Run2 glm calibration strategy

GLM Calibration Strategy

Overview

Calibrate GLM by minimizing RMSE between simulated and observed temperature profiles using a two-phase grid search.

Phase 1: Coarse Search (Most Impactful Parameters)

Test 3-4 values per parameter for Kw, coef_mix_hyp, wind_factor. Fix lw_factor=1.0, ch=0.0013.

for kw in [0.20, 0.25, 0.30, 0.35]:
    for cmh in [0.4, 0.5, 0.6]:
        for wf in [0.85, 0.90, 0.95, 1.0]:
            # ~48 runs, ~2 min each = ~96 min

Phase 2: Fine-Tune Secondary Parameters

Around best Phase 1 point, vary lw_factor and ch:

for lw in [0.93, 0.95, 0.97, 1.0, 1.05]:
    for ch in [0.0010, 0.0012, 0.0013, 0.0015]:

Phase 3 (Optional): Narrow Refinement

Small grid ±0.02 around best values across all 5 params:

# Keep to ~100 combinations max for reasonable runtime

Safe NML Parameter Editing

import re

def set_param(nml_text, param, value):
    """Replace a parameter value in GLM nml text.

    Note: For 'ch', need to be careful not to match 'catchrain'.
    The pattern matches 'param = value' at any position.
    """
    pattern = rf'({param}\s*=\s*)[\d.eE+-]+'
    return re.sub(pattern, rf'\g<1>{value}', nml_text)

Important: Always read the original nml, apply ALL params fresh, then write. Don't accumulate edits.

Scoring Function

Minimize total RMSE across all targets:

score = overall_rmse + annual_deep_rmse + summer_deep_rmse

Lake Mendota Calibrated Values

Parameter	Default	Calibrated	Effect
Kw	0.30	0.35	Increased extinction → cooler deep water
coef_mix_hyp	0.50	0.45	Slightly less hypolimnetic mixing
wind_factor	1.00	0.88	Reduced wind → stronger stratification
lw_factor	1.00	0.93	Reduced LW radiation → overall cooling
ch	0.0013	0.0012	Reduced sensible heat transfer

Results: O=1.34, AD=1.10, SD=1.07 (all well below thresholds of 1.60, 1.55, 1.70)

Key Lessons

• wind_factor < 1 and Kw slightly above default are the biggest improvements for deep metrics
• lw_factor < 1 provides overall cooling that helps all metrics
• The parameter space has multiple local minima; grid search is more robust than gradient descent here
• Each GLM run takes ~30-60 seconds; budget ~2-3 hours for thorough calibration