Morphology and dynamics of non-precipitating marine fair weather cumulus clouds

Non-precipitating marine fair weather cumulus clouds or cumulus humilis have a large impact on the boundary layer structure and on the earth's radiation budget. Due to their marine location, short life time and small spatial scales, their observations still remain sparse. These clouds are classified as forced, active and passive clouds based on their role of venting mixed layer air into free troposphere, with each type having a distinct morphological and dynamical structure. The current deployment of the Atmospheric Radiation Measurement (ARM)'s Mobile Facility (AMF) at the island of Graciosa in the Azores gives an opportunity to sample this important component of the climate system. AMF instrumentation include a vertically pointing 95 GHz Doppler cloud radar, laser ceilometer and balloon borne soundings among several other instruments. Nine cases of marine fair weather cumulus clouds totaling to 114 hours are analyzed to identify ~550 cumuli. Statistics of in-cloud vertical velocity mean, variance and skewness are developed in addition to those of hourly cloud fraction, cloud boundaries, inversion strength and mass-flux. The observed cumuli during one of the cases (26 June 2009) were further classified to identify fifty six forced, three active and four passive clouds, with the mean in-cloud vertical velocity being 0.22 ms-1, 0.38 ms-1 and -0.48 ms-1 within forced, active and passive cloud respectively. Although the preliminary results are based on few samples, they are remarkably consistent with the expectations based on theory and highlight the need to distinguish between different fair weather cumulus cloud types. An attempt will be made to classify all the observed cumuli into the subtypes not only to develop climatology of the mean dynamical structure of each subtype but also to highlight their different morphological structure.