GLM Calibration Guide
Overview
GLM calibration involves adjusting physical parameters to minimize the difference between simulated and observed water temperatures. The goal is typically to achieve RMSE < 2.0°C.
Key Calibration Parameters
| Parameter | Section | Description | Default | Range |
|---|
| ----------- | --------- | ------------- | --------- | ------- |
Kw | &light | Light extinction coefficient (m⁻¹) | 0.3 | 0.1 - 0.5 |
coef_mix_hyp | &mixing | Hypolimnetic mixing coefficient | 0.5 | 0.3 - 0.7 |
wind_factor | &meteorology | Wind speed scaling factor | 1.0 | 0.7 - 1.3 |
lw_factor | &meteorology | Longwave radiation scaling | 1.0 | 0.7 - 1.3 |
ch | &meteorology | Sensible heat transfer coefficient | 0.0013 | 0.0005 - 0.002 |
Parameter Effects
| Parameter | Increase Effect | Decrease Effect |
|---|
| ----------- | ----------------- | ----------------- |
Kw | Less light penetration, cooler deep water | More light penetration, warmer deep water |
coef_mix_hyp | More deep mixing, weaker stratification | Less mixing, stronger stratification |
wind_factor | More surface mixing | Less surface mixing |
lw_factor | More heat input | Less heat input |
ch | More sensible heat exchange | Less heat exchange |
Calibration with Optimization
from scipy.optimize import minimize
def objective(x):
Kw, coef_mix_hyp, wind_factor, lw_factor, ch = x
# Modify parameters
params = {
'Kw': round(Kw, 4),
'coef_mix_hyp': round(coef_mix_hyp, 4),
'wind_factor': round(wind_factor, 4),
'lw_factor': round(lw_factor, 4),
'ch': round(ch, 6)
}
modify_nml('glm3.nml', params)
# Run GLM
subprocess.run(['glm'], capture_output=True)
# Calculate RMSE
rmse = calculate_rmse(sim_df, obs_df)
return rmse
# Initial values (defaults)
x0 = [0.3, 0.5, 1.0, 1.0, 0.0013]
# Run optimization
result = minimize(
objective,
x0,
method='Nelder-Mead',
options={'maxiter': 150}
)
Manual Calibration Strategy
- Start with default parameters, run GLM, calculate RMSE
- Adjust one parameter at a time
- If surface too warm → increase
wind_factor - If deep water too warm → increase
Kw - If stratification too weak → decrease
coef_mix_hyp - Iterate until RMSE < 2.0°C
Common Issues
| Issue | Likely Cause | Solution |
|---|
| ------- | -------------- | ---------- |
| Surface too warm | Low wind mixing | Increase wind_factor |
| Deep water too warm | Too much light penetration | Increase Kw |
| Weak stratification | Too much mixing | Decrease coef_mix_hyp |
| Overall warm bias | Heat budget too high | Decrease lw_factor or ch |
Best Practices
- Change one parameter at a time when manually calibrating
- Keep parameters within physical ranges
- Use optimization for fine-tuning after manual adjustment
- Target RMSE < 2.0°C for good calibration