Akerboom, M

Jewell, SA

Norman, K

Ochoa-Rodriguez, S

Onof, C

Sandford, C

Wang, L

Spiral - Imperial College Digital Repository

• Overall performance of super-resolution (SR) product is good: it can well capture storm dynamics and small scale rain structures (Fig.3). • Quantitatively, SR QPEs generally perform well in terms of accumulation (often better than traditional radar products), but show a tendency to underestimate high rainfall rates, more so than the other radar products (Fig. 4). • Sporadic problems due to issues such as wind drift and higher noise power lead to unstable performance (Fig. 5).  time Evaluation of the Met Office super-resolution  C-band radar rainfall product over London 1. WHY HIGH RESOLUTION RAIN DATA? 3. PILOT AREA AND DATASET The quality and added value of the super-resolution QPEs for urban hydrological applications were evaluated using as case study 6 storm events observed in the Cranbrook catchment (8.5 km2; NE London, Fig.1), for which local dense rain gauge (RG) and runoff records, and a hydraulic model  were available (Fig.2). [1] Jewell, S. A. & Gaussiat, N. (2015). An assessment of kriging-based rain-gauge-radar merging techniques. Quarterly Journal of the Royal Meteorological Society. [2] Sandford, C. (2015). Correcting for wind drift in high resolution radar rainfall products: a feasibility study. Journal of Hydrology (In Press). S. Ochoa-Rodriguez1*, C. Sandford2, K. Norman2, L.-P. Wang3, S. Jewell2, M. Akerboom1,4, C. Onof1 1Imperial College London (*sochoaro@ic.ac.uk); 2Met Office, UK;  3KU Leuven, BE; 4TU Delft, NL High resolution rainfall estimates are required for urban hydrological applications in order to well represent the high spatial variability, fast runoff processes and short response times of urban catchments. As part of the RainGain project and with the purpose of meeting the stringent requirements of urban-scale hydrological applications, a high spatial resolution (100 m/5 min) radar rainfall product has been developed and trialled over the Greater London Area. 2. SUPER-RESOLUTION C-BAND RADAR PRODUCT The UK Met Office (UKMO) operates  a network of 18 C-band radars across the UK. These radars are normally operated in “long-pulse” mode and provide QPEs at resolutions of  500 m and 1 km in space and 5 min in time. Higher spatial resolution or “super-resolution” (100 m / 5 min) QPEs over the Greater London Area were generated using data from two dual polarisation C-band radars (Fig.1) operating in “short-pulse” mode with a pulse length of  0.5 microseconds (75 m) and half power beam width of 1 degree. QPEs from each radar were interpolated onto a 100 m resolution grid every 5 minutes, and the use of advection and rain-gauge merging techniques to improve rainfall accumulation estimates are being investigated [1,2].  Fig. 1: UKMO C-band radars used to generate super-resolution QPEs Fig. 2: Pilot area Cranbrook catchment: Sewer network and  monitoring system 4. RESULTS 5. CONCLUSIONS The super-resolution C-band radar product shows great potential to provide high-resolution high-accuracy QPEs suitable for urban hydrological applications. However, its performance is currently unstable due to issues such as wind drift [2] and signal-to-noise ratio (SNR). 100 m: 19/09/2014 500 m: 19/09/2014 1 km: 19/09/2014100 m: 19/09/2014 500 m: 19/09/2014 1 km: 19/09/2014Accumulations (mm)Max Intensity (mm/h)RD Accumulations (mm) RG Accumulations (mm) RD Intensity (mm/h) RG Intensity (mm/h) Flow Depth (mm) time Fig. 3: Super resolution QPEs (100 m/5 min) vs. traditional radar QPEs (500 m – 1 km/5 min) and local rain gauges during intense event on 19/09/2014: (a) Event accumulations; (b) Max. intensity; (c) Areal average rain rate profiles; (c) Simulated vs. observed water depths Fig. 4: Areal Av. radar-rain gauge accumulation and intensity scatterplots (combined results of all storm events) Rain Rate (mm/h) Fig. 5: Examples of problems affecting quality of SR product: Top- rain void; Bottom- shift  (a) (b) (c) (d) 

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Evaluation of the Met Office super-resolution C-band radar rainfall product over London

Ochoa Rodriguez, S

Evaluation of the Met Office super-resolution C-band radar rainfall product over the Greater London Area Susana Ochoa-Rodriguez, Caroline Sandford, Katie Norman, Li-Pen Wang, Sharon Jewell, Maria Akerboom, Christian Onof Urban catchments are characterised by high spatial variability and fast runoff processes and are therefore very sensitive to the spatial and temporal variability of precipitation. Consequently, high resolution precipitation estimates are required in order to properly represent urban runoff processes. A high spatial resolution (100 m / 5 min) radar rainfall rate product has been developed and trialled over the Greater London Area in order to meet this requirement as part of the North-West Europe Interreg IVB funded project RainGain. Quantitative precipitation estimates (QPEs) were generated using data from two operational dual polarisation C-band radars operating in “short-pulse” mode with a pulse length of  0.5 microseconds (or 75 m) and half power beam width of 1 degree. QPEs from each radar were interpolated onto a 100 m resolution grid every 5 minutes, and the use of advection and rain-gauge merging techniques to improve rainfall accumulation estimates was investigated. The quality and benefit of these high resolution QPEs for urban hydrological applications is evaluated using as case study 6 storm events observed in a small urban catchment in North-East London for which dense rain gauge and sewer flow records, as well as a recently-calibrated high-resolution urban drainage model were available. Results show that the high-resolution radar product performs well in terms of accuracy and captures the small-scale structure of rainfall fields. When comparing the use of higher spatial resolution radar QPEs and the 1 km / 5 minute operational Met Office radar QPEs as input for urban hydrological modelling, improvements are only tangible in the case of highly spatially variable storms. Further work is required to better understand the interaction and dependence between rainfall input and the resolution of hydrological models.  

Evaluation of the Met Office super-resolution C-band radar rainfall product over London

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