ecmodel

Scatterometer use in NWP at ECMWF

Physical characteristics

Contents

  • Spectral model
  • Horizontal resolution: TCO1279 (~9 km Gaussian grid)
  • Vertical resolution: 137 vertical levels
  • Analysis times: 00,12 Z

Data assimilation method

  • Since 29th June 2004, ECMWF have been running an early delivery stream (DA) and a delayed cut-off stream (DCDA). The DCDA is the equivelent of the Met Office update run and is used to generate the background for the next cycle. The early delivery stream is the equivelent of the Met Office main forecast run and is used to generate the operational forecasts.
    • DA: 4-D VAR, 6-h time window.
    • DCDA: 4-D VAR, 12-h time window.
  • Time window:
    • DA: T ± 3 hr
    • DCDA: T-3 hr – T+9 hr
  • Time constraints (model runtime):
    • DA: 1 hr after time window ends
    • DCDA: 5 hr after time window ends

Scatterometers assimilated

Scat name Product Models assimilated
ASCAT-A Wind retrieval from the ASCAT L1 BUFR 25km products using the CMOD5.N GMF Global Deterministic Model , Global Ensemble Prediction System, Seasonal Forecast
ASCAT-B Wind retrieval from the ASCAT L1 BUFR 25km products using the CMOD5.N GMF Global Deterministic Model , Global Ensemble Prediction System, Seasonal Forecast
ASCAT-C Wind retrieval from the ASCAT L1 BUFR 25km products using the CMOD5.N GMF Global Deterministic Model , Global Ensemble Prediction System, Seasonal Forecast

 

Monitoring

External monitoring web pages

Generic Quality Control

Blacklisting

  • All observations where model sea ice is greater than 0.01%
  • All observations with SST less than 273.15 K according to OSTIA products.
  • ASCAT-A/B/C: All wind speeds greater than 35 m/s
Ambiguity removal
ASCAT-A/B/C:Normally two ambiguous pairs of U and V wind components are found at each location. In 4D-Var both ambiguous winds are used and the most appropriate solution is dinamically determined by comparison with ECMWF background fields.
Bias correction
ASCAT-A/B/C:A sigma nought bias correction is applied to ASCAT backscattering measurements before the wind inversion. The bias correction is computed by the use of the ocean calibration method, by comparison to ECMWF first-guess winds. The sigma nought bias correction is dependent of the beam and the incidence angle. A wind speed bias correction is applied to ASCAT wind after the wind inversion. It is wind speed and incidence angle dependent.
Thinning
ASCAT-A/B/C:

  • Prior to assimilation observations are thinned. In the across track direction only predefined wind-vector cells are selected. For ASCAT, from 42 cells (two swaths of 21 cells each) only cells 1, 5, 9, 13, 17, 21, 22, 26, 30, 34, 38, 42 are selected resulting in about 100 km of resolution.
  • Winds are assimilated in 30 min time-slots.
Background check

  • Background check: very relaxed first-guess check
  • Variational quality control is applied in the second and third outer loops. The weight of the scatterometer observations in the analysis is based on the probability of gross error. Data with high probability of gross error are considered `rejected’ and flagged accordingly.

Specific Quality Control

Global model

Scat name ASCAT-A/B/C
Operational since June 2007 (ASCAT-A), July 2013 (ASCAT-B), Dec 2019 (ASCAT-C)
Observation error U/V 1.5 m/s
Wind speed range Observed and background winds < 35m/s
Bias corrected? Sigma nought and wind speed bias correction applied
Crosstrack cells used Nodes 1,5,9,13,17,21,22,26,30,34,38,42
QC thresholds Rank-1 distance to the cone < 1.5

Observation Operator

Observations are assimilated as equivalent-neutral 10m winds.
Lowest model level

  • Height of the lowest model level: ~10m

Interpolation

  • Model wind fields are horizontally interpolated to the observation location using a 4-point bi-linear interpolation.
  • The vertical interpolation between the lowest model level and the surface is performed using the formulation of Geleyn that is based on Monin-Obukhov similarity theory.

History of Changes

The list includes the main scatterometer or model changes implemented operationally in the ECMWF model.

References

Hersbach, H. and Janssen, P. (2007). Preparation for assimilation of surface-wind data from ASCAT at ECMWF. ECMWF Research Department Memorandum R60.9/HH/0750. Available from ECMWF on request.
Hersbach, H. (2008). CMOD5.N: A C-band geophysical model function for equivalent wind, ECMWF Technical Memorandum 554, UK. Available from: http://www.ecmwf.int/en/elibrary/9873-cmod5n-c-band-geophysical-model-function-equivalent-neutral-wind
Hersbach, H., (2010). Assimilation of scatterometer data as equivalent-neutral wind, ECMWF Technical Memorandum 629. Available from: http://www.ecmwf.int/en/elibrary/9874-assimilation-scatterometer-data-equivalent-neutral-wind
IFS-documentation (2012). Available from https://software.ecmwf.int/wiki/display/IFS/Official+IFS+Documentation