Abstract.Atmospheric subsidence over the subtropical southeast Pacific (SEP) leads to a low-level anticyclonic circulation, a cool sea surface and a cloud-topped marine boundary layer (MBL). Observations in this region from a major field campaign during October and November 2008, the VOCALS Regional Experiment, provide ample data to characterize the lower atmospheric features over the SEP. The observations are also useful to test the ability of an area-limited, high-resolution atmospheric model to simulate the SEP conditions. Observations and model-results (where appropriate) improve the characterization of the mean state (Part 1) and variability (Part 2) of the lower troposphere including circulation, MBL characteristics and the upsidence wave.Along 20 • S the MBL is generally deeper offshore (1600 m at 85 • W) but there is also considerable variability. MBL depth and variability decrease towards the coast and maximum inversion strength is detected between 74-76 • W. Weather Research and Forecasting (WRF) simulations underestimate MBL height the most near the coast but improve offshore. Southeasterly trades prevail within the MBL although the wind speed decreases toward the coast. Above the MBL along the coast of Chile, flow is northerly, has a maximum at 3 km, and extends westward to ∼74 • W, apparently due to the mechanical blocking exerted by the Andes upon the westerly flow aloft. Mean MBL features along northern Chile (18-25 • S) are remarkably similar (e.g., MBL depth just below 1 km) in spite of different SST. Observed diurnal cycles of the temperature at the coast and further offshore exhibit a number of conspicuous features that are consistent with the southwestward propagation of an upsidence wave initiated during late evening along the south Peru coast. Furthermore, the passage of the vertical motion resultsCorrespondence to: D. A. Rahn (darahn@gmail.com) in either constructive or deconstructive interference with the radiatively-forced diurnal cycle of MBL depth. Interference is clearly seen in the soundings at Iquique which are driven by a strong upsidence wave contrary to the radiation-driven cycle, leading to a diurnal cycle opposite of the other sites. Because WRF simulations have a lower MBL height, the speed of the simulated gravity wave is slower than observations and accounts for most of the discrepancy between observed and simulated phase speeds.
Prevailing wind along the west coast of South America is equatorward, driven by the southeast Pacific anticyclone. The wind induces strong coastal upwelling that supports one of the most important fisheries in the world. This region lacks a dense network of in situ observations, so the high resolution (0.313 • ) NCEP Climate Forecast System Reanalysis is used here to present a synoptic climatology of the coastal wind along the Chile/Peru coast. Covariability between the alongshore pressure gradient and alongshore wind, which was previously identified for synoptic time scales near central Chile, is generalized for the whole coast and over annual time scales. Particular attention is paid to three prominent upwelling regions: Pisco (14.8 • S), Punta Lengua de Vaca (30.0 • S), and Punta Lavapie (36.4 • S). Previous work has identified local maxima at these points but these are embedded in a broader low-level jet that exhibits a marked seasonal cycle of strong wind days due to the migration of the anticyclone and is associated with a shift of both the mean wind and a more frequent recurrence of strong wind events. Alongshore wind near Pisco is normally distributed year-round with a seasonal shift in the mean. Larger variability in the mean and distribution is found at Lavapie, associated with the seasonal change in storm tracks. The synoptic evolution that drives high-wind events at each location is characterized. A midlevel trough and surface cyclone precede wind maxima at each location and are followed by strong midlevel ridging and a strengthened surface anticyclone.
Observations from commercial aircraft [e.g., the Aircraft Meteorological Data Relay (AMDAR) automated weather reports] have been increasing dramatically. Two main applications of the aircraft data are use in short-term forecasts and assimilation into numerical weather prediction models. Now that more than 10 years of measurements exist, using this dataset to construct a description of the long-term climatological behavior (a “climatology”) of the lower atmosphere is explored with two main objectives. The first objective is to examine strengths and weaknesses of using the dataset to construct a climatology of the lower atmosphere. Unlike the traditional twice-daily radiosonde launches, the high frequency of observations at major airports allows for an unprecedented set of diurnal information at many locations globally. The second objective is to obtain a climatology of the lower atmosphere of Southern California, specifically at Los Angeles, San Diego, and Ontario, during the spring and summer when the boundary layer is well defined and easily detected. The June 2001–14 climatology reveals that the deepening of the boundary layer overnight is consistent with a cloud-topped boundary layer. Whereas the average boundary layer height decreases right after sunrise at San Diego, at Los Angeles the deeper boundary layer persists about 4 h after sunrise and then decreases rapidly over 2 h as the onshore sea breeze strengthens. Morning intrusions of the marine air inland are easily detected at Ontario in some months but are practically nonexistent during July and August.
Abstract. The VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) was a major field experiment conducted in spring of 2008 off southern Peru and northern Chile, aimed at better understanding the coupled climate systems of the southeast Pacific. Because of logistical constrains, the coastal area around 30 • S was not sampled during VOCALS-REx. This area not only marks the poleward edge of the subtropical stratocumulus cloud regime (thus acting as a source of transient disturbances) but is also one of the most active upwelling centers and source of surface ocean kinetic energy along the Chilean coast. To fill such an observational gap, a small, brief, but highly focused field experiment was conducted in late spring 2009 in the near-shore region around 30 • S. The Chilean Upwelling Experiment (CUpEx) was endorsed by VOCALS as a regional component.CUpEx included long-term monitoring, an intensive twoweek field campaign and off-shore research flights. Our goal was to obtain an atmospheric/oceanic dataset with enough temporal and spatial coverage to be able to document (a) the mean diurnal cycles of the lower-troposphere and upper-ocean in a region of complex topography and coastline geometry, and (b) the ocean-atmosphere response to the rapid changes in coastal winds from strong, upwelling-favorable equatorward flow (southerly winds) to downwelling-favorable poleward flow (northerly winds). In this paper we describe the measurement platforms and sampling strategy, and provide an observational overview, highlighting some key mean-state and transient features.
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