Ensemble prediction in AROME from MICADO

Contains:

Surfex modification: stochastic surface perturbations

(cycle cy38t1op1 - F.Bouttier Oct 2013)

• Purpose: implement stochastic surface perturbations for ensemble prediction. This code will be used in the MF AROME ensemble prediction system.
• Description:
  • initial perturbation of surface fields are prepared before the forecast by an external program that manipulates the Surfex initial conditions file to
    • (1) alter some prognostic fields, and
    • (2) append some perturbation patterns for physiographic fields.
  • The code modifications perform two tasks:
    • to read and apply perturbations to fields that cannot be easily perturbed directly in the initial file.
    • to preserve initial perturbations of quasi-constant fields (ISBA veg, lai, cv,albedo, Z0), even when there is a decadal update of physiographic fields from PGD.
  • Details:
    • Both are achieved by storing the field perturbations in new modd_isban module fields XPERTVEG, XPERTLAI, XPERTCV, XPERTALB, XPERTZ0.
    • The perturbations are read from the initial file in compute_isba_parameters.
    • The same albedo perturbation pattern is applied to all ISBA albedoes.
    • The same Z0 perturbation pattern is applied to all ISBA roughness parameters, except vegetation Z0.
    • The physiographies for non-ISBA surfaces (ice, snow, town, flood, etc) are not perturbed.
  • The surface perturbation code is activated by two namelist switches:
    • NAM_ISBAn/LPERTSURF: perturbations of ISBA (veg,lai,cv,albedo,Z0). Requires fields PERTALB and PERTZ0LAND from initial file.
    • NAM_SEAFLUXn/LPERTFLUX: multiplicative perturbation of Ecume fluxes. Requires field PERTSEAFLUX from initial file.
  • When these switches are off, the code is bit-reproducible, and standard initial files can be used.
• Modified files:
  compute_isba_parameters.F90,
  coupling_isban.F90,
  coupling_seafluxn.F90,
  ecume_flux.F90,
  ecume_seaflux.F90,
  garden.F90,
  modd_isban.F90,
  modd_seafluxn.F90,
  modn_isban.F90,
  modn_seafluxn.F90,
  modn_teb_gardenn.F90,
  read_seafluxn.F90,
  vegetation_update.F90.

This contribution was tested in AROME CY38t1 (that contains SURFEX V7_1 (to be verified)), on several months of forecast.

It would be good if adapted files were provided to test it in OFFLINE mode, and to enter the base of automatic tests STRATO in section CAS_PART.

It’s very specific and will be counted as and intermediate merging.