Coastal clouds along northern Chile: climatology and trends

The coast of northern Chile (~70 °W, 19-24 °S) lies at the boundary between two remarkably contrasting climatic environments. To the east, the extremely arid Atacama Desert and the high Andes Cordillera (~ 6,000 m ASL) are located no farther than 300 km from the coast. To the west, the subsidence associated to the semi-permanent South-East (SE) Pacific Anticyclone and the cold SE Pacific waters combine to produce a cool and humid marine boundary layer frequently capped by a stratocumulus (Sc) layer at its top. The large extension and the persistence of this Sc layer make it an important factor of the global energy budget, acting as cooling agent due to its effect in the Earth's mean albedo. As such, the SE Pacific Sc cloud layer has been the subject of several observational and modeling studies aiming at improving the understanding of its dynamics and the representation of these clouds in climate models (Mechoso et al., 2014).

While closely related to the Sc clouds existing offshore, the low-level clouds along the northern Chilean coast are influenced also by circulations induced by the sea-desert-mountain configuration and the attendant regional variations in topography and surface properties. As a result, coastal clouds in this region show a marked diurnal, synoptic and seasonal variability. Description of this variability is of importance for several applications including surface and air transportation, solar energy resources, tourism, etc.

Some previous works have addressed climatological features and tendencies of several meteorological variables measured at the coast of northern Chile. These include a 29-year climatology of the routine radiosonde data at Antofagasta (23.4 °S, 70.4 °W) by Muñoz et al. (2010), an analysis of the inter-annual and long-term tendencies in precipitation, subsidence inversion height and total cloudiness by Schulz et al. (2011), a documentation of a cooling trend in near-surface temperatures by Falvey and Garreaud (2009), and a preliminary description of the variability of coastal clouds by Quintana and Berríos (2007) and Berríos (2008). The present work aims at providing a more comprehensive description of the climatology and trends of coastal clouds in this region, together with those of related variables like temperature, humidity, winds and surface and subsidence inversion properties.

The analysis is based mainly on 45 years (1969-2013) of hourly routine meteorological observations at Cerro Moreno Airport in Antofagasta. The data consist of standard surface meteorological variables, including air temperature, relative humidity, atmospheric pressure, wind direction and velocity, and sunshine hours. Clouds are described in up to 4 layers in terms of cloud base height and fractional cover. Observations are performed by professional observers of the Chilean National Weather Service supporting the aeronautical operations at this major airport. A companion report to this meeting (Muñoz et al., 2015) validates cloud observations at this site against recently available automatic measurements with a ceilometer and a total sky imager. Routine 12 UTC radiosondes are launched daily from the same site, so that tendencies of upper-air variables and surface and subsidence inversion properties are documented as well. In order to extend the spatial representation of the analysis, similar surface observations available at the airports of Iquique (20.5 °S) and Arica (18.3 °S) are analyzed and described as well. The data is characterized in terms of their diurnal and annual cycles, inter-daily and inter-annual variability, and long-term trends. Besides documentation of the variability, the purpose is to assess the physical connection between the joint evolutions of the different variables at these several scales.

In terms of tendencies, preliminary results for the Antofagasta site reveal a lowering of cloud base heights and a slight reduction in mean cloud fraction. The latter is not necessarily an indication of a decrease in cloudiness, but, on the contrary, may be due to a more frequent observation of low-fraction clouds. Surface variables, on the other hand, show a cooling trend together with an increase of the lifting condensation level, associated to a slight negative trend in relative humidity. Radiosonde data shows a steady increase in the frequency of surface inversions and a slight decrease of subsidence inversion base heights. The full climatology of these variables at the 3 coastal sites will be reported at the meeting, together with an assessment of the physical relationships among the various trends observed.

Acknowledgments

Partial funding by Project Fondecyt 1130111 is acknowledged. Preliminary work on these topics by Patricia Berríos and Rosa Zamora is acknowledged.

References

Berríos, P., 2008: Estratocúmulos en la costa norte de Chile: variabilidad y tendencia. Undergraduate Thesis in Meteorology, U. of Valparaíso, Chile, 118 pp.

Falvey, M., R. Garreaud, 2009: Regional cooling in a warming world: recent temperature trends in the southeast Pacific and along the west coast of subtropical South America (1979-2006). Journal of geophysical research, vol. 114, D04102.

Mechoso R. et al., 2014: Ocean-cloud-atmosphere-land interactions in the southeastern Pacific. Bulletin of the American Meteorological Society, 95, 357-375.

Muñoz, 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.

Muñoz, R. C., J. Quintana, J. M. Ceppi, 2015: Cloud fractions and cloud base heights at a subtropical coastal site: comparison between routine observations and automatic measurements with ceilometer and total sky imager. 13th Symposium on the Coastal Environment, 4-8 January, 2015, Phoenix, Arizona, USA.

Quintana, J., P. Berríos, 2007: Study of the coastal low cloud in the northern coast of Chile: variability and tendency. 4th International Conference on fog, fog collection and dew. La Serena, Chile, 22-27 July 2007.

Schulz, N., J. P. Boisier, P. Aceituno, 2011: Climate change along the arid coast of northern Chile. International Journal of Climatology. DOI: 10.1002/joc.2395