ERA5-derived South Asian monsoon low-pressure system catalogue, v5.5 (1940–2025)

ERA5-derived South Asian monsoon low-pressure-system catalogue v5.5 containing 1,678 LPSs from 1940 to the end of 2025. Every retained event has a complete hourly time axis.

Brief methodology

1. Candidate centres are identified from hourly ERA5 vorticity and supporting pressure, wind, humidity and temperature fields.
2. Candidates were linked through time, physically unrealistic gaps were removed, plausible endpoints were connected, and the surviving paths were projected onto a continuous hourly geometry constrained to at most 20 m s⁻¹ propagation speed and at most 125 km from observed detector centres.
3. Event existence uses the same pressure, vorticity, duration, observed-support and trajectory-coherence rules everywhere. An event needs at least three mature observed centres in the broad 65–100°E, 0–32°N release domain, so systems may originate elsewhere and later approach or cross India.
4. Intensity is obtained from the 95th percentile of 10-m wind-vector anomaly within 125 km after subtracting the mean 300–500 km environmental wind vector. Thresholds use six-hour persistence and were calibrated against official IMD southwest-monsoon reports for 2015–2025. Precipitation is not used to select or classify events.

Identity and interpretation

track_id is the single physical-event identifier and should be used for grouping, event counts, plotting, matching and climatology. position_source distinguishes observed detector fixes from supported interpolated centres. Event-level support and peak-intensity values are repeated on each row of an event.

All unsuffixed meteorological fields are sampled at the final published centre. IMD-equivalent categories are ERA5-derived: 1 low, 2 depression, 3 deep depression, 4 cyclonic storm, 5 severe cyclonic storm, 6 very severe or extremely severe cyclonic storm, and 7 super cyclonic storm. Instantaneous persistent categories are null at interpolated positions.

Column guide

Identity, time and published position

• track_id (int64; identifier) — Hourly-complete strength-qualified physical-event identifier; use for grouping, counts, plotting, matching and climatology.
• time (timestamp[ns]; UTC) — Valid time of the published centre in UTC.
• year (int64; calendar component) — UTC calendar year of time.
• month (int64; calendar component) — UTC calendar month number of time (1–12).
• day (int64; calendar component) — UTC day of month of time.
• hour (int64; calendar component) — UTC hour of day of time (0–23).
• lon (double; degrees east) — Longitude of the final published centre.
• lat (double; degrees north) — Latitude of the final published centre.
• life_stage (string; category or text) — Categorical life-cycle stage assigned to this hourly position.
• position_source (string; category or text) — Whether the published centre is an observed detector fix or a supported posterior/interpolated position.

Compact event-support summaries

• posterior_gap_hours (double; hours) — Length of the candidate-free gap represented by a posterior/interpolated row; zero on observations.
• posterior_fraction (double; 0–1) — Fraction of this event's published rows that are posterior/interpolated; repeated on every event row.
• track_duration_hours (double; hours) — Inclusive number of hourly positions in the physical event; repeated on every event row.
• track_observed_support_hours (double; hours) — Number of observed detector positions in the event; repeated on every event row.
• track_occupancy_fraction (double; 0–1) — Observed positions divided by inclusive event duration; repeated on every event row.

Circulation, precipitation, wind and pressure

• max_vort (double; 10⁻⁵ s⁻¹) — Maximum core relative vorticity inside the dynamical sampling radius sampled at the final published centre.
• mean_vort (double; 10⁻⁵ s⁻¹) — Mean core relative vorticity inside the dynamical sampling radius sampled at the final published centre.
• max_vort_smoothed (double; 10⁻⁵ s⁻¹) — Maximum spatially smoothed core relative vorticity inside the dynamical sampling radius sampled at the final published centre.
• mean_vort_850 (double; 10⁻⁵ s⁻¹) — Mean 850-hPa relative vorticity inside the dynamical sampling radius sampled at the final published centre.
• mean_vort_700 (double; 10⁻⁵ s⁻¹) — Mean 700-hPa relative vorticity inside the dynamical sampling radius sampled at the final published centre.
• mean_vort_500 (double; 10⁻⁵ s⁻¹) — Mean 500-hPa relative vorticity inside the dynamical sampling radius sampled at the final published centre.
• mean_vort_deep (double; 10⁻⁵ s⁻¹) — Mean of published-centre relative vorticity at 850, 700 and 500 hPa.
• precip_1hr (double; mm) — Area-mean ERA5 precipitation during the valid hour inside the precipitation sampling radius sampled at the final published centre.
• precip_24hr (double; mm) — Area-mean trailing 24-hour ERA5 precipitation inside the precipitation sampling radius sampled at the final published centre.
• max_wind (double; m s⁻¹) — Maximum ERA5 10-m wind speed inside the wind sampling radius sampled at the final published centre.
• mean_wind (double; m s⁻¹) — Mean ERA5 10-m wind speed inside the wind sampling radius sampled at the final published centre.
• p95_anomaly_wind_125km_ms (double; m s⁻¹) — 95th percentile of ERA5 10-m wind-vector anomaly magnitude within 125 km after subtracting the mean 300–500 km environmental wind vector.
• background_u_300_500km_ms (double; m s⁻¹) — Eastward component of the mean ERA5 10-m environmental wind vector in the 300–500 km annulus used by the circulation-wind metric.
• background_v_300_500km_ms (double; m s⁻¹) — Northward component of the mean ERA5 10-m environmental wind vector in the 300–500 km annulus used by the circulation-wind metric.
• min_mslp (double; hPa) — Minimum mean-sea-level pressure inside the inner pressure radius sampled at the final published centre.
• ring_mslp_p60 (double; hPa) — 60th percentile of mean-sea-level pressure in the environmental annulus sampled at the final published centre.
• pressure_deficit_hpa (double; hPa) — Environmental-annulus MSLP 60th percentile minus the local MSLP minimum, floored at zero sampled at the final published centre.
• closed_isobars_2hpa_local (double; count) — Integer pressure-depth proxy equal to floor(pressure deficit / 2 hPa) sampled at the final published centre.

Humidity, temperature and surface context

• q850_mean_gkg (double; g kg⁻¹) — Mean 850-hPa specific humidity inside the dynamical sampling radius sampled at the final published centre.
• rh850_mean_pct (double; %) — Mean 850-hPa relative humidity inside the dynamical sampling radius sampled at the final published centre.
• t850_mean_k (double; K) — Mean 850-hPa air temperature inside the dynamical sampling radius sampled at the final published centre.
• t850_inner_minus_annulus_k (double; K) — Mean inner-region 850-hPa temperature minus the environmental-annulus mean sampled at the final published centre.
• q700_mean_gkg (double; g kg⁻¹) — Mean 700-hPa specific humidity inside the dynamical sampling radius sampled at the final published centre.
• rh700_mean_pct (double; %) — Mean 700-hPa relative humidity inside the dynamical sampling radius sampled at the final published centre.
• t700_mean_k (double; K) — Mean 700-hPa air temperature inside the dynamical sampling radius sampled at the final published centre.
• t700_inner_minus_annulus_k (double; K) — Mean inner-region 700-hPa temperature minus the environmental-annulus mean sampled at the final published centre.
• q500_mean_gkg (double; g kg⁻¹) — Mean 500-hPa specific humidity inside the dynamical sampling radius sampled at the final published centre.
• rh500_mean_pct (double; %) — Mean 500-hPa relative humidity inside the dynamical sampling radius sampled at the final published centre.
• t500_mean_k (double; K) — Mean 500-hPa air temperature inside the dynamical sampling radius sampled at the final published centre.
• t500_inner_minus_annulus_k (double; K) — Mean inner-region 500-hPa temperature minus the environmental-annulus mean sampled at the final published centre.
• rh_deep_mean_pct (double; %) — Mean of published-centre relative humidity at 850, 700 and 500 hPa.
• q_deep_mean_gkg (double; g kg⁻¹) — Mean of published-centre specific humidity at 850, 700 and 500 hPa.
• t850_minus_t500_k (double; K) — Published-centre 850-hPa temperature minus 500-hPa temperature.
• t700_minus_t500_k (double; K) — Published-centre 700-hPa temperature minus 500-hPa temperature.
• t_inner_minus_annulus_deep_k (double; K) — Mean inner-minus-environment temperature contrast across 850, 700 and 500 hPa.
• orography_m (double; m) — Surface elevation sampled at the final published centre.
• land_fraction (double; 0–1) — ERA5 land fraction, from 0 (water) to 1 (land) sampled at the final published centre.

Calibrated IMD-equivalent intensity

• imd_category (double; category code) — Final persistent ERA5-derived IMD-equivalent numeric intensity code; not an official IMD classification.
• imd_label (string; category or text) — Text label for imd_category.
• event_peak_imd_category (int64; category code) — Maximum persistent ERA5-derived IMD-equivalent category reached by the event, repeated on every event row.
• event_peak_imd_label (string; category or text) — Text label for event_peak_imd_category, repeated on every event row.

Licence and source-data note

The catalogue files are released under CC BY 4.0. ERA5 source data remain subject to the Copernicus/ECMWF terms that apply to ERA5.