The 8-hour Tide in the Mesosphere and Lower Thermosphere over the UK Charlotte Beldon Centre for Space, Atmospheric & Oceanic Science University of Bath [email_address] 12/9/5/05

Outline Introduction to mesosphere and lower thermosphere Atmospheric tides The UK meteor radar The 8-hour tide over the UK Comparisons with other latitudes Conclusions

Introduction to mesosphere and lower thermosphere

Atmospheric tides

The UK meteor radar

The 8-hour tide over the UK

Comparisons with other latitudes

Conclusions

The mesosphere and lower thermosphere Inaccessible to direct measurement but includes the meteor region Plays a crucial role in linking the upper and lower atmosphere Sensitive to climate change

Inaccessible to direct measurement but includes the meteor region

Plays a crucial role in linking the upper and lower atmosphere

Sensitive to climate change

Atmospheric tides Some of the largest waves in the MLT are solar tides. The ‘square wave’ shape of tidal heating results in tides with periods of 24 -, 12 -, 8 - and 6 -hours

Some of the largest waves in the MLT are solar tides.

The ‘square wave’ shape of tidal heating results in tides with periods of 24 -, 12 -, 8 - and 6 -hours

Tidal excitation Solar heating of: Atomic oxygen by EUV at ~ 150 km Molecular oxygen by NUV at 100 – 150km Ozone by UV at 30 – 70 km Water vapour in the troposphere by IR For the 8-hour tide - Non-linear interactions between the 12- and 24- hour tides Source of the 8-hour tide is uncertain Water vapour Ozone Molecular oxygen

Solar heating of:

Atomic oxygen by EUV at ~ 150 km

Molecular oxygen by NUV at 100 – 150km

Ozone by UV at 30 – 70 km

Water vapour in the troposphere by IR

For the 8-hour tide - Non-linear interactions between the 12- and 24- hour tides

Source of the 8-hour tide is uncertain

The UK meteor radar Based in Castle Eaton (52.6 ºN, 2.2 ºW ). Operating since 1988 – very long dataset Bounces a radio pulse off ionised meteor trails which drift with the atmosphere Measures horizontal wind velocities representative of ~ 90 – 95 km with a resolution of 1 hour

Based in Castle Eaton (52.6 ºN, 2.2 ºW ).

Operating since 1988 – very long dataset

Bounces a radio pulse off ionised meteor trails which drift with the atmosphere

Measures horizontal wind velocities representative of ~ 90 – 95 km with a resolution of 1 hour

The 8-hour tide All tides are highly variable Over the UK the 12-hour tide is largest but the 8-hour tide can sometimes be as large for short periods

All tides are highly variable

Over the UK the 12-hour tide is largest but the 8-hour tide can sometimes be as large for short periods

The 8-hour tide over the UK Largest amplitudes during autumn & winter-time Later phases during summer Phase quadrature (2 hour phase difference)

Largest amplitudes during autumn & winter-time

Later phases during summer

Phase quadrature (2 hour phase difference)

Inter-annual variability High degree of inter-annual variability Seasonal pattern similar from year-to-year Amplitudes can change by ~6 ms -1 from year to year e.g., successive Octobers Variability caused by: - changes in the strength of excitation - changes in the propagation environment

High degree of inter-annual variability

Seasonal pattern similar from year-to-year

Amplitudes can change by ~6 ms -1 from year to year e.g., successive Octobers

Variability caused by:

- changes in the strength of excitation

- changes in the propagation environment

Comparisons with the other latitudes At 90 – 95 km autumnal maximum, but with no winter maximum UK: autumnal maximum and large winter amplitudes Lower middle latitudes : large winter amplitudes and no autumnal maximum Arctic: Measured by meteor radar in Esrange, Sweden (69 ºN, 21 ºE )

At 90 – 95 km autumnal maximum, but with no winter maximum

UK: autumnal maximum and large winter amplitudes

Lower middle latitudes : large winter amplitudes and no autumnal maximum

Measured by meteor radar in Esrange, Sweden (69 ºN, 21 ºE )

Conclusions The 8-hour tide is a persistent feature of the MLT over the UK Climatology: amplitudes largest in autumn, smallest in summer High inter-annual variability - although general seasonal structure remains the same Seasonal behaviour changes from UK to Arctic latitudes Future work : vertical wavelengths as diagnostic of tidal excitation Higher frequency tides e.g., the 6-hour tide

The 8-hour tide is a persistent feature of the MLT over the UK

Climatology: amplitudes largest in autumn, smallest in summer

High inter-annual variability - although general seasonal structure remains the same

Seasonal behaviour changes from UK to Arctic latitudes

Future work : vertical wavelengths as diagnostic of tidal excitation

Higher frequency tides e.g., the 6-hour tide

Charlotte Beldon Centre for Space, Atmosphere and Oceanic Science, Dept. Electronic and Electrical Engineering University of Bath Thank you [email_address]