EFFECTS
ON WEATHER AND CLIMATE
OF TOPOGRAPHY AND BUOYANCY IN THE
POLAR ATMOSPHERE
AND OCEANS
1.
Why is mesoscale modelling of the Polar Regions important?
Many atmospheric flows and processes in the Polar Regions occur on the mesoscale
(10-100km), e.g. mountainous coastal margins (extremely sharp gradients
in roughness and elevation) and katabatic winds (and their interaction with
barrier winds). Additionally, only recently has it been shown that Coriolis
effects in combination with stable stratification can be significant over
length scales less than 30km (e.g. Capon 2003). Understanding the coupling
processes between the atmosphere and the ocean and sea-ice is also extremely
important. For example, coastal polynyas (gaps in the sea-ice) create a
large heat flux between the ocean and the atmosphere, which is a source
of oceanic overturning. We have investigated these fine scale features through
a combination of numerical mesoscale modelling, analytical modelling, laboratory
work, and applying observations. Our studies should help improve the parameterization
and interpretation of these mesoscale processes in weather or climate prediction
models.
2.
Mesoscale meteorology and climatology of southern Greenland
An
investigation into the characteristics of stable flows over southern Greenland
for westerly, easterly, southerly and northerly approach flows using mesoscale
and synoptic scale numerical simulation, meteorological analysis, satellite
observations, and surface observations was completed (Orr et al.
2004b). Good agreement was shown between the surface observations and the
numerical simulations. For westerly and easterly winds detached jets with
wind speeds of order 2-3 times that of local winds and a transverse scale
of order the Rossby Deformation radius LR form (Hunt et al. 2003,
2004). The strong winds and cold temperatures of the low-level wind jets
lead to enhanced heat loss in the ocean around southern Greenland. Typically
latent heat fluxes were slightly greater than the sensible heat flux, but
combined they typically contributed a strong heat flux of between -300 and
-400 Wm-2. Additionally the jets often curve markedly to the left due to
the Coriolis force, resulting in a strong, localized wind stress curl. Due
to Ekman motion this would create a cyclonic or divergent gyre in the ocean.
Together the large sea-air heat flux and strong wind stress curl force oceanic
downwelling. Pickard et al. (2003) showed that downwelling in the
Irminger Sea (to the south-east of Greenland) was associated with the 'western
tip jet' (see Figure 1). However, the western tip jet is a relatively rare
phenomenon. Results here suggest that downwelling associated with these
jets might be associated at places other than the Irminger Sea and possibly
more common than previously though. It was also noted that the approach
flow rises/falls over southern Greenland for easterly/westerly winds, leading
in both cases to more cloud on the western side.
The influence of Greenland on stable flows was investigated theoretically
by extending orographic flow analysis for mountains (such as the Antarctic
Peninsula) where B/LR >1 (e.g. O´lafsson and Bougeault 1997) to
larger mountains, such as Greenland, where B >> LR (O´lafsson
2000, Orr et al. 2004b). Here B is the half-length of the mountain.
Figure 1: UK Met Office UM 4.5 computed 10m-wind speed (ms-1) and sensible heat flux (Wm-2) over southern Greenland. A detached jet is evident at the southern tip of Greenland, propagating large distances across the ocean and cooling it at the same time, potentially a source of open-ocean deep convection (Pickart et al. 2003). DMI and GC-Net surface observations are shown in red.
3.
Mesoscale meteorology and climatology of the Antarctic Peninsula
We proposed a mechanism (Orr et al. 2004c) whereby the stronger westerly
circumpolar winds over the past 40 years (Marshall 2002) impact on the mountains
of the Antarctic Peninsula inducing increased northerlies and a greater
transport of warm air into this region. Consequently there is a reduction
in the sea-ice extent, further amplifying the local warming. This would
cause the significant warming trend as has been observed along the western
side of the Peninsula in the last 50 years (Vaughan et al. 2001).
Note that this trend could well be reversed since it is associated with
an oscillation. This was demonstrated by numerical (see Figure 2(a)) and
laboratory (see Figure 2(b)) meteorological modelling, with the hypothesis
supported by observational evidence. The increase in the strength of the
westerlies also makes the probability of the westerly flow passing over
the Peninsula and descending on the lee (easterly) side more likely, transporting
relatively warm air to the eastern side of the Antarctic Peninsula. Perhaps
this is contributing to the large (summer) warming observed over the north-eastern
Peninsula (e.g. at Esperanza) and the break up of the Larsen Ice Shelf.
Figure 2: (a) Numerical and (b) laboratory meteorological modelling demonstrating how when westerly winds impinge on the western side of the Antarctic Peninsula, air below the height (1.5-2km) of the Peninsula is advected in a southerly direction. ys defines the position of the stagnation point from the base of the cape and B is the length of the cape. In both (a) and (b) the Froude number is approximately 1/3 and the ratio ys/B is approximately 0.6.
Figure 2(b) is from a five week laboratory project using the Coriolis turntable, Grenoble, France, which we completed during 2003. The purpose of the experiment was to further understand these types of rotating flows (e.g. Hunt et al. 2003, 2004; Orr et al. 2004a) by visualising the flow using high resolution Correlation Imaging Velocimetry (CIV), and measure local density fluctuations, as it is perturbed by a bottom feature (in particular a cape (e.g. the Antarctic Peninsula), a roughness strip (e.g. coastal margins of polar regions), and a elongated barrier (e.g. Greenland)) under continuous and two-layer stratifrication.
4.
Transition of the atmospheric boundary layer to varying surface heat flux
The boundary layer response over land and sea areas of the Polar Regions
to varying surface heat flux is significant but not well understood. Both
strong localized heating, for example from coastal polynyas (e.g. Renfrew
and King 2002), and cooling, for example warm winds over tundra passing
over a relatively cold ocean (e.g. Smedman et al. 2003), occur. An
investigation into the atmospheric boundary layer response to an abrupt
(t>0), but uniform, change in surface heat flux was completed (Owinoh,
Hunt, Orr et al. 2004). Analytical solutions for changes in temperature,
mean wind and shear stress profile were obtained for the surface layer of
the boundary layer, showing that low-level jets can form under both strong
heating and cooling. The results were verified by fine scale computations
using UM 4.5. See Figure 3. It was found that the UM 4.5 code needed to
be improved to simulate these transition flows. These improvements have
been implemented in the idealised mode of UM 5.3. The analytical solutions
were compared to observations of a low-level jet which developed as relatively
warm air over land travelled over the cold Baltic Sea (Smedman et al.
2003).
Figure 3:Time variation of velocity computed using UM 4.5 for (a) strong
surface cooling of -500 Wm-2 with h/|LMO|~ 75 and (b) moderate surface heating
of 350 Wm-2 with h/|LMO| ~ 50. Here f = 1.3x10-4s-1 and u*˜0.3ms-1.
A low-level jet is shown developing in (a). Due to technical difficulties
with the UM 4.5 Fortran code it was not possible to apply the model to conditions
of strong heating. However other laboratory experiment and direct numerical
simulation data verified that low-level jets develop under conditions of
strong heating.
5.
Mesoscale air flow over orography
Hunt et al. 2003, 2004 uses a shallow-layer perturbation model to
provide an overall theoretical and climatological framework for mesoscale
air flow over orography with changing elevation, roughness and temperature,
particularly where there are sharp changes in surface conditions on the
scale of order 1-10km such as coastlines or edges of ice sheets. A noticeable
feature of these flows are wind jets driven by frictional-Coriolis-buoyancy
(FCB) forces, which are likely to be widespread around coasts when the wind
is at an angle or parallel to the coast, especially when the lower troposphere
is stably stratified or there is a strong inversion layer (Orr et al.
2004a). The FCB jets are low-level, occurring within the boundary layer.
Their peak velocity can be up to 40% greater than the upstream speed. The
jets extend a distance LR from the coastline. However the wind speeds near
the coastline vary rapidly with the peak at around 1 - 3 km (or of order
the height of the inversion layer h0) from the coast. For a meteorological
case study it was shown that the shallow-layer model represents the major
features of the surface induced flow perturbations computed by UM 5.1 at
a resolution of 2 km (see Figure 4). With onshore winds at an acute angle,
the peak wind is approximately located along the coast. These winds are
associated with variations in the height of the inversion layer of order
100 m or more that are sufficient to produce strong variations of cloudiness,
fog, and precipitation with distance from the coast, depending on the orientation
of the wind (see Figure 5). These effects can be quite simply understood
in terms of low-level convergence and divergence of winds under influence
of Coriolis forces. Numerical codes can only capture them using very fine
scale meshes of the order of 2 km resolution. As well as variations in cloudiness
they are associated with increased wind stress, storm surge, ocean up/downwelling,
etc. Hence a practical application of this study is to improve their parameterisation.
Figure 4: Case study
for 23 July 2002 over the Dover Straits region of the English Channel. (a)
10m-wind speed (ms-1) computed using UM 5.1 with a horizontal resolution
of 2km. 'GLV' is Greenwich Light Vessel, which showed winds of up to 15ms-1
during most of the day. The heavy black line is the idealised land/sea mask
used for (b). (b) Flow field (ms-1) calculated using the shallow-layer model
over the idealised terrain representing the Dover Straits region under the
same conditions as the results in (a).
Figure
5: Visible satellite image at around 1200 GMT 25 January 2001. The MSLP
analysis showed a low-pressure system anchored west of UK, creating a strong
well-defined south-south-westerly flow. A large amount of cloud is evident
over the English Channel which abruptly disappears inland over southern
England. Clear skies are evident over the Dover Straits region of the English
Channel, with cloud abruptly beginning inland of the Dutch and Belgium coastlines.
Mechanisms such as the sea-breeze, sea-land transitions, orographic effects,
or 'burn-off' due to absorption of solar radiation in the cloud layer cannot
solely be responsible for the observed increasing/decreasing cloudiness
inland when the coast is on the right/left as the airflow approaches the
sea. These effects are associated with FCB jets.
6. Work in progress
Investigate the characteristics of airflow over the Antarctic Peninsula and the large (summer) warming observed over the north-eastern Peninsula.
Investigate the effect of varying surface heat flux on the turbulent boundary layer on slopes.
References
Hunt,
J. C. R., Orr, A., Cresswell, D. and Owinoh, A., 'Coriolis
effects in mesoscale shallow layer flows', Proceedings of the International
Symposium on Shallow Flows (Eds. G. H. Jirka and W. S. J. Uijttewaal),
Delft, June 16-18, 117-124, 2003.
Hunt,
J. C. R., Orr, A., Rottman, J. W. and Capon, R., 'Coriolis effects
in mesoscale flows with sharp changes in surface conditions', Q. J. Roy.
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Hunt, J. C. R., O´lafsson, H. and Bougeault, P., 'Coriolis effects on orographic and mesoscale flows', Q. J. R. Met. Soc., 127, 1-33, 2001.
Marshall, G. J., 'Analysis of recent circulation and thermal advection in the northern Antarctic Peninsula', International Journal of Climate, 22, 1557-1567, 2002.
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O´laffson, H., 'The impact of flow regimes on asymmetry of orographic drag at moderate and low Rossby numbers', Tellus, 52A, 365-379, 2000.
Orr,
A., Hunt, J. C. R., Capon, R., Sommeria, J., Cresswell, D.,
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the atmospheric boundary layer flow over flat terrain', Boundary Layer
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Pickart, R. S., Spall, M. A., Ribergaard, M. H., Moore, G. W. K. and Milliff, R. F., 'Deep convection in the Irminger Sea forced by the Greenland tip jet', Nature, 424, 152-156, 2003.
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Vaughan,
D. G., Marshall, G. J., Connolley, W. M., King, J. C. and Mulvaney, R.,
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