Montero‐Martínez et al. (2025) How does the influence of wind on the fall speed of raindrops change with altitude?

Identification

• Journal: Atmospheric Research
• Year: 2025
• Date: 2025-10-13
• Authors: Guillermo Montero‐Martínez, Narda Isabella Miranda-Sánchez, Paloma Josselyn Ramírez-López, Fernando García‐García
• DOI: 10.1016/j.atmosres.2025.108561

Research Groups

• Instituto de Ciencias de la Atmósfera y Cambio Climático, Universidad Nacional Autónoma de México, Ciudad de Mexico, Mexico
• Licenciatura en Ciencias de la Tierra, Facultad de Ciencias, Universidad Nacional Autónoma de México, Ciudad de Mexico, Mexico
• Instituto Politécnico Nacional (IPN), Escuela Superior de Ingeniería y Arquitectura (ESIA) Unidad Ticomán, Ciudad de México, Mexico

Short Summary

This study investigates how horizontal wind intensity affects raindrop fall speed and its variability at two distinct altitudes in Mexico. It finds that while mean fall speed changes are not statistically significant, wind significantly increases the dispersion of fall speeds, with a more pronounced effect at lower-altitude coastal sites due to higher air density.

Objective

• To characterize how horizontal wind intensity affects raindrop fall speed (vf) and its dispersion for raindrops with diameters between 0.8 mm and 6 mm at a coastal and a continental site in Mexico.
• To assess the limitations of PWS100 disdrometers in ensuring accurate precipitation measurements under windy conditions.

Study Configuration

• Spatial Scale: Measurements were collected at two sites in Mexico:
  • Chamela-Cuixmala Biosphere Reserve (CHAM): Coastal site at 19.50°N, 105.04°W, 91 m above sea level (a.s.l.), 7 km from the Pacific Ocean.
  • Juriquilla Observatory (JQRO): Continental site in Central Mexico at 20.70°N, 100.45°W, 1945 m a.s.l.
  • Raindrop diameters (D) analyzed ranged from 0.8 mm to 6 mm.
• Temporal Scale:
  • CHAM: March 2017 to November 2019 (64 rainfall events).
  • JQRO: February 2015 to December 2018 (133 rainfall events).
  • Data collected with a time resolution of one minute.
  • Wind intensity categorized into: Calm (w < 2 m/s), Light (2 ≤ w < 5 m/s), Moderate (5 ≤ w < 8 m/s), Strong (8 ≤ w < 13 m/s), and Gale (w ≥ 13 m/s).

Methodology and Data

• Models used:
  • Best (1950) equation for theoretical terminal velocity (vt) adjusted for altitude.
  • Foote and du Toit (1969) and Beard (1976) parametric equations for comparison of vt estimates.
  • Statistical tests: t-student tests for mean and standard deviation contrast, Hedges effect size (g) for post-hoc analysis.
• Data sources:
  • Optical present weather sensors (PWS100 disdrometers) for raindrop size and fall speed.
  • WindSonic 4 two-dimensional ultrasonic anemometers for horizontal wind speed and direction.
  • TR-525M tipping-bucket rain gauges for rainfall accumulation.
  • MODIS data and National Hurricane Center (NHC) reports for tropical cyclone context.
  • Climatological data from Comisión Nacional del Agua (CNA).

Main Results

• Comparison of PWS100 and rain gauge data showed significant biases when horizontal wind speed (w) exceeded 10 m/s, indicating instrumental limitations under strong wind conditions.
• Average raindrop fall speed (vf) decreased with increasing wind intensity compared to theoretical terminal velocity (vt), but these differences were not statistically significant.
• Horizontal wind significantly increased the dispersion (standard deviation, σv) of raindrop fall speeds, with this effect strengthening as wind intensity increased.
• The effect of wind on vf varied with raindrop size: small raindrops (D < 1 mm) exhibited both super-terminal and sub-terminal behavior, while mid-sized and large raindrops (D > 1 mm) predominantly showed sub-terminal behavior.
• The impact of wind on raindrop movement was significantly more pronounced at the coastal site (CHAM) compared to the continental site (JQRO). This is attributed to higher air density at the coastal site (1.17 kg/m³ at CHAM vs. 0.97 kg/m³ at JQRO), leading to approximately 20% greater wind kinetic energy and power over raindrops.
• The percentage of terminal raindrops decreased with increasing wind intensity, with a notably higher proportion of non-terminal (predominantly sub-terminal) raindrops observed at the coastal location.

Contributions

• Provides a comprehensive analysis of the influence of horizontal wind intensity on raindrop fall speed and its variability across different raindrop sizes and altitudes (coastal vs. continental).
• Quantifies the significant increase in fall speed dispersion due to wind, highlighting its importance even when mean fall speed changes are not statistically significant.
• Demonstrates that the wind's impact on raindrops is amplified at lower-altitude coastal sites due to higher air density and associated wind power.
• Identifies critical limitations of PWS100 disdrometers and rain gauges for accurate rainfall measurements under strong wind conditions (w > 10 m/s).
• Emphasizes the need to incorporate wind effects into numerical modeling and remote sensing instrumentation to improve rainfall estimation and understanding of microphysical processes.

Funding

• UNAM-DGAPA-PAPIIT (IN103723) program.
• Scholarships from UNAM-DGAPA-PAPIIT (IN103723) and the Graduate and Degree Support Program of the ICAyCC.

Citation

@article{MonteroMartínez2025How,
  author = {Montero‐Martínez, Guillermo and Miranda-Sánchez, Narda Isabella and Ramírez-López, Paloma Josselyn and García‐García, Fernando},
  title = {How does the influence of wind on the fall speed of raindrops change with altitude?},
  journal = {Atmospheric Research},
  year = {2025},
  doi = {10.1016/j.atmosres.2025.108561},
  url = {https://doi.org/10.1016/j.atmosres.2025.108561}
}