Mixing height algorithm - BLVIEW - CL61

BL-View for CL61 User Guide

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Merged gradient and profile fit method

BL-View uses the gradient method and the profile fit method to produce data. The gradient method can detect up to 3 aerosol layers and performs well in many situations, particularly for shallow nocturnal boundary layers at less than 500 m (1640 ft) above ground level. The profile fit method detects only a single layer, and successfully estimates mixing heights when the aerosol backscatter signal is weak, local gradients are weak and diffuse, and the boundary layer evolves rapidly with time during morning and evening transitions.

The merged gradient and profile fit method merges the strengths of the gradient and profile fit methods, but retains the ability to detect multiple layers in a backscatter profile. The merged method uses the following rules to select the mixing height from the gradient and profile fit retrievals:

  • The gradient method’s lowest retrieval for cloud profiles and shallow, high quality boundary layers.
  • Profile fit retrieval in all other cases (the gradient method's lowest retrieval is not displayed unless it differs by more than 1000 m (3280 ft).
  • Second and third gradient retrievals are displayed in all cases, as they may indicate residual layers or other aloft aerosol layers.

The algorithm identifies reliably the various boundary layers, such as the nocturnal, convective, marine, and residual layers, and differentiates the mixed layer from other aerosol layers detected by BL-View. It also provides an outlier removal method, cloud filter, and other changes to improve performance for evening boundary layer transitions and to increase flexibility and robustness for handling data from both ceilometers.

The algorithm determines the mixing height by fitting an idealized backscatter profile to observed range-corrected ceilometer backscatter profiles. Clouds and precipitation produce backscattering profiles that deviate substantially from an idealized profile, which results in poor mixing height estimates. The algorithm can produce valid retrievals even if the backscatter profile deviates significantly from the idealized profile.

When a ceilometer detects multiple aerosol layers, the algorithm attributes 1 aerosol layer as the mixing height. When there are multiple aerosol layers present, the lowest layer is a reasonable first guess for attributing one of these layers as the mixing height.