Large-scale urbanization and potential acceleration of the earth's water cycle greatly increase the vulnerability of lives and property to natural disasters, such as flash floods.  Especially, heavy development decreases the response time of urban watersheds to rainfall and increases the chance of localized flooding events over a small spatial domain.  Successful warning of urban floods requires high spatiotemporal resolution, accurate precipitation estimation because of the rapid flood response as well as the complex hydrologic and hydraulic characteristics in an urban environment.  Radar observation of high temporal and spatial resolutions has shown great potential to drive flood forecasting by incorporating distributed hydrological models.  Derivation of quantitative rainfall estimates from radar observations is known as a challenging process.  This paper reviews various aspects in radar rainfall mapping in urban coverage using X-band radar networks.  By reducing the maximum range and operating at X-band, one can ensure good azimuthal resolution with a small-size antenna and keep the radar beam closer to the ground.  The networked topology helps to achieve satisfactory sensitivity and fast temporal update across the coverage.  Strong clutter is expected from buildings in the neighborhood which act as perfect reflectors.  The reduction in radar size enables flexible deployment, such as rooftop installation, with small infrastructure requirement, which is critical in a metropolitan region.  Dual-polarization based technologies can be implemented for real-time mitigation of rain attenuations.  Particularly, the specific differential propagation phase (K_dp) has higher sensitivity at X-band compared to the non-attenuating frequencies.  It is attractive to use the specific differential propagation phase to reliably estimate rainfall because it is immune to rain attenuation, calibration biases, partial beam blockage, and hail contamination.  Preliminary results will be presented to demonstrate the capabilities and potentials of an X-band dual-polarization radar network for urban flood warning.  The above results are discussed in the context of an example of urban X-band radar network deployed in the metropolitan Tokyo area for flood and landslide monitoring.

Diagram of the conceptual model for radar rainfall mapping and urban hydrological application for flooding risk assessment and monitoring in the metropolitan Tokyo area.