Coastal lomas cloudiness

Coastal Chile and Peru lomas

The Pacific coastal regions of South America from Ecuador to central Chile are exposed to low-level stratus clouds for much of the year.  These clouds vary in their depth and inland-penetration with a strong diurnal cycle.  In addition, the coastal topography modifies the frequency of these clouds, with some locations being frequently cloudy while others are virtually cloud-free.

Fig 1 shows a schematic of the key areas we discuss here.  This coastal region shows a latitudinal rainfall gradient, with rainfall increasing rapidly as one proceeds northward along the Ecuadorean coast.  Likewise, coastal rainfall increases southward over central Chile.  Between approximately Antofagasta and Trujillo, Peru – a distance of roughly 2000 km, there is virtually no conventional rainfall along the coastal strip.  Drizzle from stratus or sprinkles from dissipating storms that formed farther inland over the Andes does occasionally occur, but is rarely measurable (greater than 1 mm for example).

Although the MODIS-based climatologies (Fig 2-3) provide an excellent depiction of the location of the cloud-impacted regions along the Pacific coast, they do not depict the maximum extent of stratus encroachment inland.  The earliest MODIS observation time, 1030 local time, is about 4.5 hours after sunrise, when thin stratus may have dissipated due to boundary layer deepening and direct heating of the clouds by visible solar radiation.  The GOES imagery, available every 15 minutes, affords a better visualization of the daytime diurnal variation of low clouds along the coast.

Fig 4 shows the mean cloudiness for a 60-day period from the GOES-16 satellite launched in November 2016, but becoming fully operational in late 2017.  The daytime imagery has been averaged into two hour periods to provide a large sample size since the 2-month period is relatively short.