Spatial Variability of Liquid Water Path in Marine Low Cloud: The Importance of Mesoscale Cellular Convection

Abstract

Liquid water path (LWP) mesoscale spatial variability in marine low cloud over the eastern subtropical oceans is examined using two months of daytime retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the NASA Terra satellite. Approximately 20 000 scenes of size 256 km × 256 km are used in the analysis. It is found that cloud fraction is strongly linked with the LWP variability in the cloudy fraction of the scene. It is shown here that in most cases LWP spatial variance is dominated by horizontal scales of 10–50 km, and increases as the variance-containing scale increases, indicating the importance of organized mesoscale cellular convection (MCC). A neural network technique is used to classify MODIS scenes by the spatial variability type (no MCC, closed MCC, open MCC, cellular but disorganized). It is shown how the different types tend to occupy distinct geographical regions and different physical regimes within the subtropics, although the results suggest considerable overlap of the large-scale meteorological conditions associated with each scene type. It is demonstrated that both the frequency of occurrence, and the variance-containing horizontal scale of the MCC increases as the marine boundary layer (MBL) depth increases. However, for the deepest MBLs, the MCC tends to be replaced by clouds containing cells but lacking organization. In regions where MCC is prevalent, a lack of sensitivity of the MCC type (open or closed) to the large-scale meteorology was found, suggesting a mechanism internal to the MBL may be important in determining MCC type. The results indicate that knowledge of the physics of MCC will be required to completely understand and predict low cloud coverage and variability in the subtropics.

Buoyancy reversal, decoupling and the transition from stratocumulus to shallow cumulus topped marine boundary layers

“…There is also clear distinction between the low initial q þ t (low cloud fraction) cases in the MIXED and DECOUPLED series. Observational studies of low clouds usually show LWP distributions classified according to cloud fraction (e.g., Barker et al 1996;Zhou et al 2006;Wood and Hartmann 2006). The LWP distributions for high cloud fraction cases shown by these studies are similar to those found in the high cloud fraction cases in the MIXED and DECOUPLED series.…”

Characteristics of cloud liquid water path from SEVIRI onboard the Meteosat Second Generation 2 satellite for several cloud types

“…Marine-boundary-layer clouds are a major source of uncertainty for cloud radiative feedbacks, as stated in several publications (see Chlond et al, 2004, Seethala and Horváth, 2010or Wood and Hartmann, 2006. Therefore, the climate modelling community would greatly benefit from accurate LWP measurements of marine-boundary-layer clouds.…”

“…For the retrieval of LWP from SEVIRI, the common assumption that liquid water content (LWC) is constant with height was applied. Wood and Hartmann (2006) applied in their study on LWP in marine low clouds a LWC profile that linearly increases with height as is often observed in marine stratiform cloud cover. The profile assumption was applied to MODIS measurements in their case which is quite comparable to the SEVIRI instrument and LWP retrieval.…”

Evaluation of SCIAMACHY ESA/DLR Cloud Parameters Version 5.02 by Comparisons to Ground-Based and Other Satellite Data

“…In reality, a constant LWC profile shape is not representative of a typical inhomogeneous cloud (Wood and Hartmann, 2006). have provided an error estimate if a constant LWC profile is assumed in the forward model.…”