Whitecapping plays an important role in many air-sea exchange and upper ocean processes.Traditionally,whitecap coverage is parameterized as a function of wind speed only.At present,the relative speed of ocean current to wind is considered to be important in the air-sea exchange parameterization which is the function of wind speed only.In this paper,the effects of ocean surface velocity (current velocity and wave induced velocity) and the wave parameters on whitecap coverage through relative speeds are investigated,by applying a 2-parameter whitecap coverage model to the Atlantic Ocean.It is found that the impacts of both current and wave on whitecap coverage are considerable in the most part of the Atlantic Ocean.It is interesting that the effect of wave is more significant than that of current.
A four dimensional variational data assimilation (4DVar) based on a dimension-reduced projection (DRP-4DVar) has been developed as a hybrid of the 4DVar and Ensemble Kalman filter (EnKF) concepts. Its good flow-dependent features are demonstrated in single-point experiments through comparisons with adjointbased 4DVar and three-dimensional variational data (3DVar) assimilations using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5). The results reveal that DRP-4DVar can reasonably generate a background error covariance matrix (simply B-matrix) during the assimilation window from an initial estimation using a number of initial condition dependent historical forecast samples. In contrast, flow-dependence in the B-matrix of MM5 4DVar is barely detectable. It is argued that use of diagonal estimation in the B-matrix of the MM5 4DVar method at the initial time leads to this failure. The experiments also show that the increments produced by DRP-4DVar are anisotropic and no longer symmetric with respect to observation location due to the effects of the weather trends captured in its B-matrix. This differs from the MM5 3DVar which does not consider the influence of heterogeneous forcing on the correlation structure of the B-matrix, a condition that is realistic for many situations. Thus, the MM5 3DVar assimilation could only present an isotropic and homogeneous structure in its increments.
Although the annual global sea-air CO2 flux has been estimated extensively with various wind-dependent-k parameterizations,uncertainty still exists in the estimates. The sea-state-dependent-k parameterization is expected to improve the uncertainty existing in these estimates. In the present study,the annual global sea-air CO2 flux is estimated with the sea-state-dependent-k parameterization proposed by Woolf(2005) ,using NOAA/NCEP reanalysis wind speed and hindcast wave data from 1998 to 2006,and a new estimate,-2.18 Gt C year-1,is obtained,which is comparable with previous estimates with biochemical methods. It is interesting to note that the averaged value of previous estimates with various wind-dependent-k parameterizations is almost identical to that of previous estimates with biochemical methods by various authors,and that the new estimate is quite consistent with these averaged estimates.
