glm-calibration

GLM Calibration Guide

Overview

GLM calibration involves adjusting physical parameters to minimize the difference between simulated and observed water temperatures. The goal is to satisfy the task's stated evaluation metrics, not just to lower a single aggregate RMSE.

Suggested Skill Flow

This skill works best after glm-basics has confirmed the file layout and editable parameters. The job of this skill is to find a candidate parameter set that passes the task metrics. Once a candidate setting appears to pass, stop calibration and hand off immediately to glm-output for the final verifier-matching metrics and /root/metrics.json.

Task Checklist

For tasks that restrict the editable parameter set and require an exact self-evaluation:

• Only adjust the parameters explicitly allowed by the task
• Leave protected parameters and initialization profiles unchanged
• Stop only when every required task metric passes; do not use global RMSE alone as the stopping condition
• If a configuration already passes every required task metric, stop the calibration search immediately
• Do not launch a broad refinement search after finding a passing configuration
• If the task requires /root/metrics.json, hand the final reporting step to glm-output instead of a separately improvised evaluator

Important Tips

• In many case, the surface fit becomes acceptable before the deep water is fixed.
• Single-knob moves can improve whole-lake RMSE while leaving the task-defining deep subsets too warm.
• Do not optimize only the global RMSE. Recompute the full task metric set after each promising run.

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

    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)
    subprocess.run(['glm'], capture_output=True)
    return calculate_task_metrics(sim_df, obs_df)['overall_rmse']

result = minimize(
    objective,
    [0.3, 0.5, 1.0, 1.0, 0.0013],
    method='Nelder-Mead',
    options={'maxiter': 150}
)

Manual Calibration Strategy

1. Start from the provided seed and compute the exact deep-band metrics, not just overall RMSE.
2. First test changes that improve the deep layer over the full record rather than only cooling the whole lake; Kw and coef_mix_hyp are usually the first levers to inspect.
3. Once the annual deep metric improves, use smaller adjustments to wind_factor and lw_factor to reduce whole-lake bias without undoing the deep-water gains.
4. Leave ch for final fine-tuning after the main structure is already close.
5. Prefer a short hypothesis-driven search: keep candidates that improve both deep metrics together, discard one-knob moves that only make the global RMSE look better, and stop as soon as one candidate passes every required task metric.
6. After the first passing configuration is found, rerun GLM once at that setting, switch to glm-output, write /root/metrics.json, and end the task if the exact final metrics also pass.

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
Annual deep improves but summer deep still fails	Deep water cooled too weakly during stratified months	Keep Kw high and lower lw_factor slightly

Best Practices

• Keep parameters within the published ranges
• Use the deep-band metrics as the stopping condition, not surface fit alone
• Save time by evaluating a few structured candidates instead of a long blind optimizer
• After you find a passing parameter set, do not keep refining it blindly
• Rerun GLM once at the chosen setting, then use glm-output to write the exact metrics file immediately
• If glm-output confirms that the exact final metrics pass, stop there instead of reopening calibration