UTD preliminary data analysis

The UTD team proposes to work on the evaluation of the scanning lidar accuracy for different configurations and atmospheric conditions. This analysis is carried out by comparing the 200S radial velocity with the sonic data acquired by the met-tower and the profiling lidars. The sonic and profiling lidar data are projected over the 200S line-of-sight in order to produce an equivalent radial velocity.

For this analysis an "effective" range gate is evaluated, i.e. the projection of the range gate over the horizontal plane and mean wind direction. This parameter is considered to be the sensitive measurement volume of the lidar. The vertical extent of the lidar range gate is also evaluated, which is a parameter to estimate the effect of wind shear on the radial velocity.

The error analysis will be performed by considering different stability regimes, turbulence intensity, wind shear and veer.

After this error analysis for the radial velocity of single lidars, the retrieval of 3D velocity components from 3 intersecting and synchronous lidars will be carried out. For this analysis a crucial point is the definition of the effective measurement volume determined by the intersection of the 3 range gates.

For the proposed analysis the following experiments will be investigated:

• Dual/Triple Doppler Stares - Sonic + Lidar Supersite 3/21/2015-3/24/2015
• Dual/Triple Doppler Stares - Sonic + Lidar Supersite + Radar Site 3/24/2015-4/1/2015

Tower power issue solved (we hope). Thanks to some excellent weather and Bruce's sleuthing we feel we have the breaker issue solved. There was a worn wire inside one of the electrical boxes at ~125m that was shorting out. Tower vibrations and temperature swings over 40 years. It's amazing there aren't more problems"  (knock on galvanized steel").

BAO Tower Update 2015-05-27
200 and 300m T/RH not working. Starting Sat May 23 1900 UTC
Probably will not be replaced.
Lost NCAR and some standard BAO tower data over weekend. Back up  Tuse May 26.

Tower data outage

The breaker that provides power for the tower instruments and the network link to Boulder tripped some time about 2015-05-25 00:12z.
Bruce has reset the breaker and things seem to be coming back to life.
The outage lasted about 42 hours.

(From email from Bruce Bartram) 

Weibull distributions from the tower

As part of Paul's explorations of the differences between the NW and SE sonics and the profiling lidars, he has prepared wind speed distributions for the 50m, 100m, and 150m levels. All levels fit the Weibull distribution very well. The Weibull parameters are summarized in a table below.

The differences between the NW and SE booms are very subtle.

Tower wakes and accelerations

We can easily understand that a meteorological tower will affect the measurements taken within the wake of the tower. Here at the BAO, we are suspicious of measurements on the SE boom when the winds are from the NW, and similarly, we look for wakes in the NW boom data when the winds are from the SE. However, we should also remember that flow around a tower like the BAO can also generate accelerations for certain wind directions as well around the tower as well. The IEC standard 61400-12-1 Annex G specifies boom lengths (as a function of tower dimension and tower solidity) to ensure that measurements are taken outside of the zones of these wakes and accelerations.

CFD simulations (caveat: neutral stability) for flows around towers like the BAO can illustrate these issues. An example is shown below, from Stickland et al. 2012 (EWEA), for simulations of flow around the FINO3 tower (*not* the BAO).

As flow approaches from the left, the flow decelerates in an induction zone (cool colors are flow less than 100% inflow speed), but also accelerates (warm colors) around the sides of the tower.

As a reminder, here is what our tower looks like. Last week, I checked with Dan Wolfe regarding the tower solidity (as the degree of acceleration and deceleration depends on tower solidity), and such measurements have not yet been done for the BAO.

 
But what do our data show? Rob has pointed out below that the ratios of the sonics on the two booms show some evidence of both wakes and accelerations. Paul Quelet at CU has also looked at this issue, comparing the sonics to each other and to the profiling windcube (WC) lidars at the lidar supersite 135 m south of the tower. Because of the lidar comparison, Paul looks at 2-min averages of the tower sonic data at 50 m, 100 m, and 150 m:

It's interesting that the NW boom, on average, is always faster than the SE boom data, even when the NW boom should be waked. When the SE boom is waked (wind directions ~ 330), the differences are large as would be expected.

Then, when comparing the tower sonics to the lidars south of the tower, some other interesting differences emerge. Let's look at the NW boom data first:

We're comparing the WC to the NW boom sonic wind speeds. When the flow is from the south-east, the NW boom is clearly in the wake and the lidar speeds are much higher. Conversely, when the winds are from the NW, the WC measures low...but at 130 m away to the south, the WC should not be sampling tower wake. So when we see the WC measuring "low" at wind directions ~ 320, are we actually seeing the sonics measuring accelerations around the tower? Or is the tower wake propagating further downwind than we would expect?

The picture from the SE boom is different. First, the data are noisier, with lower wind speeds generally introducing more variability into these 2-min averages.
  

Second, the really puzzling thing is that the WC is still seeing higher winds than the sonic when the winds are from the south-east. The sonic isn't waked from this direction - why would the sonic wind speeds be low (or the WC winds be high)? Further, the wake effect we would expect to see when the winds are from the northwest isn't distinct.

Paul is continuing to work on this analysis, using higher time resolution data and stratifying these effects by atmospheric stability.

A major caveat to this quick analysis is that the sonic data, as yet, have not been corrected for any tilt in the booms. A systematic tilt issue could modify these results significantly. 

Thanks, Paul, for all your work on this so far!


 

As I recall, Julie had previously pointed out a few instances of tower wake effects as seen in the differences between the SE and NW sonic pairs. As I started digging into the sonic data it became apparent that there were many cases where there were huge differences in wind speeds between the SE and NW sonics.
The plot above shows the mean relative difference between the SE and NW sonic wind speeds as a function of the SE sonic wind direction at the 250 m level. The other levels look similar. The plot was obtained by averaging 1-sec sonic data from March 6 through May 16. In this case the max occurs when the NW sonic is in the tower wake, and the minimum occurs when the SE sonic is in the tower wake. As you can see the differences can be huge.
What I find really interesting is the fact that the difference changes sign as you move away from the wake zones. One interpretation might be that the wind speed is slightly higher than the free stream in a region just outside of the wake zone. Is that possible? Has such a thing been observed or modeled?

Platform operations

Click here for summary of motion platform operations. Updated daily.

Nighttime motion platform operations at BAO Area 51.

BAO Tower instrumentation 2015-05-14
Due to a tripped breaker some of the tower sonic and T/RH data maybe be missing.
~ Times are 1537-1730 UTC

Link to tower data viewer has changed

As we discussed in the call Wednesday morning, the NCAR data browser seemed to be down. It has just moved to http://datavis.eol.ucar.edu/ncharts/projects/CABL/qc_geo_notiltcor. I've corrected the link at right.