# GPSRO Usage at Deutscher Wetterdienst (DWD)

## Deterministic models assimilating RO

- ICON: non-hydrostatic grid-point model, icosahedral-triangular grid.
- Hybrid height-based vertical coordinates. Horizontally/vertically self-nesting.
- Horizontal resolution:
- Global domain: R03B07 (2949120 grid-points, ~13 km average mesh size), 90 levels, lid at 75 km.
- Europe nest (two-way): R03B08 (659156 grid-points, ~6.5 km average mesh size), 60 levels (~22.5 km).

- 8 forecast runs per day:
- to 180 hours at 00 UTC, 12 UTC (main forecast runs),
- to 120 hours at 06 UTC, 18 UTC
- to 30 hours at 03 UTC, 09 UTC, 15 UTC, 21 UTC (boundary conditions for LAM)

## Data assimilation method

- 3D-Var PSAS with a window size of 3 hours (synoptic time T +/- 1.5h).
- Cutoff-time for main run analysis: T + 2h 14min.
- Cutoff times for late assimilation runs (from synoptic time):
- 00, 12: 11h11m
- 03, 15: 08h36m
- 06, 18: 06h01m
- 09, 21: 04h36m

## RO quantity assimilated

- Bending angle.
- 1D operator, based on implementation by Michael Gorbunov.
- Uses MSIS-90 climatology above model top.
- No tangent point drift. We use an effective tangent point derived from rays between 3 km and 15 km impact height, not the nominal tangent point.
- Vertical thinning depends on impact parameter:
- 3 – 14 km: 400 m
- 14 – 15 km: 500 m
- 15 – 20 km: 700 m
- 20 – 45 km: 1000 m

- Thinning is complemented by superobbing (with exponential smoothing).
- No bias correction

## Observation error characteristics

- Assigned observation error depends on impact height h and latitude (the errors vary smoothly in latitude between these bounds):
- Tropics:
- 1.5% of observed bending angle for h > 12 km,
- 20% at 0 km,

- Polar regions:
- 0.9% of observed bending angle for h > 9 km,
- 10% at 0km,

- Additive representativity error near (statically modeled) cold-point tropopause: 2% in tropics, 1.2% in polar regions.
- Global absolute lower limit of 2*10^-6 rad.
- Errors provided in the BUFR will be ignored.
- Observations errors are assumed uncorrelated. (Diagonal R)

## Impact

- Adjoint sensitity studies are not possible with ICON or the former GME.
- Only forecast impact studies were done before operational implementation and published later (H. Anlauf et al., Atmos. Meas. Tech., 4, 1105-1113).