Mesoscale weather features in NYC Revealed by CCNY lidar

Local and mesoscale weather features in New York City are diagnosed using vertical profiles of the tropospheric aerosol content recorded by the City College of New York (CCNY) vertically pointing backscatter lidar. The lidar operates at three wavelengths (355, 532, and 1064 nm), providing 1-minute soundings from 500 m to 15 km with vertical resolution of 10 m . During periods of operation (mostly clear spring and summer weekdays), the lidar system runs continuously but is automatically shut down when a colocated radar detects aircraft above. Aerosol optical thickness is calculated from the range corrected power of the lidar signal after Rayleigh scattering of the molecular atmosphere has been subtracted.

Several characteristic profiles are observed and related to the concurrent and antecedent weather situations. These include:

1. At least some time on 65% of the days, visibility decreases upward to the top of the atmospheric boundary layer (ABL). This indicates the ABL is well mixed and has high relative humidity at the top, causing expansion of aerosols due to deliquesence. Visibility increases abruptly above the ABL when it is capped by a subsidence inversion. 2. The height and optical thickness of the ABL often increase shortly before onset of the sea breeze, but decrease after the sea breeze has become established. This suggests that the sea breeze front, when viewed in cross section, is marked by a vortex flow pattern similar to that at the leading edge of outflow from a thunderstorm downburst. 3. During established sea breeze and “back door” cold front regimes, visibility is high if fog is not present and gradually increases upward through the ABL. This indicates the ABL is stably stratified and poorly mixed 4. Discrete haze layers sandwiched in clear air above the ABL occur in about 40% of the cases. Most can be backtracked using trajectory analysis to near surface sources or to forest fires upwind. No such sources of aerosols exist for clean air layers above the ABL.