Radiation Calculation on a Coarser Grid - Very First Results With LMK

Thorsten Reinhardt, Deutscher WetterdienstThorsten.Reinhardt@dwd.de

Motivation

• Standard calling frequency of radiation scheme: 1 hr, but more frequent calling would be costly

• Column assumption at small grid sizes questionable

• Calling radiation only every hour questionable

Cloud

dx=2.8 km (LMK grid size)assume cloud 7 grid sizes big, moving at v=10.9 m/s

assume cloudmoving at v=10.9 m/s

Cloud

then the cloud has moved 7 gridpoints after 30 min,if dx=2.8 km

assume cloudmoving at v=10.9 m/s

actual cloud

then the cloud has moved 7 gridpoints after 30 min,if dx=2.8 km

But radiative effect is taken into account still at old position!

cloud radiative effect

Therefore:

Calculate radiation scheme on a coarser grid.

Call radiation scheme more frequently.

Approach: calculate radiation on a coarser grid

LMK grid

grid for radiation

Procedure1) Average input variables onto coarser grid

2) Calculate radiation on coarser grid

3) Re-distribute onto LMK grid

4) Smooth output with discrete filter

5) Apply albedo correction

Current implementation: maximal grid size of coarse grid: (nboundlines+1) x LMK gridAveraging (Step 4) and albedo correction (Step 5) can be switched on/off

LMK Testsuite 2.2c for July 2004

Changes compared to Testsuite 2.2:

• Radiation on a coarser (2x2) grid• Call radiation very 15 min instead of every hour

First Results:

First Results:

Overall: nearly neutral impact

• Bottom and top radiative fluxes, averaged: neutral• Verification against 24-h precipitation sums: ok• Eye-inspection of visualized output: ok• Verification against synop data: not yet done• Pointwise, of course, also bigger differences

Outlook:It might be better to vary continously position of coarse grid vs. fine grid.