Morphology of HCN and CN in Comet Hale-Bopp (1995 O1)

We compare images of Comet Hale-Bopp (1995 O1) in HCN and CN taken near perihelion (April 1, 1997) to determine the origin of CN in comets. We imaged the J=1→0 transition of HCN at λ=3 mm with the BIMA Array. Data from two weeks around perihelion were summed within four phase bins based on the rotational period of the comet. This increases both the signal-to-noise ratio and the u-v coverage while decreasing the smearing of the spatial features. The similarly phased narrowband CN images were taken at Lowell Observatory within the same range of dates as the HCN images. We find that there is a better correlation between HCN and CN than between HCN and the optically dominant dust. If the CN in jets does have a dust source it would have to have a very low albedo and/or small particle size. The production rates are consistent with HCN being a primary parent of CN, although there are discrepancies between the HCN destruction scalelength and the CN production scalelength which we discuss.     

Observations and Simulations of Cold Air Pooling in Valleys

A series of surveys was carried out with an instrumented vehicle along the A361 in south-west England between 20 January and 4 April 2007, primarily during clear calm nights in order to observe the formation of pools of cold air in valleys. High resolution simulations were performed for three of these cases using the Met Office Unified Model (MetUM®), which was shown to successfully reproduce many of the features of the observed along-route variations in air temperature. Analysis of the observations and the results of the simulations suggest that the cooling of air within these moderate valleys was the result of sheltering, which reduces the turbulent flux of heat down from aloft. Valleys that experience a large degree of cooling are those that become fully decoupled, allowing the near-surface air within the valley to cool rapidly in response to the surface longwave radiative cooling. The behaviour of the valley cooling intensity is shown to be similar to that observed in idealised simulations of the sheltering mechanism (Vosper and Brown, 2008, Boundary-Layer Meteorol 127:429–448).