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Validation of Propagation & Mesoscale weather models in the littoral environment: A report of the Streaky Bay Experiment Dr A.S. Kulessa Radio Frequency Technology Group
Aims of the experiment • The measurement of surface & elevated duct structure in coastal environments • The modelling of sea breeze formation and resulting refractive index structure using mesoscale weather models • The validation of mesoscale weather models • The validation of PEM / hybrid propagation codes for shipboard and airborne radar / ESM ops.
Collaborating organisations • DSTO (EWRD) • DSTO (ISRD) • AFRL – O. Cote • Flinders University • Airborne Research Australia • CW Labs • The experiment was largely funded by the RF Hub and also by the AFRL.
Atmospheric mechanisms that either degrade or enhance radar/radio transmissions
Microwave propagation through a coastal maritime atmosphere • Streaky Bay RF propagation Experiment • The experiment featured: • Land based Emitter near Mt Westall • Airborne receiver flying between Mt Westall and Franklin Islands • Airborne Measurements of meteorological parameters – refractive index • Some mesoscale numerical weather prediction modelling
Mechanism for duct formation due to a sea breeze circulation • Propagation speed of sea breeze front depends on large scale forcing • Sea breeze extension depends on large off shore wind component • Onset of sea breeze depends on solar radiation available • Development of sea breeze depends also on the prevailing winds. • Duct formation; high level elevated, lower level, stronger elevated duct, surface duct. • Evaporation duct gets stronger as wind speed increases in the surface layer. • Complex refractivity structure ranging from sub – refractive over land to elevated ducts, surface ducts and “nested” ducts over the sea.
Ground Station Transmission parameters:fre10GHzpulse width 1ms, prf 1kHz, pol H Transmitter: pulse generator, TWT amp, horn antenna Power supplied by a generator Airborne Platform: GROB G109B Superheterodyne receiver: frequency range 2-18GHz, + horn antenna. Receiver tuned to 10GHz, measures signal pulse width and time between pulses and pulse amplitudes. Mounted in equipment pod located under the starboard wing of the GROB G109B RF Equipment
Aircraft flight profile • Flights occurred between Mt Westall and the Franklin Islands • Flight pattern: Sawtooth with two ascents and two descents • Maximum height 750 metres • Minimum height 20 metres
Case 1: Propagation through a maritime surface duct. (Advection duct)
Comparison of measured refractivity profile with modelled refractivity profile • Blue curve: measured profile • Red curve: modelled profile • Model specifics: • Non-hydrostatic mesoscale • Area 120x120 km • Grid size 2x2 km • Variable vertical resolution • 10 metres at the bottom – • 100’s metres at the top • 24 layers • Initial geostrophic wind field • Initial radiosonde ascent from nearby Ceduna • Other initial inputs: soil type, land surface temp., sea • surface temp, soil moisture content
Evaporation duct estimation • No direct measurement available for this experiment • Sea surface temperature + wind speed measurements were made to infer an evaporation duct model from Evaporation duct statistics collected during past experimental campaigns.
Coverage diagrams and propagation predictions • Tx is positioned at a height of 50m • Extended propagation is evident due to the strong surface duct (Note duct height ~ 120 – 140 metres) • Signal in the duct 10dB – 20dB stronger at distances >40km
Case 2: Propagation through super-refractive layers • Total flying time : 2.95 hours • Boat located midway between Mt Westall and the Franklin Islands • HNK at minimal altitude at Franklin Islands and at the boat. • One ascent and one descent between Mt Westall and the boat. • One ascent and one descent between the boat and Mt Franklin per leg. • 10 legs between the boat and Mt Franklin • Maximum height of HNK: 650 metres • Minimum height : 20 metres
Temperature & humidity variations • Evidence for a weak temperature inversion in the second half of the run and also a dew point temperature inversion. • The wind was directed from the sea during the run. • Is it enough to produce ducting conditions ? • - super-refractive conditions ?
Some conclusions • We were able to successfully establish a ground to air radio link. • We were able to measure surface duct formation due to advection off the South Australian coast • The measurements look good when compared to surface duct structure predicted from the FOOT3D mesoscale model • Comparison between TERPEM and measured signal levels looks good as well. • We measured a super-refractive atmosphere and also the corresponding propagation effects.
Future work • More experiments in littoral environments in order to make validation more conclusive. • - require a longer term experiment (or more short experiments) that capture different weather patterns and hence different atmospheric dynamics in the same area • - require experiments in other areas (different sea surface conditions and different land types. • Consider many realisations of mesoscale models in order to determine sensitive input parameters. • Application of Mesoscale models near the equator (i.e. tropical regions) • Validation with other data sets. • Check the modelling against data taken overseas, e.g. NZ, USA