In this session, we will start focusing on the winds in the lower layers of the atmosphere, but not quite reach the heights where the wind turbine - turbines stand. Here is the outline. We will first describe in more details the layers of the troposphere. Then, after a brief parenthesis on oceanography, we will describe how winds make that transition from geostrophy to the weaker values near the surface. Then we will describe how the state and elevation of the surface influences the wind on intermediate scales; that is, from a few - a few hundred kilometers to a kilometer or so. First, let's focus on the troposphere, the layer in which meteorological phenomena occur. It extends from the surface to about 10 kilometers in the mid-latitudes and about 15 between the tropics. Previously we stated that the winds were well approximated as geostrophic; that is,along isobars with the Coriolis force balancing the pressure force. To be more precise, this is true in the free atmosphere, far enough from the surface that the drag due to the surface is not directly affecting the flow. The wind close to the surface, whether it is the ground or a body of water, has to be different. At the surface, we know that the boundary condition will require that the wind go to zero. So the region where we have this transition from the winds aloft to weak winds near the ground is the boundary layer, also called the planetary boundary layer. Its depth can vary from 100 meters to three kilometers. It varies depending on the time of day, the solar heating, depending on the temperature and the type of surface below. And it depends, of course, on the winds above. The surface exerts a drag on the flow. This cannot be occurring through just molecular viscosity, which only acts on scales of millimeters or centimeters. This drag comes from turbulent fluctuations in the wind - rolls, billows, eddies in the lowest layers of the troposphere. These turbulent fluctuations makes the temperature in the boundary layer and the transfer momentum downwards. So an important quantity is the turbulent flux of momentum. This is strong near the ground and it diminishes with height and it vanishes at the top of the boundary layer. Well, within the boundary layer, we identify a region called the surface layer in which the turbulent flux of momentum is nearly constant. This surface layer is typically about 10 percent of the height of the boundary layer. In the next sessions, we will describe winds in this surface layer, which is where wind turbines stand. Today we will talk of some aspects of the boundary layer above the surface layer and some local effects. The structure of the boundary layer is complex. Turbulent motions are nearly always present and they are driven by two mechanisms; instabilities, due to the wind shear because of the winds aloft and convection, induced by heating from the surface. Convection is the vertical motion due to differences in density. Warm air rises, cool air sinks. This figure represents a schematic of the boundary layer and it's diurnal cycle over land, where the heating strongly varies during the day. The horizontal axis is time, the vertical axis is height. During the day, the sun heats the air near the surface, initiating convection and a convecting - convective mixed layer develops, which fills the boundary layer. At night, this convection stops. Air near the ground cools and restratefies. We will not go into these details today, just keep in mind that the structure of the boundary layer is complex and it varies in time. Today, let's just consider the transition from the geostrophic winds aloft to the weaker winds close to the surface. This is an opportunity to say a few words of an amazing adventure which began in Norway at the end of the nineteenth century. In 1893, Nansen, who was 32, set off on the Fram, a vessel specially designed to survive being trapped in the ice. He set off towards the North Pole. At one point, he left the Fram on skis with another man and made his way up to 86 degrees north; the furthest north, at that time, man had ever gone. Once back in Norway, in 1896 - three years after he had left - he shared his observations that icebergs did not drift to the - in the direction of the wind blowing above the surface, but to some angle to the right of the wind. The meteorologist, Bjerknes, suggested to a Ph.D. student, Ekman, to study this. And now Ekman devised a model which accounted for this and which was published in 1905. The - the model describes how the ocean current near the surface is pushed in the direction of the wind and how the Coriolis force deviates it to the right. With some assumptions, he obtains an analytical formula describing a spiral; but the central point is that he explains how the combination of friction from the wind and Coriolis force produces a current near the surface which is overall to the right of the wind. Let's come back to the atmosphere. In the free troposphere, we have geostrophic balance. Seen from the top, you see here the pressure force and the wind, such that its corresponding Coriolis force balances the pressure force. Once we enter the boundary layer, the drag from the surface is felt and so the balance is now a three-way balance between the pressure force, the Coriolis force and drag of friction, which opposes the wind. In consequence, the wind is weaker - or we could say sub-geostrophic. As the Coriolis force is consequently weaker, the wind tilts towards the lower pressure. Winds pointing towards pressure - towards pressure lows is what you expect in the absence of the Coriolis force. So this is consistent with our intuition. If we come back to the 3-D view, you should imagine the wind making a transition from geostrophic in the free troposphere and gradually becoming sub-geostrophic and crossing the isobars in the boundary layer. Let's explore a bit further the implication of this last point. In the free troposphere, the wind is very close to geostrophic; on a pressure map, say at five kilometers height. The winds are parallel to the isobars and they are stronger where the isobars are more tightly packed together. In contrast, boundary layer winds will have weaker amplitudes and their orientations will be cross-isobaric; they will tend to converge in low pressure centers and diverge in high pressure regions or in cyclones. This induces vertical motions in the air above. Air will rather tend to sink above in cyclones and rise above cyclones with consequences for cloudiness. As the air rises, it cools because of decreasing pressure and at some point, part of the water vapor condenses into droplets, forming clouds. Conversely, you find clear skies above - in cyclones. In the next sessions, you will discuss what goes on in the lowest part of the boundary layer; in the surface layer. We will leave that aside for now and I will just say a few words on local effects on intermediate scales. In the previous session, we described low pressure systems which have typical size of a thousand kilometers. Now let's consider effects of the surface below on scales ranging between the synoptic scale down to 10 kilometers or so. First, orography can influence winds and channel it. A famous example in France is the mistral, which blows down the Rhone Valley in Southern France. At certain times, the rogue scale flow is in a favorable configuration as shown to the right. A low - that's the D for depression in French - over Italy and then in cyclone west of Spain favoring northerly winds in Southern France. Now there are several major topographic obstacles blocking the wind and there is the Rhone Valley between them. The wind is funneled through the Valley, producing episodes of strong northerly winds in Provence. This is the mistral. These winds are generally cold and they are associated to clear skies. Such winds are not anecdotical. The wind atlas for France bears a clear signature of this wind. On these maps, you see regions of strong winds over Northern Europe. These are tied to storms that go through these regions under what we call the storm track. But there is another, more localized region with strong wind potential according to these maps. It corresponds to the regions in Southern France and Northern Spain where the winds are channeled by topographic obstacles. So these effects matters very much for wind energy. Let's move on to other local effects. On scales of a few tens of kilometers, land sea breezes are an example of a local circulation that sets up - due to contrast in temperature that sets up between sea and land. Land has a much smaller heat capacity than water and hence it heats up more during the day. This creates temperature contrast between the sea at 10 to 15 degrees Celsius and land, which may reach 25 degrees Celsius on a summer day, for instance. The warmer air rises, the cooler air sinks and a circulation sets up with gentle winds flowing inland towards - near the surface. The reverse happens at night, but the temperature contrasts and hence the winds are much weaker then. Similar circulations induced by differential heating happen over topography. If there's a large scale environment with weak pressure gradients, that's more favorable for these effects to be noticeable. In the morning, the top of mountains or hills heat up more quickly and an upslope wind sets up. These are called anabatic winds. With time, a three-dimensional circulation sets up, which includes up-valley winds which feed the upslope winds. Reversely, at night, the air at the mountain top and slopes cool - cools down quicker and sinks downslope. Such winds are called katabatic winds. To conclude, let's come back to how orography modifies the flow, but on smaller scales this time; not on the scale of a whole mountain range like the Alps, but on the scale of a small mountain or hill. When there is wind blowing, the obstacle will force the air to pass above it and accelerate the wind at the top of the obstacle. So when it is possible, setting up wind turbines on hill tops is good for the energy that you will harvest. Surely you are familiar with landscapes having turbines on the hill crests. Even without this acceleration above the obstacle, the hill is a means to elevate your turbine to higher altitudes where the winds are stronger. How the wind varies in the first hundred meters or so, that is in the surface layer, is the topic for our next session.
