Marine boundary layer over the subtropical southeast Pacific during VOCALS-REx – Part 1: Mean structure and diurnal cycle

Abstract

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.

Advances and limitations of atmospheric boundary layer observations with GPS occultation over southeast Pacific Ocean

“…3b, yields a low bias in the ECMWF analysis. The correlation is lower (∼0.54) and the 1-sigma uncertainty is ∼70 m. The systematic low bias in ECMWF ABL height is consistent with previous weather and climate model studies over the region (e.g., Bretherton et al, 2004;Rahn and Garreaud, 2010;Wyant et al, 2010;von Engeln and Teixeira, 2011;Wang et al, 2011).…”

“…Such radiosonde ABL heights derived from MRG method are consistent with the ABL heights derived from the radiosonde inversion base from the same region (e.g., Fig. 4a of Rahn and Garreaud, 2010 and Fig. 10 of Wyant et al, 2010) as well as the cloud top height measurements from the airplane cloud radar during the VOCALS campaign (e.g., Fig.…”

Limited-are a modelling of stratocumulus over South-Eastern Pacific

“…Similarly to the subtropical region off the California coast, such conditions lead to a cold well-mixed boundary layer topped by a persistent stratocumulus deck (Rahn and Garreaud, 2010;Toniazzo et al, 2011). Due to a large area of coverage and persistence of stratocumulus clouds, this region significantly affects planetary albedo.…”

Modelling the effects of gravity waves on stratocumulus clouds observed during VOCALS-UK

“…Rather, we look at the sensitivity of the Sc to lifting and lowering of the layer by a "kinematic gravity wave", the details of which are specified from observed GOES10 imagery, to examine the physics behind the clearing of the Sc. We point out that the gravity wave packets under consideration in this paper have a different source to the "upsidence" wave described by Garreaud and Muñoz (2004) and Rahn and Garreaud (2010), which is due to mechanical blocking by the Andes of the westerly flow above the boundary layer. Such effects have been investigated using LES by Jiang and Wang (2012) by applying upsidence-subsidence waves of time periods from 6 to 48 h, and it was found that, perhaps because of the relatively long wave period, precipitation is significant in the clearing of the cloud.…”

“…Large-scale divergence was specified to be 3 × 10 −6 s −1 , which is consistent with both QuikSCAT and WRF simulations for the period in question (Rahn and Garreaud, 2010) and helps to maintain the inversion.…”