D. P. Wareing

The Convective Storm Initiation Project

The Convective Storm Initiation Project (CSIP) is an international project to understand precisely where, when, and how convective clouds form and develop into showers in the mainly maritime environment of southern England. A major aim of CSIP is to compare the results of the very high resolution Met Office weather forecasting model with detailed observations of the early stages of convective clouds and to use the newly gained understanding to improve the predictions of the model. A large array of ground-based instruments plus two instrumented aircraft, from the U.K. National Centre for Atmospheric Science (NCAS) and the German Institute for Meteorology and Climate Research (IMK), Karlsruhe, were deployed in southern England, over an area centered on the meteorological radars at Chilbolton, during the summers of 2004 and 2005. In addition to a variety ofground-based remote-sensing instruments, numerous rawinsondes were released at one- to two-hourly intervals from six closely spaced sites. The Met Office weather radar network and Meteosat satellite imagery were used to provide context for the observations made by the instruments deployed during CSIP. This article presents an overview of the CSIP field campaign and examples from CSIP of the types of convective initiation phenomena that are typical in the United Kingdom. It shows the way in which certain kinds of observational data are able to reveal these phenomena and gives an explanation of how the analyses of data from the field campaign will be used in the development of an improved very high resolution NWP model for operational use.

Atmospheric temperature measurements made by rotational Raman scattering

Rotational Raman scattering of light from the second harmonic of a Nd:YAG laser is used to measure atmospheric temperature at altitudes of 3 to 20 km. The method relies on taking the ratio of light passed by two filters that differ slightly in their passbands, and is therefore insensitive to variations in atmospheric transmission. The calibration of the experiment relies on only spectroscopic measurements and not on normalizing to nearby radiosondes, as has been the previous practice with this kind of experiment. Comparisons with radiosonde profiles show excellent agreement to within the precision of the measurements and the variability of the atmosphere. The main application of the experiment lies in its ability to measure a time series of temperature profiles with good height resolution throughout a night. Examples of such series that show the passage of a tropopause fold above the lidar are presented.

Upper stratospheric and mesospheric temperatures derived from lidar observations at Aberystwyth

From lidar observations of relative atmospheric density above Aberystwyth (52.4°N, 4.1°W) upper stratospheric and mesospheric temperatures have been derived for a total of 93 nights between December 1982 and February 1985. Excellent agreement was found between radiances synthesised from these temperatures and those measured by satellite-borne instruments. Summer temperatures showed a smooth and regular variation with altitude and reasonably good agreement with the CIRA (1972) model atmosphere. By contrast, winter temperatures showed a much greater variability with altitude and greater changes from night to night, with the frequent occurrence of a large amplitude wave-like perturbation in the mesosphere with about 15 km vertical wavelength and amplitude about 20K between 60 and 80 km. Pronounced warmings of the stratosphere were observed during the three winters of observation. During the warming event occurring in early February 1983 the stratopause temperature increased to 303K at 43 km, while the major warming event of late December 1984/early January 1985 produced a stratospheric temperature gradient of 16K km−1 between 34 and 36 km. During the latter event a distinct local temperature minimum at 32.6 km was observed on New Years Eve, this descending to 29 km by the following night and being accompanied by a lowering of the stratopause from 43 to 38.5 km in the same period. These results demonstrate the ability of the present technique to resolve the high stratopause temperatures and steep stratospheric temperature gradients which occur during stratospheric warmings, in marked contrast to the limited resolution achieved by satellite experiments.

Lidar observations of ice crystals associated with noctilucent clouds at middle latitudes

Lidar observations at Aberystwyth (52.4°N, 4.1°W) have shown layers of enhanced backscatter at heights just below the mesopause during four nights in late June and early July, 1993. The characteristics of these layers and their association with noctilucent clouds indicate that they are comprised of ice crystals. An examination is made of the temperature prior to the formation of a layer, the rates of descent of layers, and their integrated backscatter coefficients. The results are discussed in terms of the size of the crystals giving rise to backscatter, the ambient water mixing ratio, and the equivalent water vapour represented by these crystals.

Lidar measurements of Mt. Pinatubo aerosols at Aberystwyth from August 1991 through March 1992

Measurements of stratospheric aerosols were conducted at Aberystwyth (52.4°N, 4.0°W) from 14 August 1991 to 30 March 1992, using a dual-polarisation lidar at 532 nm. The backscatter ratios show that volcanic aerosols, present below 22 km in August, built up steadily during the autumn and extended to higher altitudes. During 7–9 December a large increase in aerosol amounts was observed, with maximum backscatter ratios of 17 and aerosols reaching 34 km. Thereafter, peak backscatter ratios remained around 8, with aerosols extending up to 28 km. Optical depths increased tenfold during the period of measurements, from .02 in August to about 0.2 in March. A notable feature of the aerosol cloud was a layer of depolarising particles, first seen near 19 km on October 1st, which gradually subsided during the course of the autumn and winter.

Observations of streamers in the troposphere and stratosphere using ozone lidar

We describe here a DIAL lidar system for atmospheric ozone measurements atAberystwyth, Wales, together witha method for deriving a stratospheric ozone profile from a single laserwavelength. Lidar measurements are used todepict the passage of three mesoscale ozone disturbances in the troposphereand stratosphere. In the troposphere, twosmall fold-like structures are shown beneath and at the edge of streamers ofhigh potential vorticity in ECMWFanalyses. MST radar measurements at the same time show that one of these foldswas actively turbulent, causingmixing of stratospheric and tropospheric air. In the stratosphere, a streamerof low-latitude air drawn into a filamentby a breaking Rossby wave event was observed crossing the lidar site.