Path-Average Rainfall Estimation from Optical Extinction Measurements Using a Large-Aperture Scintillometer

R. Uijlenhoet,J.-M. Cohard,M. Gosset

doi:10.1175/2011jhm1350.1

Abstract

Abstract The potential of a near-infrared large-aperture boundary layer scintillometer as path-average rain gauge is investigated. The instrument was installed over a 2.4-km path in Benin as part of the African Monsoon Multidisciplinary Analysis (AMMA) Enhanced Observation Period during 2006 and 2007. Measurements of the one-minute-average received signal intensity were collected for 6 rainfall events during the dry season and 16 events during the rainy season. Using estimates of the signal base level just before the onset of the rainfall events, the optical extinction coefficient is estimated from the path-integrated attenuation for each minute. The corresponding path-average rain rates are computed using a power-law relation between the optical extinction coefficient and rain rate obtained from measurements of raindrop size distributions with an optical spectropluviometer and a scaling-law formalism for describing raindrop size distribution variations. Comparisons of five-minute rainfall estimates with measurements from two nearby rain gauges show that the temporal dynamics are generally captured well by the scintillometer. However, the instrument has a tendency to underestimate rain rates and event total rain amounts with respect to the gauges. It is shown that this underestimation can be explained partly by systematic differences between the actual and the employed mean power-law relation between rain rate and specific attenuation, partly by unresolved spatial and temporal rainfall variations along the scintillometer path. Occasionally, the signal may even be lost completely. It is demonstrated that if these effects are properly accounted for by employing appropriate relations between rain rate and specific attenuation and by adapting the pathlength to the local rainfall climatology, scintillometer-based rainfall estimates can be within 20% of those estimated using rain gauges. These results demonstrate the potential of large-aperture scintillometers to estimate path-average rain rates at hydrologically relevant scales.

Highlights

Large-aperture boundary layer scintillometers are becoming standard, commercially available, tools for estimating the turbulent sensible heat flux in the atmospheric surface layer over scales of hydrological and meteorological interest, from a few hundreds of meters to several kilometers (e.g., de Bruin et al 1995; Chehbouni et al 1999; Meijninger and de Bruin 2000; Cain et al 2001; Lagouarde et al 2002; Meijninger et al 2002b; Beyrich et al 2002)
Moumouni et al (2008) have presented a detailed analysis of the main features of raindrop size distributions observed during the African Monsoon Multidisciplinary Analysis (AMMA) field campaign with the mentioned optical spectropluviometer installed about 7 km east of the scintillometer path
1.31 based on the Z–R relations listed in Battan (1973), whereas the exponents are somewhat smaller. This implies that, in Benin, relatively small optical extinctions correspond already to significant rain rates or, vice versa, that relatively large rain rates give rise to moderate extinctions. This is related to the fact that all rainfall in Benin is characterized by drop size distributions with a deficit of small drops and relatively low-density rain with large mean drop sizes (Moumouni et al 2008)

Summary

Introduction

Large-aperture (near infrared) boundary layer scintillometers are becoming standard, commercially available, tools for estimating the turbulent sensible heat flux in the atmospheric surface layer over scales of hydrological and meteorological interest, from a few hundreds of meters to several kilometers (e.g., de Bruin et al 1995; Chehbouni et al 1999; Meijninger and de Bruin 2000; Cain et al 2001; Lagouarde et al 2002; Meijninger et al 2002b; Beyrich et al 2002) Their measurement principle is based on the estimation of the path-average structure parameter of the refractive index of air from the measured variance of the logarithmic intensity fluctuations (scintillations) of the received signal, which are linearly related to each other under certain conditions (Wang et al 1978). The measurement principle is based on the use of a power-law relation to convert the average extinction per unit of pathlength estimated from the link to a path-average rain rate

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