Cloud fractions and cloud base heights at a subtropical coastal site: comparison between routine observations and automatic measurements with ceilometer and total sky imager

Located right at the Tropic of Capricorn and at the eastern margin of the SE Pacific Anticyclone, the Cerro Moreno Airport (23.4 °S, 70.4 °W) constitutes a privileged observation site for the persistent stratocumulus clouds capping the marine boundary layer offshore. An extended record of hourly observations of cloud heights and cloud fractions exists at this site, together with corresponding daily (12 UTC) routine radiosondes. The climatology and trends of this data are presented and discussed in a companion report to this meeting (Muņoz et al., 2015). In the present work we focus on the inter-comparison between routine observations of cloud properties and those determined with automatic instruments recently installed at the site.

Cloud macroscopic features like type distinction and fractional cover have resisted automation and even up to now this type of data most frequently comes from the expert judgment of professional weather observers. This is the case also for the site under study for which 45 years of hourly cloud observations are available, including type, fraction and base heights for up to 4 cloud layers. Overall quality of this data is good as attested, for example, by the physical consistency of a cloud-conditioned climatology constructed for this site by Muņoz et al. (2011). Even during the night, observers can relatively well discern cloud properties, aided by the extreme clearness of the skies above the boundary layer in this region. However, by necessity, human cloud observations are discretized in values and time, rendering them less valuable for more quantitative studies, especially those addressing phenomena with short time scales, like cloud dissipation and re-formation.

In order to improve the characterization of clouds at the Cerro Moreno site, we installed recently a CL31 ceilometer (August 2013) and a TSI-440A total sky imager (December 2013). The former instrument generates continuous measurements of cloud base height, while the latter takes one picture of the full sky hemisphere every 30 seconds (during daytime), from which an estimate of cloud cover fraction can be computed. The present work compares the data generated by these instruments with the corresponding human observations, in terms of cloud frequency of occurrence, cloud fractional cover, and cloud base height. Diurnal, synoptic and seasonal variability and co-variability of both datasets will be assessed. Furthermore, the feasibility of estimating winds at the cloud layer by using successive TSI images is explored.

Acknowledgments

Partial funding by Project Fondecyt 1130111 is acknowledged.

References

Muņoz, R., R. Zamora, J. Rutllant, 2011: The coastal boundary layer at the eastern margin of the Southeast Pacific (23.4 S, 70.4 W): cloudiness-conditioned climatology. Journal of Climate, 24, 1013-1033.

Muņoz, R., J. Quintana, M. Falvey, 2015: Coastal clouds along northern Chile: climatology and trends. 13th Symposium on the Coastal Environment, 4-8 January, 2015, Phoenix, Arizona, USA.