Special Issue paper
The potential of one-hour refractivity changes from an operational C-band magnetron-based radar for NWP validation and data assimilation
This article is protected by copyright. All rights reserved.
Quarterly Journal of the Royal Meteorological Society
- Accepted manuscript online: 2 JUL 2013 12:20PM EST
- Manuscript Accepted: 29 JUN 2013
- Manuscript Revised: 13 MAY 2013
- Manuscript Received: 24 JUN 2012
- Cited By
- Boundary-layer humidity;
- convective initiation;
Refractivity changes (ΔN) derived from radar ground clutter returns serve as a proxy for near-surface humidity changes (1 N unit ≡ 1% relative humidity at 20°C). Previous studies have indicated that better humidity observations should improve forecasts of convection initiation. A preliminary assessment of the potential of refractivity retrievals from an operational magnetron-based C-band radar is presented. The increased phase noise at C-band, exacerbated by the unknown position of the target within the 300-m gate, make it difficult to obtain absolute refractivity values, so we consider the information in one-hour changes. These have been derived to a range of 30 km with a spatial resolution of ~4 km; the consistency of the individual estimates (within each 4km × 4km area) indicate ΔN errors are about 1 N unit, in agreement with in-situ observations. Measurements from an instrumented tower on summer days show that the one-hour refractivity changes up to a height of 100 m remain well correlated with near-surface values. The analysis of refractivity as represented in the operational Met Office Unified Model at 1.5-km, 4-km and 12-km grid lengths demonstrates that as model resolution increases, the spatial scales of the refractivity structures improve. It is shown that the magnitude of refractivity changes is progressively underestimated at larger gridlengths during summer. However, the daily time series of one-hour refractivity changes reveal that, whereas the radar derived values are very well correlated with the in-situ observations, the high-resolution model runs have little skill in getting the right values of ΔN in the right place at the right time. This suggests that the assimilation of these radar refractivity observations could benefit forecasts of the initiation of convection.