Geoscience Reference
In-Depth Information
Recently Krishnamurthy et al. (2011) reported Doppler lidar measurements of turbulent
kinetic energy dissipation rate, integral length scale and velocity variance assuming a
theoretical model of isotropic wind fields during the T-REX Project. Corrections to address
the complications inherent in volumetric averaging of radial velocity over each range gate,
noise in the data and the assumption made regarding the effects of smaller scales of motion
were considered and tested. Comparisons between the lidar and tower measurements
supported the soundness of the lidar measurements of boundary layer turbulence.
7. Doppler radar and lidar measurements of boundary layer phenomena
7.1 Storm outflows
One of the most difficult forecasting problems remains the identification of when and where
convective cells develop. Two facets of this problem are (a) how first-generation convective
cells are triggered in an environment which has been previously quiescent for a period, but
which becomes more and more unstable; and (b) for convective cells that persist, how and
where subsequent generations of cells are triggered by the propagation of cold outflows
from existing cells. Over the years there have been many studies addressing these issues
using radar data (see for example Bader et al., 1995). Indeed, it is well known that first
generation convective cells may be triggered by orographic uplift, and by land surface
heterogeneity caused by variations in the temperature and moisture fields.
Collier and Davies (2004) describe a study of the pre-storm environment for a case study using
a Doppler lidar located at Northolt, North West London, a C-band weather radar sited at
Chenies north of London and the S-band Chilbolton radar. It was noted that the Doppler lidar
and the weather radar data complement each other. Figure 6 shows Chenies and Chilbolton
radar images, a PPI from the Doppler lidar and a LDR PPI from the Chilbolton radar. An
outflow boundary is evident in all the images. A reversal of the wind direction at low levels is
shown near the lidar site. The Chenies radar shows a thin line of broken echoes about 12 km to
the north and north west of the main area of convective rain. The Chilbolton radar Linear
Depolarisation Ratio (LDR) suggests that the radar targets in the outflow region are probably
not raindrops, but may be particulate matter (straw, dust). The Doppler radial velocities
observed by Chilbolton are consistent with the lidar measurements in the figure.
Similar measurements of an outflow have been reported by Collier et al. (2008). This study
illustrates the difficulty of measuring an outflow using a radar, in this case the DLR C-band
radar. In Figure 7a an outflow from a thunderstorm over the Rhine Valley is partially
observed by the radar, but the details are not clear as there is some confusion with ground
clutter. Figure 7b shows Doppler lidar measurements of the vertical velocities made from
Achern in the Rhine Valley. Here the outflow is clear. It is about 800 m deep, and a cap
cloud is observed near the leading edge. The peak kinetic energy dissipation rate was
calculated to be 0.18 m
2
sec
-3
.
7.2 Observing smoke plumes
Combined observations of smoke plumes using lidar and radar have not been extensively
reported. Such plumes may be generated from wild fires, or from prescribed (planned) burns.
The plumes may contain lofted debri as the primary source of targets, although smoke and
condensed water droplets may also be evident. Banta et al. (1992) used Doppler radar and
