Abstract
Numerical modeling of sea fog is highly sensitive to initial conditions, especially to moisture in the marine atmospheric boundary layer (MABL). Data assimilation plays a vital role in the improvement of initial MABL moisture for sea fog modeling over the Yellow Sea. In this study, the weather research and forecasting (WRF) model and its three-dimensional variational (3DVAR) data assimilation module are employed for sea fog simulations. Two kinds of background error (BE) covariances with different control variables (CV) used in WRF-3DVAR, that is, CV5 and multivariate BE (CV6), are compared in detail by explorative case studies and a series of application experiments. Statistical verification metrics including probability of detection (POD) and equitable threat scores (ETS) of forecasted sea fog area are computed and compared for simulations with the implementations of CV5 and CV6 in the WRF-3DVAR system. The following is found: (1) there exists a dominant negative correlation between temperature and moisture in CV6 near the sea surface, which makes it possible to improve the initial moisture condition in the MABL by assimilation of observed temperature; (2) in general, the performance of the WRF-3DVAR assimilation with CV6 is distinctly better, and the results of 10 additional sea fog cases clearly suggest that CV6 is more suitable than CV5 for sea fog modeling. Compared to those with CV5, the average POD and ETS of forecasted sea fog area using 3DVAR with CV6 can be improved by 27.6% and 21.0%, respectively.
Highlights
Sea fog occurs within the marine atmospheric boundary layer (MABL)
Data assimilation is absolutely necessary for numerical modeling of sea fog, because modeling result is highly sensitive to initial condition, especially to initial moisture status. e weather research and forecasting (WRF)-ARW model and WRF-3DVAR module have been widely employed for sea fog modeling, and the WRF-3DVAR has already been proven to be able to effectively improve the initial conditions for sea fog simulations over the Yellow Sea
The default domain-dependent background error (BE) of the WRF-3DVAR is CV5, which does not consider the relationship between moisture and other control variables
Summary
Sea fog occurs within the marine atmospheric boundary layer (MABL). It is a type of fog that forms over an ocean surface and spreads over coastal areas. It can seriously affect many maritime activities (e.g., transport, navigation, fishery, and rescue) due to its low horizontal visibility [1, 2]. Due to the cold sea surface temperature (SST), an atmospheric inversion layer can develop in the MABL over the Yellow. Erefore, the sensitivity of sea fog modeling to the initial condition is largely related to temperature and moisture in the MABL [6,7,8,9,10,11,12].
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