Abstract
We investigate the time evolution of the Local Boundary Layer (LBL) for the
first time over a mountain ridge at Nainital (79.5° E,
29.4° N, 1958 m a.m.s.l.) in the central Himalayan region, using a
radar wind profiler (RWP) during November 2011 to March 2012, as a part of
the Ganges Valley Aerosol Experiment (GVAX). We restrict our analysis to
clear–sunny days, resulting in a total of 78 days of observations. The
standard criterion of the peak in the signal-to-noise ratio
(S ∕ N) profile was found to be inadequate in the
characterization of mixed layer (ML) top at this site. Therefore, we
implemented a criterion of S ∕ N > 6 dB for the
characterization of the ML and the resulting estimations are shown to be in
agreement with radiosonde measurements over this site. The daytime average
(05:00–10:00 UTC) observed boundary layer height ranges from
440 ± 197 m in November (late autumn) to 766 ± 317 m above
ground level (a.g.l.) in March (early spring). The observations revealed a
pronounced impact of mountain topography on the LBL dynamics during March,
when strong winds (> 5.6 m s−1) lead to LBL heights of
650 m during nighttime. The measurements are further utilized to evaluate
simulations from the Weather Research and Forecasting (WRF) model. WRF
simulations captured the day-to-day variations up to an extent (r2 = 0.5), as well as the mean diurnal variations (within
1σ variability). The
mean biases in the daytime average LBL height vary from −7 % (January)
to +30 % (February) between model and observations, except during March
(+76 %). Sensitivity simulations using a mixed layer model (MXL/MESSy)
indicated that the springtime overestimation of LBL would lead to a minor
uncertainty in simulated surface ozone concentrations. However, it would lead
to a significant overestimation of the dilution of black carbon aerosols at
this site. Our work fills a gap in observations of local boundary layer over
this complex terrain in the Himalayas, and highlights the need for year-long
simultaneous measurements of boundary layer dynamics and air quality to
better understand the role of lower tropospheric dynamics in pollution
transport.
Citation
ID:
165109
Ref Key:
singh2016atmosphericboundary