Relationships between marine boundary layer clouds and meteorological factors

In order to develop marine boundary layer cloud parameterizations for global climate and weather prediction models, we are studying the subgrid-scale variability of liquid water path (LWP) of marine boundary layer clouds in areas which correspond to the typical grid size of such models using geostationary satellite 1 km mesh visible data. The ERA-Interim and ERA-40 data are used in addition to the satellite data to investigate the relationships between meteorological factors and subgrid-scale statistics of LWP PDFs related to boundary layer clouds.

We already reported that meteorological factors related to stability between 850hPa and 1000hPa show high correlations with cloud amount and with the homogeneity, skewness and kurtosis of PDFs of LWP of marine boundary layer clouds (Kawai and Teixeira (2010, J.C. in press) ). We also found that when a Gaussian distribution is assumed for the shape of the probability density function (PDF) of total water content, the observed behavior of the PDFs of LWP can be explained quite well.

In this presentation, some intriguing results related to the relationships between marine boundary layer clouds and the meteorological field will be discussed.

1. It was found that relative humidity at 850hPa and cloud amount of marine boundary layer clouds have clear negative correlation; The higher the relative humidity at 850hPa, the less the low cloud amount. At first sight it may seem opposite to what could be expected. These results will be interpreted.

2. The correlation coefficients between different stabilities at low level and the statistical properties (homogeneity, skewness and kurtosis) of the LWP PDFs are higher than the correlation between the same stability factors and cloud amount. The relationships between cloud amount and different stabilities have been studied and are well- known (e.g. LTS by Klein and Hartmann (1993) and EIS by Wood and Bretherton (2006)). It is interesting that there are cloud properties that have clearly higher correlation coefficients with the meteorological field than cloud amount.

3. The correlation coefficients between stabilities at low level and low level cloud amount deteriorate dramatically from monthly data to daily anomaly data (anomaly is calculated from monthly average). Though correlation coefficients between cloud amount and stabilities have been shown in some papers and the coefficients are remarkably high, the coefficients are often calculated for relatively large areas (~10°x10°) and/or for long term (several months). It is worth investigating how good these correlations are for areas which correspond to the size of model grid (200km x 200km) and for short term (several hours). Our results could provide important implications for the case when such relationship or mechanism of cloud processes is considered in detail or such relationships are utilized as simple parameterizations of marine boundary layer clouds.