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Boundary-Layer Meteorol (2011) 139:167191 DOI 10.1007/s10546-010-9580-2
ARTICLE
Radiation Fog. Part I: Observations of Stability and Drop Size Distributions
Jeremy Price
Received: 19 January 2010 / Accepted: 20 December 2010 / Published online: 22 January 2011 Crown Copyright 2011
Abstract Data from several cases of radiation fog occurring at the Met Ofce eld site at Cardington, Bedfordshire, UK have been analyzed with a view to elucidating the typical evolution in its static stability from formation to dissipation. Typically the early stages of radiation fog are characterized by a stable thermal prole and a relatively shallow depth. However, when the fog reached approximately 100m depth it was observed to become optically thick (to longwave radiation), with a subsequent change over several hours to a saturated adiabatic stability prole. At this time turbulence levels were seen to increase significantly. The mechanisms involved appear to be radiative cooling from fog top and a positive heat ux to the atmosphere from the soil. The importance of this change in stability for numerical modelling of fog episodes is discussed. Several case studies are made to gain some insight into how common this transition is. Droplet spectra were measured at 2-m height for many of the cases considered, and their evolution is discussed. It is found that distributions fall into an initial phase with small drop sizes (approximately 10 m diameter) and concentration,
and a mature phase with the appearance of much larger drop sizes with a mean diameter of approximately 1520 m. It is found that the appearance of the mature phase does not
coincide with the change in stability from stable to saturated adiabatic, but there is some evidence that once a saturated adiabatic prole is established, the droplet spectra variations are significantly less than for the stable period. The observed evolution of these spectra brings into question the suitability of microphysical schemes that assume constant spectral shape, drop diameter, and number density.
Keywords Fog Observations Radiation fog Stable boundary layer Visibility
J. Price (B)
Met Ofce Research Unit, Cardington Aireld, Shortstown, Bedfordshire MK42 0SY, UK e-mail: [email protected]
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168 J. Price
1 Introduction
Fog is clearly an important meteorological phenomenon to forecast correctly, since poor prediction of fog leads to greater disruption to air, sea and surface transport with...