Ceilometer-Based Analysis of Shanghai's Boundary Layer Height (under Rain- and Fog-Free Conditions)

To investigate the boundary layer dynamics of the coastal megacity Shanghai, China, backscatter data measured by a Vaisala CL51 ceilometer are analyzed with a modified ideal curve fitting algorithm. The boundary layer height z(i) retrieved by this method and from radiosondes compare reasonably overall. Analyses of mobile and stationary ceilometer data provide spatial and temporal characteristics of Shanghai's boundary layer height. The consistency between when the ceilometer is moving and stationary highlights the potential of mobile observations of transects across cities. An analysis of 16 months of z(i) measured at the Fengxian site in Shanghai reveals that the diurnal variation of z(i) in the four seasons follows the expected pattern; for all seasons z(i) starts to increase at sunrise, reflecting the influence of solar radiation. However, the boundary layer height is generally higher in autumn and winter than in summer and spring (mean hourly averaged z(i) for days with low cloud fraction at 1100-1200 local time are 900, 654, 934, and 768 m for spring, summer, autumn, and winter, respectively). This is attributed to seasonal differences in the dominant meteorological conditions, including the effects of a sea breeze at the near-coastal Fengxian site. Given the success of the retrieval method, other ceilometers installed across Shanghai are now being analyzed to understand more about the spatial dynamics of z(i) and to investigate in more detail the effects of prevailing mesoscale circulations and their seasonal dynamics.