Observations of decoupled boundary layers in the south eastern Pacific during VOCALS using aircraft, ship and model data

The VOCALS (VAMOS Ocean-Cloud-Atmosphere-Land Study) aims to develop knowledge of the South Eastern Pacific (SEP) coupled ocean-atmosphere-land system. The region plays host to the largest stratocumulus deck in the world, and as such plays an important part in the radiation budget of the planet. The project aims to improve knowledge of the significant processes on timescales ranging from diurnal to inter-annual, and on spatial scales from those involving explicit aerosol-cloud interactions to large scale global climate and forecast models. One mode of variability is the diurnal cycle of thermodynamic boundary layer structure in the region. Long wave radiative cooling from cloud top is enhanced at night, leading to an increase in turbulent activity driven from cloud top. Daytime solar insolation acts to stabilise the cloud layer, leading to a reduction in turbulent intensity in the boundary layer, and thus heat, momentum and moisture transport. This leads to a diurnal cycle in cloud parameters such as Liquid Water Path (LWP), cloud heights, and cloud fraction.

Observations collected by the NCAR C130 and the FAAM BAe146 aircraft, and the Ron. H. Brown (RHB) research vessel are used to probe the structure of the oceanic boundary layer. In some cases the cloud base and top are measured explicitly using RADAR and LIDAR onboard the C130. Thermodynamic profiles collected by aircraft and radiosonde ascents, are used to investigate the stability of the boundary layer, and to diagnose decoupled boundary layers. Each aircraft flew a number of flights along the 20 South latitude, allowing a mini-climatology of observations to be compiled. An assessment is made of the frequency and location of decoupled boundary layers in the region. A value for Lifting Condensation Level (LCL) is generated from flight level aircraft data. These data are used to supplement the profile observations of the mixing state of the boundary layer, and inference is made about the typical scale of turbulent eddies. As such the variance of LCL is used as a proxy for estimating decoupling in the boundary layer.

The Met Office Unified Model (UM) ran in NWP mode during the VOCALS campaign and provided operational forecast support. The boundary layer scheme applies a turbulence parameterisation according to the thermodynamic type of boundary layer that is diagnosed. The UM diagnoses a value for the Turbulent Mixing Height (TMH) and the Boundary Layer Depth (BLZ). TMH represents the height of the marine boundary layer temperature inversion, and BLZ represents the height that a surface generated eddy will reach. These two parameters can be used to define a model estimate of decoupling. The model performance is assessed against observations along the 20 South latitude, in terms of the diurnal cycle of boundary layer thermodynamics.