Abstract
AbstractRefractivity 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 shorter wavelengths, 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 1 h 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 4 km × 4 km area) indicates that ΔN errors are about 1 N unit, in agreement with in situ observations. Measurements from an instrumented tower on summer days show that the 1 h 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, 4 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 grid lengths during summer. However, the daily time series of 1 h 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.
Highlights
It has been generally acknowledged that water vapour is quite poorly represented in numerical weather prediction (NWP) models (Dabbert and Schlatter, 1996; National Research Council, 1998)
Given the limitations for absolute refractivity retrievals with magnetron-based C-band radars, in this article we shall investigate whether the information contained in retrievals of 1 h refractivity changes with about 4 km spatial resolution has the potential to improve the representation of near-surface humidity in NWP forecast models
Refractivity time series derived from instantaneous in situ measurements of temperature, pressure and humidity show a significant degree of variability at small scales (e.g. < 4 km)
Summary
It has been generally acknowledged that water vapour is quite poorly represented in numerical weather prediction (NWP) models (Dabbert and Schlatter, 1996; National Research Council, 1998). Given the limitations for absolute refractivity retrievals with magnetron-based C-band radars, in this article we shall investigate whether the information contained in retrievals of 1 h refractivity changes with about 4 km spatial resolution has the potential to improve the representation of near-surface humidity in NWP forecast models. This is achieved through the comparison of 1 h refractivity changes obtained from both radar retrievals and NWP model output with in situ observations from a surface weather station.
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