UltraViolet Index

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UltraViolet Index

• Craig S. Long
• National Oceanic and Atmospheric Administration
• National Weather Service
• National Centers for Environmental Prediction
• Climate Prediction Center

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Outline

• Background information on UV radiation
• Factors that affect UV radiation
• Health effects
• History of UV Index
• WMO/WHO Standards
• NWS UV Index Algorithm
• Current
• Future
• Products

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Background

• Definition
• UV Index is a scaled erythemal (skin reddening)
  dose rate integrated over the UV-B and UV-A
  spectral bands.
• WMO guidelines require forecasts to be generated
  at least for next day at solar noontime.

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Background

• UltraViolet radiation has shorter/more powerful
  wavelengths than visible light.
• The most dangerous wavelengths of UV spectrum
  (UV-C) are completely absorbed in the
  stratosphere.
• Partial absorption of UV-B
• Minor absorption of UV-A

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UV Health Effects

• Short Term overexposure to UV-B
• UV-B causes sunburn (erythema)
• Sunburn can prevent your body from eliminating
  heat efficiently and can contribute to heat
  stress
• Long Term overexposure to UV-B
• Several severe sun burns linked to melanoma
• One person dies of skin cancer per hour in the
  U.S.
• Sustained exposure linked to squamous and basal
  skin cancers non lethal
• Cataracts of eyes
• Long Term overexposure to UV-A
• Aging of skin
• Loss of elasticity (leathery look)
• Small amounts of UV radiation needed for Vitamin-D

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UV Health Effects
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Background

• Factors affecting UV radiation from TOA to
  surface
• Flux at TOA
• Earth Sun distance
• Ozone (total column)
• Solar Zenith Angle
• Latitude, Day of Year
• Clouds
• Aerosols
• Elevation
• Surface Albedo
• No Snow
• Snow

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History

• 1980s Australia started reporting yesterdays
  UV radiation levels in conjunction with huge sun
  protection campaign.
• 1992 Environment Canada began issuing a next day
  forecast of clear sky UV levels called UV
  Index.
• 1994 NWS and EPA begin issuing a next day
  forecast w/cloud effects. Use slightly different
  scale than Canadians. Allowed to use name UV
  Index.
• 1995 WMO holds meeting of experts to establish
  UV Index definition.
• 1997 WMO holds second meeting of experts to
  standardize UV Index name and forecasts
• 2000 WHO holds meeting to standardize health
  messages and exposure categories.

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UV Index Definition

• UV Index is a scaled erythemal dose rate
  integrated over the UV-B and UV-A spectral bands.

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UV Spectrum
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Weighted Irradiances
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Erythemal Dose Rate

• EDR Integral over the 290 to 400 nm range
• EDR 0.250 Watts/m2 or 250 mW/m2.
• Erythemal Dose Rate is the instantaneous flux of
  skin reddening UV radiation.
• UV Index is the EDR(W/m2) x 40.
• Or EDR(mW/m2)/25
• UV Index 10.0

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Radiative Transfer Models

• RTMs are needed to determine clear sky UV flux
  at surface under prescribed conditions.
• SZA
• Ozone
• Aerosols
• Elevation
• Albedo
• Several types of RTMs
• multiple scattering spectral models
• fast spectral models
• empirical models
• RTMs are computationally expensive
• Operational forecasts of large gridded fields
  require the creation of Look Up Tables

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Results from Spectral Radiative Transfer Model
Tropics
Extra Tropics
Ozone Hole
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Zonal Mean Ozone
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Zonal Mean UV Index (clear sky, sea level, no
aerosol)
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UV Index Forecast

• Current UV Fcst
• Ozone Forecast from GFS
• Albedo 5 (no snow)
• Elevation (6 per Km)
• Aerosol clean atmosphere
• (AOD0.2, SSA1.0)
• Clouds amounts from MOS
• Products
• 58 city bulletin
• Clear sky UV Index on global 1x1 grid
• Cloudy UV Index for CONUS and AK grids

• Future UV Fcst
• Ozone Forecasts from GFS
• Albedo
• no snow 3
• Snow gt30 (GFS Albedo)
• Elevation
• 9 1st km
• 7 2nd km
• 6 3rd km
• Aerosol
• Variable from climatology
• Cloud attenuation from GFS shortwave code
• Products
• 58 city bulletin
• Global Cloudy and Clear Sky grids

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UV Index Forecast

• 3 Forecasts in one
• Ozone Forecast
• Cloud Forecast
• Aerosol Forecast (not there yet)

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NCEP GFS Ozone Assimilation
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Global Ozone Field
Ozone Hole
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Global UV Index
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Elevation Effect
Breckenridge, CO
Yellowstone, WY
Jackson, WY
Boulder, CO
Lubbock, TX
Binghamton, NY
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Albedo

• Surface has low albedo is UV 3
• Sand is more reflective 30
• Snow is most reflective 30-90

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Albedo (snow and sand)
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Aerosol Effects

• Aerosols scatter and absorb UV radiation
• Reduce direct and Increase diffuse
• Global is usually attenuated
• Aerosol parameters include
• Aerosol Optical Depth
• Single Scattering Albedo
• Ratio of scattered /(scattered absorbed)
• Asymmetry factor
• Forward scattering vs backward scattering

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Aerosol Climatology

• Aerosol Optical Depth
• Single Scattering Albedo

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Aerosol Effect
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Cloud Forecasts

• Current Method MOS clouds
• MOS fcsts Pct Clear, Scattered, Broken, Overcast
• Regression determined by comparing MOS fcsts
  against observed cloud transmission
• Clear (0-1 tenths) 100 transmission
• Scattered(2-5 tenths) 89
• Broken(6-8 tenths) 72
• Overcast(9-10 tenths) 31
• Proposed Method From GFS Shortwave scheme
• UV Transmission UVCloud/UVNo Cloud

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UV Attenuation from GFS Clouds
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Clouds and UV Attenuation
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Clouds and UV Attenuation
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Clouds and UV Attenuation
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Clouds and UV Attenuation
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Clouds and UV Attenuation
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Products MOS Locations for CONUS and AK
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Products 58 City Bulletin
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Products Cloudy UV Index Output Grids
Eta Grid 207
Eta Grid 211
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Products UV Index Contoured Map
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Products Global UV Index
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UV Index to do list

• Generate and validate global noon time UV Index
  fcsts
• EMC must operationally output UVcloud and
  UVno-cloud surface fluxes
• Generate UV Index forecasts out to day 5
• Generate UV Index forecasts at both 00Z and 12Z
  cycles
• Generate daily UV dosage
• Diurnal variation

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