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 g lobal climate, as well as in coastal weather. As in other subtropical regio ns, the SCu form at the top of a marine boundary layer (MBL) bounded by a r elatively 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 a long a transect at 27 degreesS from the Chilean coast (71 degreesW)A to Eas ter Island (110 degreesW) during the second half of October 1999 provide a preliminary description of the low-level circulation, thermodynamic structu re, and cloudiness over the subtropical southeast Pacific. Three types of o bservations were made: (a) 15-min average of air temperature, relative humi dity, solar radiation, pressure, and wind by an automatic weather station o n 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. S everal 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 similar to 80%. A significant increase in the free-tropospheric moisture in dicates an offshore decrease in the large-scale subsidence. Consistently, t he MBL evolves from a shallow, well-mixed MBL topped by compact SCu. near t he 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 subtr opical-stratus regime to trade-cumulus regime described elsewhere. In addit ion to these "climatological" features, the ship data also reveal the large sensitivity of the MBL-trade inversion structure to synoptic-scale disturb ances 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 re ached a minimum by midafternoon.