Developments to Infra-Red Radiative Transfer Products Applications for Performance Surfaces

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1 UNCLASSIFIED - FOUO Developments to Infra-Red Radiative Transfer Products Applications for Performance Surfaces From Stephan Havemann and Jean-Claude Thelen, Met Office. Prepared by Damian Wilson, Met Office Presented by Warren Lewis, Met Office Defence R&D Capability Adviser, March 2012 UNCLASSIFIED - FOUO

2 Summary Calculation of maps of infra-red visibility at lowlevels for use as guidance products for FLIR use Uses NWP and the Fast Radiative Transfer Code capability (the same code as Neon uses), which has been updated to include the effect of cloud / fog and precipitation Integration of infra-red visibility into a webapplication product to start soon Integration of cloud / fog and precipitation updates into a new Neon release to start soon

3 Representation of cloud and precipitation The Fast Radiative Transfer Code has been updated to include the effects of cloud, precipitation and fog Scattering properties of spherical cloud droplets and raindrops can be calculated using Mie theory Particle size distributions are assumed to be consistent to those used within the MetUM Numerical Weather Prediction (NWP) model Ice particle scattering properties for ice and snow follow Baran and Francis (2004), also consistent with MetUM assumptions The NWP diagnostics of liquid and ice water contents and precipitation rates can then be used to infer the scattering properties

4 Cloud and precipitation Thermal contrast of a target viewed through an atmosphere at various ranges and heights Contrasts are much reduced within rain but are still visible Contrasts are eliminated when there is a significant thickness of cloud present Water cloud with no rain Water cloud with 6 mm hr -1 rain

5 Representation of fog A similar method to that used for aerosol The visible-wavelength visibility (v) is used to estimate the infra-red extinction over the viewing path () via the Koschmeider relationship for wavelengths (,v) = fog () [ln(0.02)/v] / (0.55m) Extinction as a function of wavelength Koschmeider relationship for visible wavelenths Extinction at visible wavelengths The values of fog () come from SBDART

6 Fog in radiative transfer Contrasts are much reduced when fog is present but are still visible Clear-sky Fog with visibility 20 km

7 Infra-red visibility prediction Infra-red visibility guidance maps are currently drawn by a forecaster with the help of a stand-alone TDA application, running on representative points within an air mass This is time-consuming and does not utilise the latest scientific developments to radiative transfer techniques Are we able to utilise NWP predictions to provide an infra-red visibility map? The current, standalone, IRVIS TDA

8 IR visibility definition We use a Koschmeider type relationship as described by Touart (1992) to calculate infrared visibility (V IR ) as a function of extinction ( IR ) in the 8-12 m broad spectral band V IR (r) = / IR (r) We need to calculate IR (r) at each point from NWP model predictions near the surface (humidity, temperature, pressure, dust, cloud and precipitation rates) and a viewing geometry Utilises the Fast Radiative Transfer Code to do this, including the precipitation elements

9 Example visibilities Dry Moist Visible-wavelength visibilities Infra-red visibilities - particularly sensitive to water vapour

10 Clustering Calculations for a complete map are time consuming. Processing time can be decreased significantly using a clustering approach where we calculate the effect of the atmosphere only for distinctly different regions Similar atmospheric regions IR visibility for 7000 clusters

11 Choice of cluster size The choice of cluster size needs to be considered carefully depending on customer requirements for speed and accuracy Un-clustered field 9 minutes calculation time 400 clusters only 15 s calculation time

12 Summary Calculation of maps of infra-red visibility at lowlevels for use as guidance products for FLIR use Uses NWP and the Fast Radiative Transfer Code capability (the same code as Neon uses), which has been updated to include the effect of cloud / fog and precipitation Integration of infra-red visibility into a webapplication product to start soon Integration of cloud / fog and precipitation updates into a new Neon release to start soon

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