Addendum to “Sea spray aerosol flux estimation based on long-term variation of wave statistics”: estimation based on long-term variation of wind statistics ☆

摘要

Summary This note provides estimates of the mean whitecap coverage and the mean sea spray aerosol flux based on long-term wind statistics from the Northern North Sea. Here the improved sea spray aerosol production flux model by Callaghan (2013) is used. The results are compared with those in Myrhaug et al. (2015) based on long-term wave statistics from the Northern North Sea and the North Atlantic. Keywords Sea spray aerosol flux ; Whitecap coverage ; Mean wind speed ; Wind statistics prs.rt("abs_end"); 1. Introduction Myrhaug et al. (2015) (hereafter referred to as MWH15) provided estimates of the mean sea spray aerosol flux based on long-term variation of wave statistics using the whitecap method applying the limiting steepness and threshold vertical acceleration criteria. Here the long-term wave statistics represented open ocean deep water waves in the Northern North Sea and the North Atlantic. This note is supplementary to MWH15 with the purpose of demonstrating how similar results for the mean sea spray aerosol flux can be obtained by using estimates of the whitecap coverage based on long-term variation of wind statistics. Moreover, the whitecap method used in MWH15 has been replaced by the Callaghan (2013) improved sea spray aerosol production flux model. The whitecap coverage, which is defined as the area of whitecaps per unit sea surface, has often been used to quantify the occurrence of breaking wind waves at sea. There are many parameterizations of whitecap coverage available in the literature; comprehensive reviews are given in Anguelova and Webster (2006) , Massel (2007) and de Leeuw et al. (2011) . Parameterizations are based on U ₁₀ and u _*. Here U ₁₀ [m?s?1] is the mean wind speed at the 10?m elevation, and u _* [m?s?1] is the friction velocity equal to the square root of the vertical flux of horizontal momentum at the sea surface. However, when plotting the whitecap coverage versus U ₁₀ and versus u _* it is often found that the data scatter is larger when plotted versus u _* than when plotted versus U ₁₀ (see e.g. Sugihara et al., 2007 ). This is attributed to the larger uncertainties in estimating u _* than measuring U ₁₀. Therefore the parameterizations in the present study are based on U ₁₀. Other important factors affecting the whitecap coverage are the stratification of the near-surface air boundary layer and the state of development of surface waves, see e.g. Sugihara et al. (2007) and Myrhaug and Holmedal (2008) . Reviews of whitecap coverage at sea and how it is linked to marine aerosol production are given by Massel (2007) , de Leeuw et al. (2011) and Callaghan (2013) . 2. Whitecap coverage and sea spray aerosol flux estimation based on long-term variation of wind statistics 2.1. Whitecap coverage estimation The following whitecap coverage ( W _c ) parameterizations will be considered here to demonstrate the use of wave statistics. The Monahan and O’Muircheartaigh (1980) (hereafter referred to as MO80) parameterization is widely used and recognized ( de Leeuw et al., 2011 ), given as fraction, equation ( 1 ) W c = 3.84 × 10 ? 6 U 10 3.41 . The Callaghan et al. (2008) (hereafter referred to as C08) parameterization is based on data collected in the North East Atlantic inside a geographical area defined by 9.5°W, 13°W, 55.5°N and 57.5°N, given in percent, equation ( 2 ) W c = 0.00318 ( U 10 ? 3.70 ) 3 ; 3.70 m?s ? 1 It should be noted that the wave statistics in BGGS07 ( Bitner-Gregersen and Guedes Soares, 2007 ) Data Sets 1 to 5 used in MWH15 is from the same ocean area, i.e. from the North Atlantic.