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Simon R. Osborne and Alan Lapworth

Abstract

In the autumn of 2016, an undular atmospheric bore passed over southern England. Observations at surface sites as the bore passed gave phase speeds of up to 30 m s−1, surface pressure rises of over 1.5 hPa, and abrupt wind direction changes of 90°, followed by slower changes of a further 180°. Use was made of this event, rarely observed in the United Kingdom, to compare surface and satellite observations of the bore with output from the operational Met Office limited-area 1.5-km numerical weather model and to investigate the bore initiation mechanism within the model. Although the model had timing errors of over an hour and orientation errors of the bore, the bore propagation was simulated fairly well, giving similar bore phase speeds and decay times. There was also a reasonable correlation between surface and satellite observations and the model. One significant difference was the longer bore wavelength in the model simulation.

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Simon R. Osborne and Graham P. Weedon

Abstract

A meteorological drought in 2018 led to senescence of the C3 grass at Cardington, Bedfordshire, United Kingdom. Observations of near-surface atmospheric variables and soil moisture are compared to simulations by the JULES land surface model (LSM) as used for Met Office forecasts. In years without drought, JULES provides better standalone simulations of evapotranspiration (ET) and soil moisture when the canopy height and rooting depth are reduced to match local conditions. During drought with the adjusted configuration, JULES correctly estimates total ET, but the components are in the wrong proportions. Several factors affect the estimation of ET including modeled skin temperatures, dewfall, and bare-soil evaporation. A diurnal range of skin temperatures close to observed is produced via the adjusted configuration and doubling the optical extinction coefficient. Although modeled ET during drought matches observed ET, this includes simulation of transpiration but in reality the grass was senescent. Excluding transpiration, the modeled bare-soil evaporation underestimates the observed midday latent heat flux. Part of the missing latent heat may relate to inappropriate parameterization of hydraulic properties of dry soils and part may be due to insufficient evaporation of dew. Dew meters indicate dewfall of up to 20 W m−2 during drought when the surface is cooling radiatively and turbulence is minimal. These data demonstrate that eddy-covariance techniques fail to reliably record the times, intensity, and variations in negative latent heat flux. Furthermore, the parameterization of atmospheric turbulence as used in LSMs fails to represent accurately dewfall during calm conditions when the surface is radiatively cooled.

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Paul Glantz, Kevin J. Noone, and Simon R. Osborne

Abstract

Comparisons of counterflow virtual impactor (CVI) and forward scattering spectrometer probe (FSSP) measurements of cloud droplet number concentrations obtained by two different aircraft in marine boundary layer stratocumulus clouds are presented. The cut sizes of the CVIs have been calculated using a mathematical model (DLH50) and using Stokes' theory (DS50). For most of the cases the agreement between DLH50 and DS50 is within 3%. Based on the results obtained with the mathematical model the agreement between CVI and FSSP concentrations is within 25% for most of the cases analyzed in this study and also within 10% for several of the cases. The reasonable agreement between CVI and FSSP concentrations supports the assumption that a droplet releases a single residual particle upon evaporation. Furthermore, in this study it is shown that droplet shattering can qualitatively be identified and it occurs often when there are elevated concentrations of drizzle droplets in the clouds. Poor agreement between the CVI and FSSP was found in a few instances. For one of these cases the discrepancy obtained between the CVI and FSSP seems to have been caused by losses in the CVI mainly by cloud droplets with larger sizes.

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