A case study of a low-level jet (LLJ) during the OPALE (Oxidant Production over Antarctic Land and its Export) summer campaign is presented. It has been observed at Dome C (East Antarctica) and is simulated accurately by the three-dimensional version of the Modèle Atmosphérique Régional (MAR). It is found that this low-level jet is not related to an episode of thermal wind, suggesting that Dome C may be a place where turbulence on flat terrain can be studied.
Low-level jets (LLJs) have been observed and studied for a long time (see e.g. Davies, 2000; Cuxart and Jimenez, 2007; Banta et al., 2003). Their interest may be related to the need for a better understanding of the atmospheric boundary layer. On the one hand, they are suspected of generating additional turbulence. On the other hand, their behaviour may have an impact among others on the management of wind turbines, and bird migration (Van de Wiel et al., 2010). Following Blackadar (1957) and Van de Wiel et al. (2010), LLJs may be due to the onset of an inertial oscillation when the turbulence force suddenly decreases at the end of day-time. LLJs have been observed over the Weddel Sea (Antarctica) (Andreas et al., 2000).
A wind speed maximum near the surface has also been observed at the South Pole during ANTCI (Neff et al., 2008). In contrast to the LLJs observed at Dome C, it is associated with events of inversion winds. Indeed, the South Pole is situated on a slope, while Dome C is not. The LLJ at Dome C is related to the pressure gradient force (PGF) extending well above the boundary layer, while at the South Pole, the wind speed maximum is caused by the downslope PGF developing only in the bulk of the inversion wind layer. Another difference is that there is no diurnal cycle at the South Pole. Consequently, a LLJ could not develop there at the end of day-time, when turbulence shuts down. Possibly a LLJ could develop at the South Pole with a rapid stabilisation of the atmosphere associated with changes in synoptic-scale conditions. A consequence of the absence of a diurnal cycle is that turbulence in the stable boundary layer of the South Pole may reach an equilibrium, while this is not the case at Dome C during summer. Note that Neff et al. (2008) mention that the behaviour of nitrous oxide (NO) below the wind speed maximum they observe is not fully understood, since it could depend (but not always) on an accumulation process of NO over a thin drainage flow whose thickness increases gradually before it reaches the South Pole. In our case no drainage flow reaches Dome C, so that the above-mentioned accumulation process does not exist.
A common point between LLJs associated with an inertial oscillation and the observations of Neff et al. (2008) is that the wind shear is zero at the jet maximum, so that turbulent transport could not exist through the jet core (the gradient Richardson number is “infinite” there). Note however that the LLJ at Dome C forms at a height where turbulence has already shut down, so that the LLJ is not strictly necessary for precluding vertical turbulent transport there. In contrast, the wind speed maximum at the South Pole is associated with the turbulent inversion winds, and could play a more important role in causing the shutdown of turbulence.
Finally, the shutdown of turbulence by a wind speed maximum remains an open question. Indeed, turbulence bursts have been simulated through a jet core in a LES (large eddy simulation) by Cuxart and Jiménez (2007), but only when the wind and air temperature near the surface are prescribed in their model.
In this note we consider a case study of a LLJ happening at Dome C during the night of 16–17 December 2011 (during the OPALE campaign) and accurately simulated by MAR. The model has been satisfactorily validated for the OPALE campaign in Gallée et al. (2015). The objective here is to focus on the driving forces of a LLJ at Dome C.
The model used is MAR (Modèle Atmosphérique Régional). It is described and set up as in Gallée et al. (2015). A summary is given here.
MAR is a hydrostatic primitive equations model using finite difference schemes (Gallée and Schayes, 1994). The terrain following normalised pressure is used to take into account topography. Turbulence is parametrised by using two prognostic equations for turbulent kinetic energy and its dissipation (Duynkerke, 1988; Bintanja, 2000). The prognostic equation of dissipation allows one to relate the mixing length to local sources of turbulence and not only to the surface. Finally, the relationship between the turbulent diffusion coefficient for momentum and scalars (Prandtl number) is dependent on the Richardson number, according to Sukoriansky et al. (2005). An explicit cloud microphysical scheme describes exchanges between water vapour, cloud droplets, cloud ice crystals (concentration and number), snow particles and rain drops (Gallée, 1995).
MAR integration domain and topography (colour). Solid line represents the 3250
The horizontal domain covers an area of about
The simulation is started on 1 November 2011 and the model is not reinitialised until the end of the experiment (end of January 2012). Thus the simulation is sufficiently long to allow the influence of lateral boundary conditions to reach the central part of the domain, in contrast to what happens in a simulation starting from prescribed initial conditions, and lasting a few hours or days only. As lateral boundary conditions are over-specified in a limited area model, they may distort its solution and cause some differences between the simulation and the observation. This point will be illustrated in the next section.
It was possible to observe LLJs occurring only at a height below the top of the tower. As LLJs occur where turbulence shuts down, this means that in these cases stabilisation of the vertical column of air is strong, i.e. when the wind shear is not too large and a strong radiational cooling of the surface occurs.
Observed and simulated LLJs at Dome C during OPALE.
Observed and simulated LLJs during the OPALE period (12 December 2011–14 January 2012) are listed in Table 1. They are obtained by searching from below the lowest wind speed maximum below the highest level of the tower. Note that the vertical resolution of the model (2 m) is higher than that of the observations (six levels, respectively at 3.5, 10.8, 18.2, 25.6, 32.9 and 42.1 m). Consequently, the estimation of the height of the LLJ in the observations may be very crude. No LLJ is simulated or observed in January 2012, but no observations at the tower were made between 1 and 9 January, and generally we did not get clear sky conditions in the first half of January 2012 (see e.g. Fig. 2a of the companion paper – Gallée et al., 2015). MAR simulated a LLJ on 15 December below the top of the tower, while it was very weak in the observation. No LLJ was simulated below the top of the tower on 26, 27 and 28 December, when MAR underestimated cloud cover and consequently overestimated day-time solar warming the day before. This caused an overestimation of turbulence and precluded the formation of a shallow inversion layer during night-time. In short, the good simulation of a LLJ by MAR or not in December 2011 was mainly the result of the good behaviour of turbulence or not in the model, which itself results mainly from the good behaviour or not of the simulated cloud cover. LLJs are more sensitive to turbulence than the winds simulated near the surface. Consequently, the evaluation of their behaviour may help us in evaluating vertical mixing of chemical species. Of course, a longer time series must be analysed in order to confirm this result. Note that statistics of observed LLJs at Dome C have already been given in Barral et al. (2014).
Hereafter we focus on a well-marked case study which is accurately simulated, in order to infer in a deeper way how to evaluate the simulation of a LLJ by a 3-D model.
We consider the same experiment as in Gallée et al. (2015)
and the observations which have been performed on a 45
Temperature (colour) and wind speed (isocontours) at the Dome C
tower, simulated by MAR on 16–17 December 2011 (upper panel) and observed
(lower panel). Local Time LT (Universal Time UT
The MAR simulation for 16–17 December 2011 is compared with observation in
Fig. 2. The LLJ is simulated at 01:00 LT on 17 December at
14
Vertical profiles of simulated temperatures, wind speeds and wind directions on 16 December 2011 at 16:00 LT and midnight.
Vertical profiles of simulated temperatures, wind speeds and wind directions are compared in Fig. 3 to the observations made at the tower for 16:00 and 24:00 LT. Temperatures are overestimated during day-time and overestimated above the LLJ during night-time. The overestimation above the LLJ during night-time may be due to an underestimation of turbulence by the E–e model. Similarly, momentum mixing seems to be well simulated during day-time, but the wind speed is underestimated at midnight above the LLJ, as the temperature. Possibly this is linked to the representation of large-scale winds in the model (see Fig. 5). Wind direction seems to be well simulated.
500 hPa geopotential (m) over Antarctica on 16 December 2011 at
12:00 UT (colour key to the right). Total cloud liquid water content (TCLW),
from 0.01 (dark blue) to 1.2
Comparison between the analysed wind speed (top) and direction (bottom) and the simulation, at 100 m a.g.l. and 300 m a.g.l. Note that Universal Time is used.
The behaviour of the MAR turbulent scheme is also discussed in Gallée et al. (2015), with the conclusion that the underestimation of turbulence may be partly due to the underestimation of DLW, which is responsible for an overestimation of the vertical stability near the surface during night-time.
We now have a look at the general conditions prevailing during this LLJ.
The synoptic-scale situation prevailing on 16–17 December in the vicinity of
Dome C and illustrated by the 500
It appears that the model captures reasonably well the wind vector above the
tower, as can be seen from a comparison with the forcing (ERA-Interim) at 100
and 300 m a.g.l. The error in the wind speed and direction may amount
respectively to 1.5 m s
Let us now look at the simulation along the slope (
We get (see Appendix for more details)
Vertical profiles of the contributions to the wind speed of PGF, advection, turbulence and horizontal diffusion.
Vertical profiles of advection, PGF, the contribution of turbulence and
horizontal diffusion, and their sum at 16:00 and 24:00 LT are shown in
Fig. 6. The last is interpreted as the tendency of the wind speed. These
profiles are roughly homogeneous along the vertical during day-time
(16:00 LT), with PGF counterbalancing roughly the turbulent contribution. A
similar equilibrium between PGF and the turbulent contribution exists at
midnight below the LLJ, but their absolute values are reinforced. The
contribution of turbulence is zero at the level of the LLJ and just above,
where turbulence production by the wind shear is almost zero. Horizontal
diffusion contributes negatively (positively) below (above) the height of the
jet core. The negative contribution in the bulk of the boundary layer could
be related to the weakening of the wind speed on the slope directed towards
negative
The wind speed
A positive PGF contribution to the wind speed, as defined in Eq. (1), means
that the PGF is responsible for an acceleration of the wind speed. The wind
is roughly from the south-south-east during day-time (Fig. 7a, 16:00 LT). It
comes from a slightly more southerly direction only above the jet level
(14
The reason why the PGF contributes to an acceleration of the wind speed up to
14
Note in Fig. 7b the weakening of the wind speed below the jet level
(14
Note also the occurrence of the wind speed maxima with downslope wind
direction just above 14
Simulated contribution of the forces to the wind speed, 14
The component of the PGF along the
Note that the height of the strong inversion layer is the smallest and the
inversion the strongest over the dome (Fig. 8), probably because of
a progressive weakening of the flow which is counteracted upstream by the
downslope contribution of the PGF along the
The contribution of the PGF to the wind speed is also compared with the air
temperature in Fig. 8b. From the discussion above it appears that the change
of sign of this contribution at Dome C (i.e. a change in the PGF contribution
from an acceleration of the wind speed below 14
In fact, the coincidence between the height of the change of sign of the PGF contribution to the wind speed and the top of the inversion layer during night-time may be due to a wind vector no longer orthogonal to the PGF in the inversion layer, but partly directed in the same direction as the wind vector. This is because turbulence there is generated by surface friction (Ekman wind) at that time. As a remnant of the wind direction change due to turbulence still exists in the upper part of the inversion layer, while turbulence contribution has already shut down, the PGF is in a position to accelerate the wind speed there.
Wind hodograph at Dome C between 16 December 2011 19:00 LT and
17 December 2011 10:00 LT. Colours represent time in hours before/after
17 December midnight (negative/positive values). Arrows are plotted
for 16 December 2011 19:00 LT and 17 December 2011 01:30 LT.
Panel labeled “MAR”: simulation at
Figure 9 illustrates the sudden shutdown of turbulence 14
Advection weakens between 18 and 21:00 LT and recovers after that time. The weakening of advection occurs mainly below 20 m a.g.l. and decreases progressively upwards. It is found that turbulence is higher to the south of (upstream) Dome C than at Dome C at 14 m a.g.l. (height of the jet core) and at 19:00 LT, while this is not the case at 22:00 LT. Also, at 19:00 LT the wind speed is higher upstream from Dome C than at Dome C. But at 17:00 LT the contribution of turbulence is smaller everywhere at 14 m a.g.l., while the wind speed is already higher upstream from Dome C. A possible mechanism upstream from Dome C could be a slight reinforcement of the wind speed during day-time by an upslope PGF, leading to a larger wind shear and turbulence there at the end of the day. While the inertial oscillation starts at Dome C due to the shutdown of turbulence, this is not yet the case upstream. Turbulence shuts down there only a few hours later. Consequently, the advection of momentum at Dome C could be weaker during a few hours at the end at the day. In short, if surface temperature is overestimated by the model, the reinforcement of the wind speed and turbulence upstream from Dome C during day-time could be overestimated by the model, and could lead to an overestimation of turbulence during a few hours at the end of day-time, a subsequent underestimation of advection at Dome C at the height of the LLJ, and an underestimation of its strength.
MAR simulates a low-level jet (LLJ) at Dome C on 16–17 December 2011, as in the observations. It is the first time that a 3-D simulation of such a low-level jet over an ice sheet has been performed. The good behaviour of the model allows us to perform an analysis of the dynamical contributions (PGF, turbulence, advection) to the simulated wind speed.
It appears that the LLJ is generated when turbulence shuts down at the end of day-time, just above the turbulent layer, where the flow is still deflected from the geostrophic wind direction, blowing from higher to lower pressures. The LLJ seems not to be due to inversion winds over Dome C, but a reinforced LLJ is simulated by the model over the slopes near Dome C, where and when the downslope PGF reinforces the synoptic-scale PGF. In contrast, the model is not able to simulate the inertial oscillation after 01:30 LT. The cause is not yet firmly identified and this would be the subject of future work.
Finally, the height of the LLJ at Dome C is strongly dependent on the height
of the turbulent layer, and thus its simulation is an indicator of the
success or not of a model in simulating the intensity of turbulence under
stable conditions. Cuxart et al. (2006) and Barral et al. (2014) show that
a model overestimating turbulence overestimates the height of the wind speed
maximum. Here a slight underestimation of turbulence by MAR possibly due to
a slight underestimation of the downward longwave radiation flux during
night-time is responsible for a possible slight underestimation of the LLJ
height. Vertical stratification of the atmosphere is strongly stable at Dome
C during night-time, even in summer. During day-time the sensible heat fluxes
are much larger than the latent heat fluxes, because of the low temperature
and the subsequent very low capacity of the atmosphere to contain water (see
e.g. King et al., 2006). Consequently, the conditions for developing
a well-mixed layer during day-time are optimal. This means that the
simulation of summer case studies at Dome C could help a lot in validating
the turbulence scheme of an atmospheric model. Due to its particular location
and available set of observations, Dome C was recently selected as the test
site for the next Gewex Atmospheric Boundary Layer Studies (GABLS4) model
intercomparison (see
Equations of horizontal motion in MAR read (Gallée and Schayes, 1994)
The OPALE project was funded by ANR (Agence National de Recherche) contract ANR-09-BLAN-0226.
Most of the computations presented in this paper were performed using the
Froggy platform of the CIMENT infrastructure
(
This work was also granted access to the HPC resources of IDRIS under the allocation 2014–1523 made by GENCI.
The IPEV-CALVA, INSU-LEFE-CLAPA and OSUG GLACIOCLIM-CENACLAM projects are acknowledged for their support. Edited by: S. Preunkert