A COMPARISON OF ABL HEIGHTS INFERRED ROUTINELY FROM LIDAR AND RADIOSONDES AT NOONTIME

The height of the atmospheric boundary layer (ABL) obtained with lidar and radiosondes is compared for a data set of 43 noon (12.00 GMT) cases in 1984. The data were selected to represent the synoptic circulation types appropriately. Lidar vertical profiles at 1064 nm were used to obtain three estimates for the ABL height (h(lid)), based on the first gradient in the back-scatter profile, namely, at the beginning, middle and top of the gradient. The boundary-layer height obtained with the radiosondes (h(s)) was determined with the dry-parcel-intersection method in unstable conditions. As a first guess for near-neutral and stable conditions, the height of the first significant level in the potential temperature profile was taken. Overall, the boundary-layer thickness estimates agree surprisingly well (regression line (h(lidb) = h(s): cc. = 0.93 and the standard error = 121 m). However, in 10% of the cases, the lidar estimate was significantly lower (difference>400 m) than the routinely inferred h(s). These outliers are discussed separately. For stable conditions, an estimate of ABL height (h(N)) is also made based on the friction velocity and the Brunt-Vaisala frequency. The agreement between h(N) and h(lidb) is good. Discrepancies between the two methods are caused by: (a) rapid growth of the boundary layer around the measurement time; (b) the presence of a deep entrainment layer leading to a large zone in which quantities are not well mixed; (c) a large systematic error of 100-200 m in the estimate of boundary-layer height obtained from the radiosonde due to the way that profiles are recorded, as a series of significant points.