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) cas es in 1984. The data were selected to represent the synoptic circulati on types appropriately. Lidar vertical profiles at 1064 nm were used t o obtain three estimates for the ABL height (h(lid)), based on the fir st gradient in the back-scatter profile, namely, at the beginning, mid dle and top of the gradient. The boundary-layer height obtained with t he radiosondes (h(s)) was determined with the dry-parcel-intersection method in unstable conditions. As a first guess for near-neutral and s table conditions, the height of the first significant level in the pot ential temperature profile was taken. Overall, the boundary-layer thic kness 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 separat ely. For stable conditions, an estimate of ABL height (h(N)) is also m ade based on the friction velocity and the Brunt-Vaisala frequency. Th e agreement between h(N) and h(lidb) is good. Discrepancies between th e two methods are caused by: (a) rapid growth of the boundary layer ar ound the measurement time; (b) the presence of a deep entrainment laye r 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-laye r height obtained from the radiosonde due to the way that profiles are recorded, as a series of significant points.