A. O'Neill

A Stratosphere-Troposphere Data Assimilation System

Abstract A data assimilation system has been developed at the UK Meteorological Office to analyze the mix of observations available in the troposphere and stratosphere. The data assimilation system is based on the analysis correction scheme used at the UK Meteorological Office for operational weather forecasting. The assimilation system is currently being used to supply near real-time analyses of meteorological fields from the troposphere and stratosphere to the Upper Atmosphere Research Satellite (UARS) Science Team. At this stage, these analyses are based on a similar set of observations to the operational analyses, so they provide an independent check of the UARS observations. In the stratosphere they are largely based on soundings from the National Oceanic and Atmospheric Administration polar orbiters. Some results from the assimilation system are presented for periods in January and August 1992. They are compared with equivalent products from the National Meteorological Center. A particular study is ...

On the Motion of Air through the Stratospheric Polar Vortex

Abstract Trajectory calculations using horizontal winds from the U.K. Meteorological Office data assimilation system and vertical velocities from a radiation calculation are used to simulate the three-dimensional motion of air through the stratospheric polar vortex for Northern Hemisphere (NH) and Southern Hemisphere (SH) winters since the launch of the Upper Atmosphere Research Satellite. Throughout the winter, air from the upper stratosphere moves poleward and descends into the middle stratosphere. In the SH lower to middle stratosphere, strongest descent occurs near the edge of the polar vortex, with that edge defined by mixing characteristics. The NH shows a similar pattern in late winter, but in early winter strongest descent is near the center of the vortex, except when wave activity is particularly strong. Strong barriers to latitudinal mixing exist above about 420 K throughout the winter. Below this, the polar night jet is weak in early winter, so air descending below that level mixes between pola...

High-resolution stratospheric tracer fields estimated from satellite observations using Lagrangian trajectory calculations

Abstract A technique is introduced by which high-resolution tracer fields may be constructed from low-resolution satellite observations. The technique relies upon the continual cascade of tracer variance from large to small scales and makes use of wind fields generated by a data assimilation scheme. To demonstrate its usefulness, the technique has been applied in a study of isentropic distributions of nitrous oxide in the winter midstratosphere, using measurements made by the Improved Stratospheric and Mesospheric Sounder instrument on the Upper Atmosphere Research Satellite. The results show that the high-resolution fields significantly increase the amount of information that is available from the satellite observations. The fields give insights into the characteristic structure and evolution of tracer distributions at scales that are normally obscured from view. Two results are particularly noteworthy. First, at the interface between low and middle latitudes there is evidence of active mixing. This mixi...

Global QBO Circulation Derived from UKMO Stratospheric Analyses

Global circulation anomalies associated with the stratospheric quasi-biennial oscillation (QBO) are analyzed based on U.K. Meteorological Office (UKMO) assimilated wind and temperature fields. Zonal winds and temperatures from the assimilation are compared with Singapore rawinsonde data (the standard QBO reference time series), showing reasonable agreement but an underestimate of maxima in the UKMO analyses. Global structure of the QBO in zonal wind, temperature, and residual mean meridional circulation (derived from thermodynamic balance and mass continuity) is isolated, showing coherent tropical and midlatitude components. Important aspects of the QBO revealed in these data include 1) out of phase maxima in temperature (and vertical velocity) between the lower and upper stratosphere, and 2) strong seasonal synchronization of midlatitude anomalies. These characteristics are also evident in long records of satellite radiance measurements.

Comparison of U.K. Meteorological Office and U.S. National Meteorological Center stratospheric analyses during northern and southern winter

Meteorological data from the United Kingdom Meteorological Office (UKMO), produced using a data assimilation system, and the U.S. National Meteorological Center (NMC), produced using an objective analysis procedure, are compared for dynamically active periods during the Arctic and Antarctic winters of 1992. The differences seen during these periods are generally similar to those seen during other winter periods. Both UKMO and NMC analyses capture the large-scale evolution of the stratospheric circulation during northern hemisphere (NH) and southern hemisphere (SH) winters. Stronger vertical and horizontal temperature gradients develop in the UKMO than in the NMC data during stratospheric warmings; comparison with satellite measurements with better vertical resolution suggests that the stronger vertical temperature gradients are more realistic. The NH polar vortex is slightly stronger in the UKMO analyses than in the NMC in the middle and upper stratosphere, and midstratospheric temperatures are slightly lower. The SH polar vortex as represented in the UKMO analyses is stronger and colder in the midstratosphere than its representation in the NMC analyses. The UKMO analyses on occasion exhibit some difficulties in representing cross-polar flow or changes in curvature of the wind field at very high latitudes. In addition to the above study of two wintertime periods, a more detailed comparison of lower-stratospheric temperatures is done for all Arctic and Antarctic winter periods since the launch of the Upper Atmosphere Research Satellite. In the NH lower stratosphere during winter, NMC temperatures are consistently lower than UKMO temperatures and closer to radiosonde temperatures than are UKMO temperatures. Conversely, in the SH lower stratosphere during winter, UKMO temperatures are typically lower than NMC and are closer to radiosonde temperature observations.

Quantification of the isentropic mass transport across the dynamical tropopause

A method is developed to quantify the quasi-horizontal, isentropic mass transport across the dynamical tropopause by small-scale filaments. This method is based on the contour advection technique and is used to calculate isentropic mass fluxes across the tropopause on the 330, 345, and 360 K surfaces from the European Centre for Medium-Range Weather Forecasts analyses for the years 1997 and 1998. The annually integrated isentropic net mass flux across the extratropical tropopause is directed from the stratosphere into the troposphere. Isentropic stratosphere troposphere exchange is, however, a two-way process, and the net flux is the residual of troposphere to stratosphere and stratosphere to troposphere transport, which can be of similar magnitude. It is shown that the isentropic mass fluxes are of the same order of magnitude as the upward vertical mass fluxes across the tropical tropopause by the large-scale circulation. Further, the extratropical lowermost stratosphere consists of a “well-ventilated” outer zone in which tropospheric and stratospheric air are heavily mixed, enclosing a more isolated inner zone. The mixing in the outer zone is equivalent to a mass exchange of 7 times the mass of the lowermost stratosphere per year in the Northern Hemisphere and 5 times per year in the Southern Hemisphere. The mixing of tropospheric and stratospheric air is strongest on the lower isentropic surfaces, and the stratosphere and the troposphere become more isolated with increasing altitude. There is greater troposphere to stratosphere transport during the spring of 1997 compared with that of 1998. This might partly explain why stratospheric ozone concentrations were lower during the spring of 1997 than during the spring of 1998.

Isentropic Mass Exchange between the Tropics and Extratropics in the Stratosphere

Abstract The isentropic mass exchange between the Tropics and extratropics in the stratosphere is investigated with a semi-Lagrangian transport model for the periods from 1 June to 31 October 1992 and from 1 December 1992 to 30 April 1993 using winds from the U.K. Meteorological Office data assimilation system. Calculations with an idealized, initially zonally symmetric tracer show that in the middle and upper stratosphere the bulk of tropical air is transported into the midlatitudes of the winter hemisphere although there exist quasi-permeable barriers in the subtropics. The transport takes place in the form of planetary-scale “tongues” of material that are drawn poleward in association with the episodic amplification of planetary-scale waves in high latitudes of the winter hemisphere. Once air of tropical origin is transported to the midlatitudes it is irreversibly mixed with the midlatitude air in the “surf zone.” Air of tropical origin can, however, hardly penetrate into the interior of the winter pol...

Mixing Processes within the Polar Night Jet

Abstract Lagrangian material line simulations are performed using U.K. Meteorological Office assimilated winds and temperatures to examine mixing processes in the middle- and lower-stratospheric polar night jet during the 1992 Southern Hemisphere spring and Northern Hemisphere winter. The Lagrangian simulations are undertaken to provide insight into the effects of mixing within the polar night jet on observations of the polar vortex made by instruments onboard the Upper Atmosphere Research Satellite during these periods. A moderate to strong kinematic barrier to large-scale isentropic exchange, similar to the barrier identified in GCM simulations, is identified during both of these periods. Characteristic timescales for mixing by large-scale isentropic motions within the polar night jet range from 20 days in the Southern Hemisphere lower stratosphere to years in the Northern Hemisphere middle stratosphere. The long mixing timescales found in the Northern Hemisphere polar night jet do not persist. Instead,...

Predictable winter climate in the North Atlantic sector during the 1997–1999 ENSO cycle

The winters of 1997/98 and 1998/99 were marked by strikingly different weather conditions over the North Atlantic ocean and adjacent continents. We use a state-of-the-art atmospheric general circulation model forced with observed sea surface temperatures (SSTs) to investigate the hypothesis that the anomalous conditions in the North Atlantic sector during the winters of 97/98 and 98/99 were related to the ENSO cycle and were therefore potentially predictable. We demonstrate that the major circulation anomalies observed in the North Atlantic sector are reproducible in both winters. We show further that these circulation anomalies were forced primarily by ENSO-related SST anomalies in the Pacific Ocean, but that SST anomalies in the Atlantic Ocean also had an influence. Our results are encouraging for the prospects of useful seasonal predictions for wintertime in the North Atlantic sector.

Evolution of the Stratosphere during Northern Winter 1991/92 as Diagnosed from U.K. Meteorological Office Analyses

Abstract Meteorological analyses, produced at the U.K. Meteorological Office by data assimilation, are used to study the circulation of the stratosphere in the Northern Hemisphere during winter 1991/92. The analyses are supplemented by Lagrangian visualizations of the circulation. The main features discussed are 1) the changes in vertical structure of the circulation, 2) the merger of anticyclones that precipitated a strong stratospheric warming, 3) vortex roll up in the upper stratosphere, 4) the entrainment of air into the polar vortex in the middle and upper stratosphere, and 5) the influence of tropospheric blocking on the lower stratosphere. The study provides a meteorological basis for the interpretation of data from the Upper Atmosphere Research Satellite.