CALOTRITON: A convective boundary layer height estimation algorithm from UHF wind profiler data

Abstract. Long series of observation of the atmospheric dynamics and composition are collected at the French Pyrenean Platform for the Observation of the Atmosphere (P2OA). Planetary boundary layer depth is a key variable of the climate system, but it remains difficult to estimate and analyse statistically by use of long series. In order to obtain reliable estimates of the convective boundary layer height (Zi) and to allow long-term series analyses, a new restitution algorithm, named CALOTRITON, has been developed, based on the observations of a Ultra High Frequency (UHF) wind profiler radar from P2OA, with the help of other instruments for evaluation. Zi estimates are based on the principle that the top of the convective boundary layer is associated with both a marked inversion and a decrease of turbulence. Those two criteria are respectively manifested by larger radar reflectivity and smaller vertical velocity Doppler spectral width. With this in mind, we introduce a new UHF- deduced dimensionless parameter which weights the air refractive index coefficient with the inverse of vertical velocity standard deviation to the power x. We then search for the most appropriate local maxima of this parameter for Zi estimates, with defined criteria and constraints, like temporal continuity. Given that Zi should correspond to fair weather cloud base height, we use ceilometer data to optimize our choice of the power x, and find that x = 3 gives the best comparisons/results. The estimates of Zi by CALOTRITON are evaluated using different Zi estimates deduced from radiosounding, according to different definitions. The comparison shows excellent results with a regression coefficient of up to 0.96 and a root mean square error of 80 m, close to the vertical resolution of the UHF of 75 m, when conditions are optimal. In more complex situations, that is when the atmospheric vertical structure is itself particularly ambiguous, secondary retrievals allow us to identify potential thermal internal boundary layers or residual layers, and help to qualify the Zi estimations. Frequent estimate errors are nevertheless observed when Zi is below the UHF first reliable gate, but also at the end of the day, when the boundary layer begins its transition to a stable nighttime boundary layer.