*CNV Section

26JAN26
Description
Change Convergence Criteria from Default values[1]

Structure
Multiple PARAMETER=VALUE records, any order

Parameter Type Description Range Default
MAXITR I Maximum number of iterations > 0 10
CHILIM R Minimum value of χ2 for convergence[2] ≥ 0 1.0
DELCHI R Minimum change in χ2 for convergence[2] ≥ 0 0.1
DXIMAX R Minimum change in retrieved element[3] per iteration ≥ 0 10.0
GAMMAX R Maximum value for Levenberg-Marquardt parameter ≥ 1 1000.0
NOIFAC R Scaling factor for L1C noise values (MIC_NOI)[5] > 0 1.0

Notes
  1. Default values are set in module cnvcom_dat.f90, but can also be modified collectively by setting the ACC Flag in the *FLG section.

  2. The χ2 statistic is the sum of the contributions from the A priori and Measurements, divided by the number of measurements, evaluated in module swprtv_sub.f90. χ2∼1 if the instrument noise is the only limiting factor.

  3. State vector elements are scaled by the a priori standard deviation, so a value 10.0 means that an element can change by up to 10σ in a single iteration

  4. The retrieval will end in one of two states (set in swprtv_sub.f90)
    CONVERGED If: The LM parameter GAMMA = 1 (its minimum value)
    and either: change in χ2 < DELCHI × min(χ2),
    or: χ2 < CHILIM
    STOPPED If either: Number of iterations > MAXITR
    or: LM parameter GAMMA > GAMMAX

  5. NOIFAC: Retrievals may be unstable due to 'overfitting' the measurements, i.e. if the assumed noise on the measurements underestimates the achievable match between the measurement and the forward model. This parameter provides a convenient way of investigating if this is the case, eg if setting a value 10 results in 'smoother' profiles.

Example
*CNV CHILIM=2.0 MAXITR=5