Atmospheric Wind Evaluation using Spectroscopic Observations of Millimetre-wave Emission (AWESOME)

Lead Research Organisation: NERC British Antarctic Survey
Department Name: Science Programmes

Abstract

We propose the world's first measurements of atmospheric winds using ground-based millimetre-wave radiometry incorporating recent advances in receiver and spectrometer technology. This remote sensing technique will provide continuous, precise observations of horizontal winds in the stratosphere and mesosphere with unprecedented detail, covering the altitudes 20-70 km where measurements are currently very limited. The equipment will be robust and readily automated for deployment in harsh, remote environments including the Polar Regions where ground-based or satellite observations of winds are extremely limited. This will open up new areas of atmospheric science investigating atmospheric dynamics, circulation, waves, and tides - highly relevant to understanding polar and global climate. Ultimately the proposed observations will lead to advances in climate models and weather forecasting. The technique of wind radiometry is analogous to a traffic 'radar gun'. However instead of detecting a transmitter signal reflected by vehicles, the wind speed is determined from the Doppler shift of rotational emission signals originating from moving molecules in the air. By taking atmospheric measurements from opposite azimuthal directions (e.g. pointing east and west or north and south) the zonal and meridional wind components are determined. While this passive technique works in adverse weather conditions and doesn't need an active transmitter source, it does require accurate frequency measurements that can now be achieved using stable, high resolution spectrometers. Wind profiling using microwave and sub-millimetre radiometry has recently been demonstrated. We propose extending the technique to the millimetre-wave where the Doppler shifts are larger and more readily measured than in the microwave and atmospheric attenuation is lower than in the sub-millimetre region. In our paper study (raising the technology readiness level to ~3-4) we will use computer-based atmospheric simulations and retrieval calculations to determine the horizontal wind profiling capability of a sensitive ground-based 230-250 GHz radiometer deployed at high-latitude locations in the Arctic and Antarctic. We will investigate applications of this observing technique in studies of atmospheric tides and wave activity that can cause abrupt weakening or even reversal of the strong eastward winds at mid- and high latitudes during winter-time sudden stratospheric warming events. These atmospheric processes can lead to strong coupling between the lower and upper atmosphere and are sometimes linked to the onset of potentially disruptive cold weather across Europe. The proposed work is relevant to five NERC research topics and will build UK expertise in millimetre-wave sensing and atmospheric retrieval.

Planned Impact

1. WHO WILL BENEFIT FROM THIS RESEARCH?

The principle beneficiaries of this work include:

a) The academic and industrial community who develop and build instruments for environmental observations and monitoring. Microwave, millimetre-wave, and terahertz technologies are advancing rapidly and the UK is leading its exploitation in potentially far-reaching applications that include industrial analysis, medicine, and security as well as the environmental sciences.

b) The academic community who work in tropospheric, stratospheric, and mesospheric science. We will present our results and make a particular effort to bridge the gaps between experimental scientists using different observing techniques to study different regions of the atmosphere and those using observations and assimilated datasets for numerical weather prediction and climate modelling.

c) Governments making policies about climate change and stratospheric ozone need to have explanations for observed changes in the environment and climate of the Polar Regions. The creation of accurate models that take into account natural variability and complex interactions between atmospheric chemistry and dynamics, the ozone layer, and the impact of increased greenhouse gases is vital for Governments revising policy concerned with the Montreal Protocol for the protection of the ozone layer and its amendments. The scope of the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5) includes the need to 'assess climate change impact on - and the role of the mesosphere in radiative forcing of the atmosphere'.

d) The general public, which shows large and widespread interest in climate change and stratospheric ozone. Clear communication and engagement on how this research focuses on these questions would be in high demand by the public and media, and would clearly benefit society in general.

2. HOW WILL THEY BENEFIT?

a) The outputs of the proof-of-concept project will primarily be technical reports on the feasibility study and peer-reviewed publications. Where appropriate the research team will engage with the wider academic community and industry e.g. through NCAS, NCEO and the Centre for Earth Observation Instrumentation (CEOI). We will interact directly with the international community involved in ground-based atmospheric observations including the Microwave Group in the Network for the Detection of Atmospheric Composition Change (NDACC).

b) The academic community will benefit from an increased knowledge of fundamental processes in the atmosphere gained from new techniques for observing winds in the stratosphere and mesosphere.

c) The proposed feasibility study is aimed at providing high quality observations of the stratosphere and mesosphere that cannot currently be made, filling a major gap in our ability to improve and verify atmospheric models that contribute towards environmental assessments used by governments to formulate policy.

d) The general public will benefit by having access to an exciting scientific / technical project via the BAS Press, PR, & Education section, thus raising awareness of STEM subjects. They will also benefit as users of weather forecasts since our work will ultimately contribute to their becoming more accurate. It is hard to quantify the financial and operational benefits to general and specialist users of more accurate weather forecasts, but any reasonable estimate suggest they will be very considerable.

3. TIMESCALES

All results will be released and available within the scientific literature within one year of the project's end (allowing for publication times).

4. RESEARCH & PROFESSIONAL SKILLS

The academic staff involved will benefit from exposure to the complementary expertise each hold. The PDRA on the project will likewise benefit from exposure to these different areas of expertise and will develop and strengthen their skills in data analysis, remote sensing, and polar science.
 
Description Meteorological and atmospheric models are being extended up to 80 km altitude but there are very few observing techniques that can measure winds in the stratosphere and mesosphere at altitudes between 20 km and 80 km to verify the model data-sets. We have demonstrated the feasibility of filling this gap in atmospheric wind measurements using ground-based passive millimetre-wave radiometry. Horizontal wind speeds have been retrieved from model simulations of Doppler-shifted atmospheric emission lines above Halley station, Antarctica. For a typical millimetre-wave radiometer observing ozone at 231.28 GHz we estimate that 12 hour zonal (east-west) and meridional (north-south) wind profiles could be determined over the altitude range 25-74 km in winter, and 28-66 km in summer. Height-dependent measurement uncertainties are in the range 3-8 m s-1 and vertical resolution ~8-16 km. Under optimum observing conditions at Halley a temporal resolution of 1.5 hours for measuring either zonal (east-west) or meridional (north-south) winds is possible, reducing to 0.5 hour for an optimised radiometer. Combining observations of the 231.28 GHz ozone line and the 230.54 GHz carbon monoxide line gives additional altitude coverage at 85±12 km. The effects of clear-sky seasonal mean winter/summer conditions, zenith angle of the received atmospheric emission, and spectrometer frequency resolution on the altitude coverage, measurement uncertainty, and height and time resolution of the retrieved wind profiles have been determined.
Exploitation Route The routes identified in the Pathways to Impact statement remain valid for this project. The postdoctoral researchers who worked on this NERC grant-funded project, and an associated HEIF-funded innovation project (SPECTRO-ICE) with the University of Cambridge, have gone on to apply skills, techniques, and experience gained working on this project in their employment at the Met Office. With their help we're developing new collaborations with a number of different Met Office groups which focus on space weather, monitoring Met Office stratospheric analyses and forecasts, developing middle atmosphere modelling, the impact of the representation of the stratosphere on the troposphere, and ozone data assimilation. We are using new and existing methods of engagement to inform them of the outcomes of our feasibility study. In the longer term, improved stratospheric and mesospheric wind observations would be relevant to the Met Office Unified Model (UM). The current operational system represents 70 altitude levels from the surface to the upper mesosphere (~85 km), covering the altitudes 30-70 km where there are limited horizontal wind observations. Furthermore, the planned extension of the UM to a "whole atmosphere" version with an upper boundary around 600 km would require accurate representation of chemical and dynamical coupling between the upper and lower parts of the atmosphere. In particular the outcomes of research from this project benefit the Met Office developments in upper atmosphere coupling and space weather; Ozone in Numerical Weather Prediction Applications; Climate Research; and Coupled Climate Chemistry Modelling.
Sectors Environment