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Wageningen Universiteit (2007)

Hydrometeorological application of microwave links : measurement of evaporation and precipitation

Leijnse, H

Titre : Hydrometeorological application of microwave links : measurement of evaporation and precipitation

Auteur : Leijnse, H

Université de soutenance : Wageningen Universiteit

Grade : PhD thesis 2007

Descriptif partiel
Chapter 1. This thesis deals with the estimation of evaporation and precipitation using microwave links. Measurement of both of these vertical water fluxes at the land-atmosphere interface at regional scales is very important in many disciplines such as hydrology and meteorology. As the scale at which microwave links operate is the scale of interest of many applications, microwave link measurements offer a clear advantage over point-scale measurements. Other instruments have disadvantages that make their measurements potentially less accurate than those from microwave links. However, microwave links are by no means perfect. Therefore microwave links, like all other instruments, will be of most benefit if used in combination with other instruments. There is great potential for the use of microwave links from very dense existing commercial cellular communication networks, especially for the estimation of rainfall, which makes research on these instruments highly worthwhile and relevant.
Chapter 2. A method to estimate areal evaporation using a microwave link (radio wave scintillometer) in combination with an energy budget constraint is proposed. This Radio Wave Scintillometry-Energy Budget Method (RWS-EBM) is evaluated for its applicability in different meteorological conditions and for its sensitivity to several variables (the structure parameter of the refractive index of air, the total available energy, the wind velocity, the effective average vegetation height and the correlation coefficient between the temperature and humidity fluctuations). The method is shown to be best suited for use in wet to moderately dry conditions, where the latent heat flux is at least a third of the total available energy (i.e. Bowen ratio ≤ 2). It is important to accurately measure the total available energy and the wind velocity, as the RWS-EBM is most sensitive to these variables. The Flevoland field experiment has provided data, obtained with a 27-GHz microwave link (over 2.2 km), a Large Aperture Scintillometer (LAS, also 2.2 km) and four eddy covariance (EC) systems, which are used to test the RWS-EBM. Comparing 92 daytime measurements (30-minute intervals) of the evaporation estimated using the RWS-EBM to that determined in alternative manners (eddy covariance and two-wavelength scintillometry) leads to the conclusion that the method provides consistent estimates (coefficient of determination r2 = 0.85 in both cases) under relatively wet conditions.
Chapter 3. The suitability of the same 27-GHz microwave link for measuring path-averaged precipitation is investigated. Theoretical analyses show that the specific attenuation of an electromagnetic signal at this frequency varies nearly linearly with the rainfall intensity, which is ideal for line-integrating instruments. The dependence of this relation on the drop size distribution and on the temperature is small, so that uncertainties in these variables do not play large roles in the estimation of rainfall intensity. Data from an experiment with this microwave link (now spanning 4.89 km) and a line configuration of seven tipping-bucket raingauges are used to test whether this instrument is indeed suitable for the estimation of path-averaged rainfall. Results from this experiment show that the attenuation due to wet antennas can have a significant effect on the retrieved rainfall intensity. However, when a two-parameter wet antenna correction function is applied to the link data, comparisons with the raingauge data show that the instrument is indeed well-suited for the measurement of path-averaged rainfall.
Chapter 4. There is actually an existing network of microwave links over nearly the entire land area of the world. These links are part of commercial cellular communication networks. An analysis of data from two such links (both operating at 38 GHz) collected during eight rainfall events over a two-month period (Oct.–Nov. 2003) during mostly stratiform rainfall in The Netherlands is presented. Comparisons between the time series of rainfall intensities estimated using the microwave links and those measured by a nearby rain gauge and a composite of two C-band weather radars show that the dynamics of the rain events is generally well-captured by the radio links. This shows that such links are potentially a valuable addition to existing methods of rainfall estimation, provided the uncertainties related to the reference signal level
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Mots clés : evaporation / precipitation / measurement / hydrology / meteorology / measurement techniques / microwaves

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