effectiveness of the CMA-TRAMS for cyclogenesis forecasts over the western North Pacific and South China Sea. Based on previous research and typhoon observation data over five years, a set of localized, objective criteria has been proposed. The analysis results indicate that the CMA-TRAMS demonstrated superiority in cyclogenesis forecasts, predicting 6 out of 22 TCs with a forecast lead time of up to 144 h. Additionally, over 80% of the total could be predicted 72 h in advance. The model also showed an average TC genesis position error of 218.3 km, comparable to the track errors of operational models according to the annual evaluation. The study also briefly investigated the forecast of Noul (2011). The forecast field of the CMA-TRAMS depicted thermal and dynamical conditions that could trigger typhoon genesis, consistent with the analysis field. The 96-hour forecast field of the CMA-TRAMS displayed a relatively organized threedimensional structure of the typhoon. These results can enhance understanding of the mechanism behind typhoon genesis, fine-tune model configurations and dynamical frameworks, and provide reliable forecasts for forecasters. [ABSTRACT FROM AUTHOR]

Previous studies showed that 4D-Var technique used for data assimilation could be modified for weather control. This study demonstrates the ability of 4D-Var to influence the future path of a tropical cyclone by calculating perturbations in WRF simulation. Given the background error covariance matrix, the initial field is improved by the vortex dynamic initialization technique. Our results show that 4D-Var can be applied to control the trajectory of simulated tropical cyclones by producing "optimal" perturbations. In the numerical simulation experiment of Typhoon Mitag in 2019, after this kind of weather control similar to data assimilation, the tropical cyclone moved obviously, and the damaging wind over the coastline weakened. The prediction results after the initial field modified by 4D-Var have a great change, and the position of the tropical cyclone moved about 0.5° southeastward after assimilation, which misses the southeast coast of China. Moreover, the damaging wind is also weakened. Since the 4D-Var is premised on the assumption that the model is perfect and does not consider the model error, then the research plan to consider model error and introduce new methods is discussed in the paper. [ABSTRACT FROM AUTHOR]

In this study, the seasonal variations of Madden-Julian Oscillation (MJO) are simulated with a 2.5-layer model governed by a set of perturbation equations, considering the effects of boundary layer friction convergence, diabatic heating, moisture feedback and background flow, under a non-resting basic state. The governing perturbation equations are written under a basic state defined by the 30-year climatology of observation (reanalysis), in which daily/monthly climatic wind, geopotential and moisture act as a background forcing influencing the evolution and propagation of MJO. Under this climatic background state, the precipitation of basic state is no longer zero but equal to the 30-year precipitation climatology, which is generally larger in the tropical region and has a dominant seasonal cycle in the subtropical region. Diabatic heating of convection is the key factor that produces the coupled Rossby-Kelvin structure of MJO and is a function of precipitation perturbation in the model. In common practice, precipitation perturbation can be derived from precipitation parametrization schemes, which usually consider only positive values during calculation because negative precipitation does not physically exist in the real world. However, the model in the study is run under a non-zero rainfall background, thus precipitation perturbation is not constraint to be non-negative. In this case, "negative precipitation" area is referred to as abnormally dry area, and leads to "diabatic cooling" (i.e. weaker diabatic heating compared to the basic state) to the troposphere.Model simulations show that the climatic state and "negative precipitation" are two indispensable factors to the MJO seasonal variations. The climatic state provides a crucial background circulation and moisture distribution that favors MJOs / Boreal Summer Intraseasonal Oscillations (BSISOs), as reported in previous studies. On top of the climatic state, our results indicate that the "negative precipitation" effect is also essential to the simulation of MJO in different seasons. The MJO seasonal variations are less obvious if the model only incorporates positive precipitation. Results also reveal that the "negative precipitation" effectively damps shortwaves. [ABSTRACT FROM AUTHOR]