Low Level Turbulence, III
Turbulence caused by relatively small systems or circulations of airflow are called mesoscale circulations. These are small patterns and confined to a local area. The turbulence experienced by pilot from these circulations is not significant by themselves but they can become hazardous in combination with other factors like mountains, valleys and low visibility and or ceilings.
Another factor is that these mesoscale circulations are not normally included in weather reports. Unless the airport is located within a valley experiencing these effects and you called them before your flight, you would not be aware if any exist. Interpreting geographic data combined with weather charts can reveal possible locations for mesoscale turbulence.
Local breezes are found around the world and for the pilot it can be a very interesting experience if he encounters one of these.
These are caused by horizontal temperature differences within a couple to a few hundred miles. The source of all these variations in heating and cooling of the surface caused by the sun and can vary due to the time of day, season and latitude. You may expect low level turbulence and wind shear.
Often times where winds of different speeds converge over the surface a shear line develops and together with updrafts and low level turbulence is the result.
Found along coastlines, heating of the land during the day will cause the land to be warmer than the sea or lake nearby. Due to this heating pressure drops (warm air expands) and a horizontal pressure gradient starts the flow of cool air from the body of water inland. This pressure drop is most of the time to low to be seen on a surface analysis map, but you may see different QNH readings if airports are located in these areas.
Wind speeds may go up to some 15 kts and is at its highest during late afternoon, subsiding afterwards. The depth can go up to 1500 to 3000 ft and above that there will be a return flow. For example: if airports are close to the beach you may experience a cross wind that abruptly changes from left to the right (or vice versa) when descending into this sea breeze circulation.
Sea breeze front
When cool air moves inland as a sea breeze it eventually will warm up and start to rise where it meets the warm air over land. At that point a small frontal zone has established with rather steep slopes. The warm air over land will be pushed upwards by this frontal zone and with some instability and moisture in the warm air cumulus clouds will develop marking the position of the front.
Wind shear, up and downdrafts (some 500 ft/m) can be expected when flying through this front. A change in OAT, wind speed and direction plus the absence of turbulence indicates to the pilot that he has passed this zone. Depending on the terrain this frontal zone may lay a few miles inland or more.
This is the opposite from a sea breeze and occurs after sunset when the land cools more rapidly than the water nearby, this causes the pressure gradient to reverse (low pressure area over the water) and the wind will blow toward the water with a return flow at altitude. This continues until sunrise but is usually weaker than the sea breeze, around 5 kts. Along mountain ranges this land breeze can be stronger due to cooling of the slopes and this intensifies the land breeze.
As with the sea breeze front, there is the possibility of a land breeze front developing a couple of miles out over the water. Expect a shear line with convective activity with cumulus clouds and maybe even up to thunderstorm level.
The slopes of hills and mountains will heat up during the day and under fair conditions air will start to rise creating a wind moving upslope (Anabatic wind) from the valley. Above the mountain tops a return flow will be found. With enough moisture and instability, fair weather cumulus will be seen over the tops.
The development of a valley breeze depends on the time of day and more importantly the orientation of the valley with respect to the sun and incoming solar radiation. During winter time and with snow covered slopes this effect will be minimized. Any trees will also disturb the valley breeze.
Quite the opposite of the valley breeze, this wind is called katabatic. The cooling of the slopes creates a pressure gradient. This wind flows down the slopes and cold air will sink into the valley and exits at a lower point creating winds sometimes up to 25 kts.
With both valley and mountain breezes you may expect some turbulence due to wind shear. When winds exceeds 15 kts at the mountain tops these circulation are disturbed creating more turbulence. Local variations in vegetation, trees and valley orientation will dictate how the end result will be.