The Pennines Experiment 20042005

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The Pennines Experiment 2004/2005
Calibration and Preliminary Results
University of Leeds
Volker
Horlacher, Stephen Mobbs
Met Office Phil Hignett, Simon
Vosper, Gabriel Rooney, Peter Sheridan, Richard
Crocker, Howard Wilson, Dave Bamber
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• Talk Outline
• Motivation of the experiment
• Previous Studies
• Experiment
• Pressure calibration and related problems
• IOP on the 18th of May
• Pressure perturbation calculation
• Summary

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• Motivation
• Occurrence of severe weather in hills and
  mountains (e.g. strong downslope winds, gravity
  wave breaking, hydraulic jumps and rotors due to
  trapped lee waves)
• Results damaging effects on the landscape in
  mountain region, severe turbulence events (e.g.
  hazard for aircrafts).
• The resolution of numerical weather prediction
  models is still not fine enough to resolve such
  small scale details
• The knowledge of the local flow behaviour is
  therefore crucial to predict such occurrences.
• Research into this problem has increased in the
  last ten years even though a few studies had
  been conducted in the 40 and 50s (e.g. Sierra
  Wave Project (1957)).
• Pennines experiment contribution to gain a
  better understanding of the flow behaviour with
  the long term aim to provide a now cast tool for
  the aviation.
• In this sense the Pennines project pursuits the
  same aim as the Falkland Island project in
  2000/2001.

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• Background
• Mechanism of gravity wave activity and
  non-dimensional flow parameters

• FL U/NL for FL lt 1 gravity waves for FL gt 1
  waves decay vertically
• FH U/NH controls non-linear processes (FH lt 1)

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• Previous Findings
• Rotors under crest of lee waves measure 1-2 km.
  Therefore it requires horizontal grid spacing of
  a few hundred metres which is computational
  expensive.
• Large wave amplitudes give concern for flow
  separation and rotors. Surface friction and plays
  an important roll as shown by Doyle and Durran
  (2002).
• Sharp temperature inversion upwind can also play
  an significant role in the formation of rotors
  (Vosper 2004).
• Surface measurements from AWS on the Falkland
  Island suggest that strong downwind flow
  acceleration is related to the existence of rotor
  motion aloft (Mobbs et al. 2004).
• The measurements also show the presence of a
  strong low-level temperature inversions (gt 5 K)
  most likely related to the difference between air
  and sea-surface temperature.

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H/zi hill height/ inversion height
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The Experiment Setup
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Instrumentation

• Duration of the experiment from November 2003 to
  April 2005
• 16 (17) Automatic Weather Stations (AWS)
• Measuring absolute pressure, temperature,
  humidity, wind speed, wind direction and the GPS
  time.
• Sample rate is 3 sec. Data is stored on flash
  medium of 32 MB which needs changing every 85
  days.
• Radiosondes twice a day from Lancaster and every
  hour during IOPs (Intensive Observation Period)
  mainly at Leeming air base.
• Two day forecast runs with 3dVOM. The output can
  be viewed at http//www.env.leeds.ac.uk/pennines/
  and http//www.convection.info/3dvom/
• The Met Office operates four 30 m mast at
  Leeming, Dishforth, Linton and Lancaster
• Measuring temperature, humidity, wind speed and
  wind direction.

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Calibration of instruments
Pressure calibration

• Pressure is temperature dependent
• Analogue Panalogue a0 a1 x bits a2 x bits
  x T a3 x bits x T2 a4 x T
• Digital Pdigital d0 d1 x p d2 x T
  d3 x T2

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• Method
• Least squares fit (varying pressure and
  temperature in order to fill up matrix).
• Ideal Climate chamber which regulates the
  pressure and the temperature
• First calibration in January 2003 Haverah Park
  (Storage space from the department.
• Second calibration in April 2004 climate chamber
  in the Physics Department

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Analogue Panal a0 a1 x bits a2 x bits x T
a3 x bits x T2 a4 x T grad temp
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Pressure Perturbation Calculation
P(i,t) Poff(i) Psyn(t) P'(i,t)
P(i,t) absolute pressure Poff(i) hydrostatic
component (caused by the heights of the
instruments above see level) P'(i,t) pressure
perturbation Psyn(t) synoptic pressure i
number of microbarographs t time
step (10min averages)
For wind speeds less 2 ms-1 (0.02 hPa)
Psyn(t)
Minimize P'2 with a least squares fit method to
gain Poff
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IOP 18th of May 2004
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Surface Measurement
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• Summary
• The pennines experiment provides another data set
  to develop techniques to predict rotor events on
  a routinely basis. In this sense the Falkland
  Island project and the Pennines project share the
  same goal.
• Vosper (2004) and Mobbs et al. (2004) have
  determined none-dimensional parameters in order
  to characterise the flow behaviour during rotor
  events. The concepts will be applied to the
  Pennines data.
• The calibration of the pressure sensor revealed
  the significance of the temperature dependency.
• The radiosonde profiles show some gravity waves
  which were most likely not strong enough to
  create any rotor events.

• Future work
• Maintaining instruments until the end of the
  experiment
• Validating and analysing the data.
• Running 3dVOM wave phase wrong

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• Doyle, J. D. and Durran, D. R. (2002). The
  dynamics of mountain-waves-induced rotor. J.
  Atmos. Sci., 59, 186-201.
• Vosper, S. B. (2004). Inversion effects on
  mountain lee waves. Q.J.R. Meteorol. Soc,
• Mobbs, S. D., Vosper, S. B., Sheridan, P.F.,
  Cardoso, R., Burton, R. R., Arnold, S. J., Hill,
  M. K., Horlacher, V., and Gadian, A. M. (2004).
  Observation of downslope winds and rotors in the
  Falkland Island. Q. J. R. Meteorol. Soc, in press.