Operational Oceanography: Modeling EM Propagation Characteristics

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Operational OceanographyModeling EM Propagation
Characteristics

• LT Erin OMarr
• 7 Sept 2007

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Purpose of the Study

• Compare and contrast upper air sounding TDA
  inputs for the purpose of modeling EM propagation
  characteristics
• In situ
• Model
• Identification of trapping layers and ducting
  layers affecting shipboard sensor propagation
  characteristics
• Surface Search Radars

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Operational Significance

• As a Navy METOC Officer, we will be assigned
  to/support various deployable assets
• Critical decisions about variations in
  capabilities of platform sensors, as well as,
  counter-detection
• Fire Control Systems
• Surface Search Radars
• Communications
• OPTEMPO in the littoral battlespace
• Coastal region is becoming evermore important
• Criticality of ship-to-ship and ship-to-shore
  radar and communications
• The environment effects system performance
• Proper characterization of the environment

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Advanced Refractive Effects Prediction System
(AREPS)

• Tactical Decision Aids (TDA)
• Qualitative assessment of system performance
  given existing environmental conditions
• AREPS is most widely used TDA for the prediction
  of radar ranges and signal propagation
  characteristics
• Advanced Propagation Model (APM)
• Extreme sensitivity to model inputs such as SST
  and RH
• Quality input quality output
• Critical area of research
• Obtain an understanding of typical duct height
  values
• Understand procedures for accurately estimating
  duct heights from routine measurements

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Review Refraction Categories
Figure and Table courtesy of Davidson,
Assessment of Atmospheric Factors in EM/EO
Propagation, pp 3-18 .
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Review Types of Ducting
Figure and Table courtesy of Davidson,
Assessment of Atmospheric Factors in EM/EO
Propagation, pp 3-21.
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Evaporation Duct

• The ED is one of an operators primary concerns
  over water
• Can significantly enhance the range and strength
  of signal propagation
• Radar
• communications
• The rapid, vertical decrease in relative
  humidity, at the surface, results in a
  simultaneous rapid decrease in the refractive
  index.
• Primarily concerned with water vapor content
  however, RH is readily measured and
  representative of the amount of water vapor
  (pressure/specific humidity) present and
  coincident gradient
• The gradient of the refractive index causes
  significant bending of the ray geometry
• The EDH fluctuates throughout the day and is
  highly dependent of Tair, Tsea, q, and the wind
  component(s)
• Local mixing above the sea surface
• Surface-based duct with typical depths of 2-30m
• EDH 10m is significant for surface radars with
  frequencies above 5GHz
• Ducts 30m are significant for almost all radar
  frequencies

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Data and Methodology

• First leg of the Operational Oceanography cruise
• Standard meteorological observations
• Rawinsondes were launched to collect upper air
  soundings
• Regional AF MM5 vertical profile forecasts
• 15km horizontal resolution, 25mb vertical
  resolution, and 3-hr time step.
• Vertical sounding data was extracted at or as
  close as possible to the soundings times and on
  the precise location of the in situ sounding
  launch point.
• Calculated Evaporation Duct Height (EDH) and it
  added to environmental profile
• Paulus/Jeske (P/J) model
• In situ SST
• Organic SWS, SWD, pressure, and Tair
• AREPS standard project
• Platform R/V Point Sur with standard 10GHz
  (X-band) radar at 15ft height.
• Target R/V Cypress Sea (small/medium sized
  vessel) with a regular ESM receiver

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Rawinsonde
MM5
Ship rawinsonde 18191313ZJUL07
Evidence of a shallow ED (17.98ft thick) present
at time of ship sounding. MM5 does not reflect
the ED however, does forecast the presence of a
subrefraction layer once SST is added (31.53ft
thick).
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Ship rawinsonde 18191313ZJUL07
Range-height cross section of probability of
detection (Pd) using a near surface radar against
a small/medium-sized vessel target. Pd is
indicated by the color scale on the bottom.
Extended near-surface ranges due to ED.
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Rawinsonde
MM5
Ship rawinsonde 20114219ZJUL07
Evidence of an ED layer at surface (30.2ft) and
various elevated ducts on the in situ sounding.
MM5 forecasts a deeper ED (58.12ft) however, no
elevated ducts.
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Extended near-surface ranges due to ED.
ED almost 2x as thick as 18JUL
Ship rawinsonde 20114219ZJUL07
Range-height cross section of probability of
detection (Pd) using a near surface radar against
a small/medium-sized vessel target. Pd is
indicated by the color scale on the bottom.
Extended near-surface ranges due to ED.
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Model has little to no skill in predicting ED
when observed SST is used for extrapolating the
ED. No further statistics were looked at.
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Quality of input into TDA- APM is very sensitive
to SST and RH. Graphic shows in situ/model RH
differences. My hypothesis Observed SST (not
shown) created a discontinuous profile and
erroneous model predictions of propagation
characteristics.
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Discussion

• Value of observed surface variables added to
  rawinsonde/model
• Knowledge of near surface atmosphere
  significantly aids the modeling of the existence,
  depth, and intensity of ducts
• Need either both measured or both modeled to
  yield proper coupling between the SST and Air T
• If adding obs SST to model sounding, must pay
  attention to the temperatures measured
• If the SST is warmer than the lowest levels, the
  atmosphere would be less stable than it
  actually is
• If model RH is off, the ED will be off
• Underprediction or overprediction!
• Appending an ED profile to an upper air profile
  takes manipulation
• Gradients at the top of the EDH profile and the
  first gradient of the upper air profile cannot be
  too discontinuous
• Focused on surface ranges, but no ducts shown in
  model
• Model uses significant levels
• Could be important for ducting situations at
  height and range

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What do we do???

• Vertical high resolution model fields
• Use a model with SST fields
• Make in situ SST observations to use
• 2m wind and Air T/another level and extrapolate
  down
• In Situ or land vertical sounding
• Need to remove levels/add levels for ED over
  water (smoothing)
• Climo?
• AREPS has the capability to automatically append
  the EDH profile to upper air refractivity
  profiles from COAMPS files
• COAMPS has the surface parameters to compute EDH
  profiles using bulk models (P/J, NPS)

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Current Research-Models

• Models of similar resolution proved to be useful
  in predicting the spatial distributions and
  diurnal variations of refractivity, but missed
  the fine vertical structure (which is critical)
• In the case of our AO, the model resolution is
  not fine enough to accurately depict the
  localized processes caused by the San Nicolas
  islands
• Further manipulation of vertical profile
• The sfc obs represent the lower 1km and the
  profile above
• Experiment have shown improvement using the
  technique.

Courtesy of Atkinson et al, 2000.
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Current Research-Predicting EDH

• Cannot be determined by rawinsondes (near-surface
  resolution is too coarse)
• Atmospheric surface layer theory
• Nomogram (TA, TS, RH, WS)
• Model predictions of path losses
• ED models developed that use bulk atmospheric
  measurements at a single altitude to blend with
  refractivity data measured at higher altitudes
• As many measurements as possible at levels 10m
  and less
• Minimum of two levels to extrapolate
• AREPS
• Automatically append the EDH profile to upper air
  refractivity profiles from COAMPS files

Courtesy of Babin et al, 1996.
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Questions?