CIMAR-5: A Snapshot of the Lower Troposphere over the Subtropical Southeast Pacific

The extensive and persistent deck of stratocumulus (SCu) off the west coast of subtropical South America plays an important role in the regional and global climate, as well as in coastal weather. As in other subtropical regions, the SCu form at the top of a marine boundary layer (MBL) bounded by a relatively cold ocean and a large-scale subsidence inversion. Nevertheless, details of the structure and variability of the lower troposphere over this region are largely unknown. Ship-based meteorological observations taken along a transect at 27°S from the Chilean coast (71°W) to Easter Island (110°W) during the second half of October 1999 provide a preliminary description of the low-level circulation, thermodynamic structure, and cloudiness over the subtropical southeast Pacific. Three types of observations were made: (a) 15-min average of air temperature, relative humidity, solar radiation, pressure, and wind by an automatic weather station on the ship deck; (b) 15-min average of vertical reflectivity and cloud base by a laser ceilometer on the ship deck; and (c) twice-daily rawinsondes. Several cloud and radiation properties were derived from GOES-8 imagery and validated against the ship-based observations. A preliminary assessment of the ability of NCEP-NCAR reanalysis and scatterometer winds in representing the atmosphere over a largely in situ data-void area is also presented. Sea surface temperature and near-surface air temperature increase gradually westward, while near-surface relative humidity remains nearly constant at -80%. A significant increase in the free-tropospheric moisture indicates an offshore decrease in the large-scale subsidence. Consistently, the MBL evolves from a shallow, well-mixed MBL topped by compact SCu near the coast; to a deeper, decoupled MBL with a cumuli rising into a patchy SCu deck near Easter Island, in a similar fashion to the transition from subtropical-stratus regime to trade-cumulus regime described elsewhere. In addition to these "climatological" features, the ship data also reveal the large sensitivity of the MBL-trade inversion structure to synoptic-scale disturbances over the subtropical Pacific. Cloud droplet effective sizes increase from the coast to open ocean. Furthermore, cloud fraction, cloud-top height, liquid water path, and optical depth all peaked during the morning and reached a minimum by midafternoon.