Climatology and anomalies¶
What the climatology is¶
To say whether this season is unusual, SnowMapper needs to know what usual looks like for each terrain unit and each day of the season. Few mountain areas have that from observations. So the model re-runs past seasons with the same engine, the same terrain units and ERA5 forcing. The result is a model climatology: for every unit, every day of the season, and every variable, it holds the 5th, 50th and 95th percentiles over the reference years (q05, q50, q95).
The reference period depends on the region. It ranges from a few recent seasons to about 25 years (back to 2000). The Info tab of each board names it.
Why a model climatology helps
Model errors that repeat every year cancel out. If the model is 30 % too wet over a range, both this season and the climatology are 30 % too wet, and the anomaly is still right. This is why anomalies are often more trustworthy than absolute amounts.
Normalised anomaly¶
- \(a = 0\): the median of past years.
- \(|a| < 0.5\): normal. This band covers roughly the range in which 90 % of years fall.
- \(a = \pm 1\): one full usual range above or below the median.
Dividing by the spread makes the anomaly comparable between a valley and a summit, and between November and March.
Spread floors. Where the climatology is "essentially no snow", the spread q95 − q05 is a fraction of a millimetre, and any trace of snow would score an extreme anomaly. The anomaly is masked where the spread is below 10 mm (SWE) or 2 cm (snow depth).
Glaciers are excluded (see Snow model).
The season¶
The season, or water year (WY), is named after the year in which it ends. In most regions it starts on 1 September: WY 2027 runs from 1 September 2026 to 31 August 2027.
Why 1 September? In the high Himalaya and the Pamir, early snowfalls in September often persist. A model started cold on 1 October discards that snow and begins the season with too little. Starting on 1 September lets the early-season pack build up. Most of September works as spin-up.
Basin and catchment curves¶
The Climatology tab aggregates units over a basin (area-weighted). It then compares the current season with the q05–q95 envelope and the median of the same basin. See the manual page.