A 3D-Var Assimilation Scheme for Vertical Velocity with the CMA-MESO v5.0

Abstract. Certain vertical motions associated with meso-microscale systems are favorable for convection development and maintenance; correct initialization of updraft motions is thus significant in convective precipitation forecasts. A three-dimensional variational-based vertical velocity (w) assimilation scheme has been developed within the high-resolution (3 km) CMA-MESO (the Mesoscale Weather Numerical Forecast System of China Meteorological Administration) model. This scheme utilizes the adiabatic Richardson equation as the observation operator for w, enabling the update of horizontal winds and mass fields of the model’s background. The tangent linear and adjoint operators are subsequently developed and undergo an accuracy check. A single-point w observation assimilation experiment reveals that the observational information is effectively spread both horizontally and vertically. Specifically, the assimilation of w contributes to the generation of horizontal wind convergence at lower model levels and divergence at higher model levels, thereby adjusting the locations of convection occurrence. The impact of assimilating w on the convective precipitation forecast is then examined using a heavy rainfall event, and the results suggest that better 6-h precipitation forecasts are obtained by assimilating w. A batch continuous run is also conducted, and the result indicates that the scheme exhibits a high degree of robustness, leading to improved equitable threat score (frequency skill score) for the first 1 h (3 h) precipitation forecasts compared to the experiment without w assimilated.