CFD-based Numerical Studies of Rainfall Disdrometer: Aerodynamic Design and Correction of Wind-Induced Errors

Investigators

Emad Habib, Witold Krajewski, and Anton Kruger  

Overview

Knowledge of precipitation amount and distribution is essential for hydrologic prediction and a wide range of engineering design applications. In addition to rainfall quantities, commonly obtained using rain gauges, information about raindrop size distribution (DSD) is also valuable. A recent instrument specially designed for DSD measurements is the 2-D video disdrometer recently deployed and operated by the IIHR Hydrometeorology group in several NASA-sponsored field experiments in Florida, Brazil, and Marshall Islands in 1998-99.

 In-situ measurements of rainfall are subject to the wind effect a major source of errors. The 2-D video disdrometer body causes distortion of the wind field resulting in modified trajectories of the raindrops. This leads to a reduction in the catchment efficiency of the instrument. This study examines the potential use of a computational fluid dynamics (CFD) technique that can be used for quantification and correction of wind-related rainfall measuring errors. The distorted wind field around and inside the instrument's body is simulated using a three-dimensional numerical model.  A modified geometry of the instrument, suggested for operational purposes, is tested numerically.  Trajectories of raindrops are simulated to investigate the wind effect on the catchment efficiency of the instrument.  A stochastic Lagrangian particle-tracking model that accounts for the turbulence effect is examined.  General guidelines related to aerodynamic aspects of the design of in-situ rainfall measuring devices are outlined.  

References

Habib, E., and W.F. Krajewski, 2001: An Example of Computational Approach Used for Aerodynamic Design of a Rain Disdrometer. Journal of Hydraulic Research (in press). 

Habib, E., Krajewski, W.F.,  Nespor, V., and  A. Kruger, 1999. Numerical Simulation Studies of Raingage Data Correction due to Wind Effect. Journal of Geophysical Research, 104(D16), 19723-19734.

Nespor, V., W.F. Krajewski, and A Kruger, 2000: Wind-Induced Error of Raindrop Size Distribution Measurement Using a Two-Dimensional Video Disdrometer. Journal of Atmospheric and Oceanic Technology: Vol. 17, No. 11, pp. 1483–1492.

Numerical Simulations of Wind Flow Around the Disdrometer Body: contour lines of horizontal flow velocity

Stochastic particle tracking model is used to simulate the trajectories of raindrops and their distortion due to wind effect