Reply to Comments on “Seasonal Variation of the Physical Properties of Marine Boundary Layer Clouds off the California Coast”

Based on the strong similarity in microphysical differences between summer–winter marine boundary layer (MBL) clouds and the closed–open mesoscale cellular convection (MCC), Liu (2010)’s comments suggest that there is plausibly a microphysical mechanism, in addition to the macrophysical mechanism discussed in Lin et al. (2009), that is responsible for the seasonal contrast of the boundary layer structure and MBL cloud properties. Given the Moderate Resolution Imaging Spectroradiometer–Clouds and the Earth’s Radiant Energy System (MODIS–CERES) retrieval of mean liquid cloud effective radius re, together with the cloud liquid water path (LWP) and cloud thickness H as presented in Lin et al. (2009), the cloud droplet number density N and drizzle rate P can be estimated using (1), (2), and (6) in Liu (2010)’s comments. Without losing generality, we assume the more commonly used be 5 1/3 and use ae 5 66.83 from Martin et al. (1994) for maritime stratocumulus clouds. See Liu’s comments and the cited references for more discussion about the relations and the coefficients. The effective radius and the calculated number density as well as the drizzle rate are shown in Fig. 1. Indeed, as predicted in Liu’s comments, the MBL clouds off the California coast in summer are not only with higher liquid water content L but also with smaller re, larger N, and smaller P than those in winter. Figure 1 additionally shows that the spatial downstream variations of these microphysical properties along the cross section for each season are similar to the seasonal contrast of these variables. The summer-to-winter variation mirrors the transition from closed to open MCCs, as discussed in Liu (2010). Considering that the microphysical sensitivity of the cloudbase precipitation rate becomes weaker for clouds with stronger precipitation (Kubar et al. 2009; Wood et al. 2009), an alternative formulation with weaker dependence on microphysical properties (Van Zanten et al. 2005) is also used to estimate the drizzle rates. The seasonal difference is very similar (not shown). The higher drizzle rate during the winter in this region is also supported by multiyear MODIS retrievals, with drizzle frequency being higher among MBL cloud scenes during off-peak season for stratocumulus clouds (Jensen et al. 2008). The coincidence of higher drizzle rate and higher inversion height during winter appears to be consistent with the positive correlation between cloud-top height and drizzle frequency, as reported in Leon et al. (2008). Note that the equations for the estimation of N and P are for cloud-scale processes and highly nonlinear. The numbers that are derived based on the long-term averaged L and re should be interpreted with caution. However, there are also important cloud properties that cannot be explained in this analogy. First, for MCCs, along the cross section in the summer, the increasing drizzle rate should be associated with decreasing cloud fraction, because in comparison with closed MCCs, open MCCs usually have about 30% less cloud fraction (Wood and Hartmann 2006). But this is not the case in the seasonal variation: higher rate of drizzle in the summer is associated with larger cloud fraction along the cross section (Fig. 5a in Lin et al. 2009). Furthermore, in the winter, * Current affiliation: Brookhaven National Laboratory, Upton, New York.