*CNV Section

16FEB25
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

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

Example
*CNV CHILIM=2.0 MAXITR=5