Nordic weather radar workshop Helsinki 3.10.-4.10.2002

Radar beam refraction modelling

Uta Gjertsen, met.no, FoU

Meteorologisk Institutt met.no

For the quantitative use of weather radar data in Norway it is essential to correct for beam shielding. The degree of beam shielding is variable due to changes in atmospheric humidity and temperature profiles. In the near future we will implement a model deriving the degree of beam blockage from actual radiosonde profiles. The model will increase the accuracy of the shielding correction and identify regions where data is missing due to anaprop. The model gives also an indication on situations with beam splitting. Beam splitting can produce large errors and is difficult to identify from NWP model or radar data alone.

Meteorologisk Institutt met.no

Today’s precipitation composite

• Spatial gauge adjustment with mean monthly gauge/radar factors • Precipitation intensity • Accumulated precipitation

Meteorologisk Institutt met.no

Accumulated precipitation

Meteorologisk Institutt met.no

Radar Oslo: Mean daily precipitation for a winter and spring season, beam blocking and topography

Meteorologisk Institutt met.no

Beamsplitting: Radar beam height as a function of distance modeled from radiosonde data 16.8.2002 (elevation = 0.5 degrees, radar Oslo)

Meteorologisk Institutt met.no

Beam shielding west of radar Oslo 16.8.2002

• Lower edge height of radar beam Blue line: measurement volume Red line: meausrement volume after beam blockage Dashed red line: measurement volume from standard atmosphere

Meteorologisk Institutt met.no

Beam splitting east of radar Oslo 16.8.2002

Meteorologisk Institutt met.no

Examples for vertical humidity and temperature profiles causing anaprop .

Condition

Trapping Superrefraction Normal Standard Subrefraction

dN/dh

< -157 -157 to -79 -79 to 0 -39 > 0

Meteorologisk Institutt met.no