Lidar data from REPARTEE II campaign

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Lidar data analysis from REPARTEE II campaign
Janet Barlow and Tyrone Dunbar, University of Reading
With thanks to Fay Davies, University of Salford and
Ewan O’Connor, University of Reading
Data courtesy of the Universities’ Facilities for Atmospheric Measurement (UFAM)
1. Introduction
This document contains a description of the methodology used to analyse the Doppler lidar
data taken during the second REPARTEE campaign, October to November 2007. Results are
given in the files <name>.xls (mixing layer heights), and Quicklook time series in
Lidar_images_from_REPARTEEII_campaign.doc. These files can be obtained by emailing
either Tyrone Dunbar ([email protected]) or Janet Barlow
([email protected]), and will also be available for download at
http://www.met.reading.ac.uk/Research/boundary_layer/people/tyrone.html .
2. Lidar Technical details
The lidar instrument used on the REPARTEE II campaign was a Halo Photonics (http://halophotonics.com) 1.5 micron scanning Doppler lidar which was provided by UFAM and
operated by the University of Salford. It was installed in the car park of the University of
Westminster building on the Marylebone Road and was running continuously for three weeks
between the 24th of October and the 14th November 2007. Due to the proximity and height of
the neighbouring buildings, the lidar was restricted to making only vertical stare
measurements i.e. directly upwards.
The lidar operates by transmitting laser light into the atmosphere and then measuring any
energy reflected back to it. The amount of energy reflected back depends upon the size and
amount of particles present in the atmosphere. Energy is reflected primarily by particles
within an order of magnitude of the lidar wavelength, and as such the lidar generally detects
particles such as aerosols and ice crystals within the atmosphere. The lidar splits the received
data time series into sections called ‘gates’ which correspond to the height from which the
backscatter data is received, which determines the vertical resolution of the data. In this case
the lidar has a vertical resolution of 30 metres, and has 66 gates measuring up to 2 km into the
atmosphere. Each received signal is relatively weak, and so the lidar accumulates the returns
from several pulses of energy to form a stronger signal. This lidar returns a measurement
every ~4 secs consisting of 20,000 ‘integrations’, or accumulated signals. The beam can be
thought of as a column of about 5 mm in diameter. Over each gate the beam therefore has a
volume of 0.0005 m3.
The lidar also has Doppler capability, and so can measure the velocity of particles parallel to
the direction of the lidar beam. It achieves this by measuring the frequency shift in the
returned signal. This frequency shift is proportional to the velocity of the particles detected by
the lidar beam, which are small enough to be assumed as passive tracers for the wind. If
pointing vertically, then the instrument gives a direct measure of rapid variations in vertical
velocity, from which we can deduce boundary layer structure and the strength of turbulent
mixing. Vertical profiles of backscatter and turbulence are determined by whichever surface
is upstream of the lidar. In order to understand the data we must therefore be aware of the
surrounding geography and the wind direction, in particular Regent’s Park directly to the
North and Hyde Park to the South-West. Should the wind flow over these parks, complicated
internal boundary layers can form (especially at night) and there may be a change in the
source distribution of aerosols.
3. Interpretation of data
As the backscatter is a measure of both the size and amount of particles that reflect the energy
transmitted by the lidar, we can only use this as an indicator of the aerosol present in the
atmosphere. It is important to bear in mind that the lidar is not calibrated to differentiate
between, for example, a dense cloud of small particles and a less dense cloud of larger
particles. For this reason, we must use our knowledge of the boundary layer to make some
assumptions about the nature of the information displayed by the backscatter data.
In the backscatter quicklook images generated from the REPARTEE lidar data, a very simple
generalisation of the colour scale could be thus: blue/white indicates relatively clear air that
has perhaps been mixed down from above the boundary layer, or has had particulates
removed through precipitation; green/yellow areas indicate higher number concentrations
and/or increased size of aerosol particles, perhaps typical of polluted air in the centre of
London; these colours can indicate precipitation if seen in streak-like formations below
clouds. Red areas almost exclusively indicate cloud. Note that these classifications are meant
only to be a qualitative guide for interpreting the images.
As mentioned previously, the lidar can measure the velocity of particles parallel to the beam
path, and these are assumed to act as tracers for the wind. As this lidar was restricted to
vertically pointing measurements, we can use the Doppler measurements of the vertical wind
velocity, w, to give us a measure of the convective turbulence throughout the layer, and so
indicate where mixing is taking place.
In the colour scheme used for the REPARTEE lidar data, green indicates relatively quiescent
air with little or no vertical motion. Yellow and red pixels generally indicate convective
updrafts extending throughout the depth of the boundary layer that are generated by surface
heating, although they can also indicate turbulence generated by the roughness of the surface
below. Blue pixels indicate downdrafts, usually associated with the updraft activity before or
after, but can be associated with mechanically driven turbulence due to the underlying urban
surface, or falling precipitation. Clouds also cause significant areas of up- and down-drafts
beneath their bases, due to cooler air descending and causing turbulence.
If we calculate the variance of the vertical wind, σw2, this can also be used as an indicator of
turbulence, and therefore mixing. Remembering that as the lidar averages its measurements
over a volume, it is possible for the average velocity to be zero, but a non-zero variance will
indicate that there is still vertical transport occurring. In this colour scheme, light blue
indicates little turbulence is present, while darker blue through green then yellow and red
indicates increasing amounts of turbulence.
A further property of the boundary layer which we have derived is the turbulent kinetic
energy dissipation rate, ε. This is the rate at which turbulent kinetic energy is converted into
internal energy (i.e. heat) by viscous stress at small scales, and is balanced by the production
of turbulence by shear and convective processes. As such, ε is a good proxy for the presence
of turbulence, and in contrast to the vertical velocity variance can be used to infer the
presence of shear (wind-driven) turbulence as well as that of buoyancy-driven convection.
Quicklooks of these variables for all the REPARTEE II campaign days are provided in the
powerpoint presentation: Lidar_images_from_REPARTEE_II_Campaign.ppt
4. Boundary layer height measurements
One of the main advantages of a high resolution remote sensing instrument such as the lidar is
that it can provide us with height measurements of different layers within the boundary layer.
However, it is important to consider what these different heights represent, and how they are
measured. Within the attached data file, REP_boundary_layer_heights.xls, there are four
different height measurements that we have derived from the lidar measured properties
described above:
Aerosol layer top - this is calculated using the gradient in the backscatter measurement, and is
detected as the first large gradient in backscatter we see going up from the surface. As the
backscatter is a good proxy measurement for the concentration of aerosol, this essentially
shows us the depth of the aerosol layer adjacent to the ground. This is most useful at night
when the lack of convection and increased humidity allow denser layers of aerosol to develop,
associated with the nocturnal stable layer.
Boundary layer top - again calculated from the gradient in the backscatter, but this time
calculated from the top of the lidar measurements downwards. This basically finds the limit to
which the lidar measures backscatter, which again we assume is equivalent to aerosol. On a
clear (i.e. non-cloudy) day this will show the top of the 'entire' boundary layer i.e. somewhere
in the entrainment zone between the cleaner troposphere and the more polluted boundary
layer below. If clouds are present then this height should be ignored; clouds rapidly attenuate
the lidar signal and this height will be roughly a few tens of metres above the cloud base,
depending on how thick the cloud is.
Convective mixing layer height - this is calculated using a threshold on the value of the
vertical velocity variance. As mentioned above, the variance gives us a measure of the
intensity of the vertical turbulence occurring, which is dominated by convection during the
day. To calculate the height we look at profiles of the variance from the ground up; if the
value of the variance is above our threshold (in this case > 0.1 m2 s-2), then we assume that
convection is mixing air from near the surface up to this height. The point at which the
variance drops below the threshold we assume to be the top of this mixing layer.
Turbulent mixing layer height - this height is calculated using a threshold value of the
turbulent kinetic energy (tke) dissipation rate, ε. As mentioned above, ε is a proxy for the
presence of turbulence; if tke is dissipating, then this must be balanced by production of tke.
In a similar way to the convective mixing layer height method, we assume that if ε in the
layer is above a certain threshold (in this case > 1.0 10-4 m2 s-1), then turbulence is mixing air
vertically to this point. The difference is that ε will show more clearly the presence of shear
mixing as well as convective mixing, for example, during the night when little/no convection
is present yet there is significant horizontal wind to cause mixing.
Clearly, care should be taken in interpreting the calculated heights, e.g. when the lidar signal
is attenuated in a thick cloud layer thus limiting the height range of observation.
5. Case studies
Two days were chosen as case studies in order to give an example of what we can see in the
lidar data. Measurements from above the boundary layer top have been removed as they were
not of interest in this study, and also to emphasise the boundary layer depth. Additionally, the
bottom three measurements taken by the lidar had to be removed due to a corruption of the
data. The measurements therefore start at ~100 m. It should also be noted that the various
height measurements were derived using 30 minute averaged data, whereas the data shown in
the figures is averaged over one minute. This averaging can cause the derived heights to
sometimes appear to be above the level of the data. The height of the BT Tower is indicated
on the plots by the dashed black line at 190 m.
6th November 2007 – convective day with few clouds present
The 6th of November was a relatively calm day with low winds and quite clear skies. Looking
at the backscatter first, the plot shows a full day, from midnight on the 5th through to midnight
on the 6th, from the surface up to 2000 metres. During the night, from 00:00 hours to about
10:00 hours, the backscatter shows two distinct layers; the green, stable surface layer which
has allowed aerosol to build up within it, and the residual layer above which is the remnants
of the mixed layer from the previous day which has less aerosol present. The top of this
aerosol layer has been picked out and marked by the downward pointing triangles. At about
10:00 hours, convection due to surface heating begins to mix aerosol from the green stable
layer upwards. During the day, the backscatter within the layer is relatively smaller and
uniform with height, indicating that the aerosol is well-mixed. A few cumulus clouds pass
over the lidar in the early afternoon, appearing as red patches. At about 17:00, convective
activity ceases, and a nocturnal stable layer forms again, trapping aerosol near the surface.
Again this is picked out by the downward pointing triangles.
The vertical velocity plots shows much the same story, with convective updrafts and
downdrafts clearly appearing from 10:00 onwards, and continuing through the day before
dying down in the late afternoon.
The velocity variance plot shows the strong convection during the day again, but also gives us
an indication of the intensity of the turbulence. This is shown by the convective mixing layer
height plotted in black. There is some vertical mixing apparent between midnight and sunrise,
which isn't obvious in the velocity plot.
The tke dissipation rate plot shows a similar story to the previous plots, with the convective
daytime layer picked out in green by the turbulent mixing layer height. Looking at the period
between midnight and sunrise again, there is a distinct layer of turbulence, and this is
probably due to strong winds during the night.
28th October 2007 – overcast day with constant cloud cover
In contrast the 28th of October was overcast and windier, and a frontal system passed over
later in the day. The backscatter plot shows the particularly low cloud-base in red, below this
it is possible that some of the regions of yellow backscatter are rain-showers. At 20:00 the
frontal system passes through.
The vertical velocity, variance and tke dissipation rate plots show that there is little
convective turbulent activity during the day, although the variance perhaps shows some
evidence of heating during the morning and early afternoon. However, both the variance and
tke dissipation rate clearly show relatively high and constant levels of turbulent activity
throughout the entire day. This is due to the shear generated by the wind, which is causing
significant mixing. All methods of deriving the height then essentially pick out the top of this
shear mixing layer, although this is limited by the fact that the lidar can only see a short
distance into the cloud layer.
UPDATE 2/9/09
We have refined the four different height calculating algorithms and have also
improved the processing of the lidar data. In particular, we are rejecting any data that
falls below a threshold in the signal to noise ratio as unreliable. This has mainly has
an effect on measurements at the point where the boundary layer meets the free
troposphere, as the signal becomes much weaker here.
Additionally, we have been comparing the variance measured by the lidar to that
measured by the lidar on the BT Tower, and have found a clear bias towards the sonic
measured variance during clear, cloud-free nights; in some cases the lidar measured
variance can be up to a third less than that measured on the BT Tower. This should
not make a large difference to the heights measured by the lidar on most days,
however, this problem is most noticeable on the 6th of November, and so the heights
measured on this day (particularly between 0300 to 0900) should be treated with
caution.
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