Seamless wind profiling of the atmospheric boundary layer

Structures and processes in the atmospheric boundary layer (ABL) span a wide range of scales which cannot be fully captured by a single measuring technique. Doppler lidars are a preferred remote sensing tool for the observation of wind profiles in the entire ABL. Typically the relevant scales of ABL flow structures increase with increasing distance from the surface.

While pulsed Doppler lidars (PDLs) are typically sufficiently sensitive to reach the top of the ABL, they leave an unobserved gap in the lowest 50 m. This is because the PDL receiver doesn’t work before the transmit pulse has left the lidar. In addition to the gap, the spatial resolution δr, which is proportional to the pulse length l, is not always satisfying in the lowest range gates, since wind gradients occuring over canopies, urban surface structures or in case of stable stratification cannot be resolved reliably. l cannot be deliberately shortened for the sake of velocity resolution δv and sensitivity s (δv ∝ l^-1, s ∝ l).

Continuous wave lidars (CDLs) are typically restricted to a maximum height of 200 m. The ranging is here achieved by focusing the beam. With this technique the range resolution δr is not constant but becomes finer the shorter the range r. At r = 10 m, a range resolution δr = few centimeters is possible. Since δr ∝ r² the resolution at 200 m is no longer superior to that of PDLs. Thus, a CDL is a perfect complement to a PDL in order to observe near-surface structures of the ABL.

We will present observations obtained with a PDL-, CDL-combination. Both systems are using an identical scanning cone with a zenith angle of 10°. This scan geometry is particularly suitable for operation close to obstacles as trees or buildings, in urban environments or in complex terrain.