Microwave links for rainfall estimation in urban environment: insights from an experimental setup in Luxembourg city
Fenicia, F. ; Pfister, L. ; Kavetski, D. ; Matgen, P. ; Iffly, J.F. ; Hoffman, L. ; Uijlenhoet, R. - \ 2012
Journal of Hydrology 464-465 (2012). - ISSN 0022-1694 - p. 69 - 78.
path-averaged rainfall - dual-frequency - attenuation - resolution - fields - gauges - radar
Although the theoretical aspects of rainfall monitoring through microwave links are quite well established, only few practical applications have evaluated this technique in an operational setting. Microwave links are of particular interest in urban areas, where high frequency measurements are needed due to the fast hydrological response of the system, and link networks are usually already in-place. This study presents the first results of an on-going experiment in Luxembourg-City, which includes two dual-frequency links and several rain gauges at intermediate locations along the links. The experimental set-up allows comparing rain rate estimates based on the individual frequencies as well as estimates based on the difference between the two frequencies. We compared several models for expressing the relationship between attenuation and rain rate, including different baseline estimation methods such as the traditional constant-baseline model and a one-parameter model based on a first order low-pass filter. The models were evaluated using a Bayesian approach and subjected to posterior scrutiny based on several diagnostics. In contrast to previous research, our results indicated that estimates based on the attenuation difference appeared poorer than the estimates based on individual frequencies. The one-parameter baseline estimation method provided consistently better results than the traditional constant-baseline method, which justifies the increased model complexity. Uncertainty of model predictions was relatively large for low intensity rainfall, which highlights one of the limitations of this technique. Models were validated in different periods and on different links, in some cases demonstrating large bias. Model parameters were generally well-identifiable, though uncertainty in the rainfall predictions appeared under-estimated in some cases.
Errors and uncertainties in microwave link rainfall estimation explored using drop size measurements and high-resolution radar data
Leijnse, H. ; Uijlenhoet, R. ; Berne, A.D. - \ 2010
Journal of Hydrometeorology 11 (2010). - ISSN 1525-755X - p. 1330 - 1344.
path-averaged rainfall - x-band radar - dual-frequency - communication-networks - video disdrometer - weather radar - axis ratios - attenuation - matrix - distributions
Microwave links can be used for the estimation of path-averaged rainfall by using either the path-integrated attenuation or the difference in attenuation of two signals with different frequencies and/or polarizations. Link signals have been simulated using measured time series of raindrop size distributions (DSDs) over a period of nearly 2 yr, in combination with wind velocity data and Taylor’s hypothesis. For this purpose, Taylor’s hypothesis has been tested using more than 1.5 yr of high-resolution radar data. In terms of correlation between spatial and temporal profiles of rainfall intensities, the validity of Taylor’s hypothesis quickly decreases with distance. However, in terms of error statistics, the hypothesis is seen to hold up to distances of at least 10 km. Errors and uncertainties (mean bias error and root-mean-square error, respectively) in microwave link rainfall estimates due to spatial DSD variation are at a minimum at frequencies (and frequency combinations) where the power-law relation for the conversion to rainfall intensity is close to linear. Errors generally increase with link length, whereas uncertainties decrease because of the decrease of scatter about the retrieval relations because of averaging of spatially variable DSDs for longer links. The exponent of power-law rainfall retrieval relations can explain a large part of the variation in both bias and uncertainty, which means that the order of magnitude of these error statistics can be predicted from the value of this exponent, regardless of the link length.
Microwave link rainfall estimation: Effects of link length and frequency, temporal sampling, power resolution, and wet antenna attenuation
Leijnse, H. ; Uijlenhoet, R. ; Stricker, J.N.M. - \ 2008
Advances in Water Resources 31 (2008). - ISSN 0309-1708 - p. 1481 - 1493.
path-averaged rainfall - dual-frequency - weather radar - band - raindrops - fields - model - shape
Issues associated with microwave link rainfall estimation such as the effects of spatial and temporal variation in rain, the nonlinearity of R¿k relations, temporal sampling, power resolution, and wet antenna attenuation are investigated using more than 1.5 years of data from a high-resolution X-band weather radar. Microwave link signals are generated for different link frequencies and lengths from these radar data, so that retrieved path-averaged rainfall intensities can be compared to true path-averaged values. Results of these simulations can be linked to the space¿time structure of rain. A frequency-dependent relation between the rainfall intensity at an antenna and the attenuation caused by its wetting is derived using microwave link and rain gauge data. It is shown that if the correct temporal sampling strategy is chosen, the effects of the degradation of power resolution and of wet antenna attenuation (if a correction is applied) are minor (i.e., MBE and bias-corrected RMSE are >¿20% and
Hydrometeorological application of a microwave link: 2. Precipitation
Leijnse, H. ; Uijlenhoet, R. ; Stricker, J.N.M. - \ 2007
Water Resources Research 43 (2007). - ISSN 0043-1397 - p. W04417 - W04417.
path-averaged rainfall - dual-frequency - attenuation - radar - band - resolution - hydrology - gauges - size
The suitability of a 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 a 4.89-km microwave link and a line configuration of seven tipping bucket rain gauges 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 rain gauge data show that the instrument is indeed well suited for the measurement of path-averaged rainfall