Climate dynamics

Abstract:

Significant differences in heating rates are found when two solar irradiance spectra are used in a line-by-line radiative transfer code. Compared with a spectrum of recent satellite data an older theoretical spectrum gives 20-40% more heating in the ozone Hartley band, important in the upper stratosphere. The spectra are implemented in a broadband radiation code to which some improvements are also made to the ozone absorption parameterization. A widely-used spectrum of ground-based data from 1960s gives somewhat lower heating rates. The effects of the changes in the spectrum, and the broad-band scheme, on the temperatures simulated by a middle atmosphere GCM are investigated. The model has previously shown a warm bias, compared with climatology, around the stratopause but this is significantly reduced when the former spectrum is substituted for the latter, and the new ozone parameterization incorporated. The change in spectrum accounts for two-thirds of the improvement. Copyright 2008 by the American Geophysical Union.

Abstract:

The atmospheric circulation changes predicted by climate models are often described using sea level pressure, which generally shows a strengthening of the mid-latitude westerlies. Recent observed variability is dominated by the Northern Annular Mode (NAM) which is equivalent barotropic, so that wind variations of the same sign are seen at all levels. However, in model predictions of the response to anthropogenic forcing, there is a well-known enhanced warming at low levels over the northern polar cap in winter. This means that there is a strong baroclinic component to the response. The projection of the response onto a NAM-like zonal index varies with height. While at the surface most models project positively onto the zonal index, throughout most of the depth of the troposphere many of the models give negative projections. The response to anthropogenic forcing therefore has a distinctive baroclinic signature which is very different to the NAM. Copyright 2008 by the American Geophysical Union.

Abstract:

A synoptic situation termed 'high-latitude blocking' (HLB) is shown to occur frequently in both the Atlantic and Pacific sectors, and to result in flow anomalies very similar to those associated with the negative phase of the Northern Annular Mode (NAM) in the respective sector. There is a weak but significant link between the occurrence of HLB in the two sectors, with Atlantic HLB tending to lead Pacific HLB by 1-3 days. This link arises from rare events in which both sectors are almost simultaneously affected by a large-scale wave-breaking event which distorts the polar trough over Northern Canada. In several cases the tropospheric wave-breaking occurs in tandem with a large-scale disturbance of the stratospheric polar vortex. There is, therefore, a physical link between the Atlantic and Pacific sectors, but analysis suggests that this does not contribute to determining the pattern of the NAM, as conventionally defined from monthly mean data. However, an alternative version of the NAM, derived directly from daily data, does appear to reflect this physical link. These conflicting results highlight the sensitivity of the NAM to the period over which data are averaged. Copyright © 2008 Royal Meteorological Society.

Abstract:

The High Resolution Dynamics Limb Sounder (HIRDLS) instrument was launched oil the NASA Aura satellite in July 2004. HIRDLS is a joint project between the UK and USA, and is a mid-infrared limb emission sounder designed to measure the concentrations of trace species, Cloud and aerosol, and temperature and pressure variations in the Earth’s atmosphere front the upper troposphere to the mesophere. The instrument is intended to make measurements at both high vertical and horizontal spatial resolutions, but validating those measurements is difficult because few other measurements provide that vertical resolution sufficiently closely in time. However, the FOPMOSAT-3/COSMIC suite of radio occultation satellites that exploit the U.S. GPS transmitters to obtain high resolution (similar to 1 km) temperature profiles in the stratosphere does provide sufficient profiles nearly coincident with those from HIRDLS. Comparisons show a good degree intercorrelation between COSMIC and HIRDLS down to about 2 km resolution, with similar amplitudes for each, implying that HIRDLS and COSMIC are able to measure the same small scale features. The optical blockage that occurred within HIRDLS during launch does not seem to have affected this capability.