Low Level Turbulence, II
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 where you would experience these effects and if you called the tower before your flight, you would not be aware if any exists. Interpreting geographic data combined with weather charts can reveal possible locations for this type of mesoscale turbulence.
Our subject here will be low level jet streams as these can cause windshear and turbulence during take-off and landing especially in certain coastal areas.
Low Level Jet Streams
These jets are mainly narrow band and usually found between 500 and 5000 feet with speeds from 20 to 80 kts. Horizontal dimensions range from several hundred miles long and wide and usually occur within a stable layer and you may expect windshear when climbing or ascending through such a layer.
Most of the low level jets are nighttime occurrences so if you fly day VFR only your exposure will be very low. But if you fly night VFR or do a lot of IFR flying you may find yourself, one fine day, experiencing one of these little jets. And bumping into unexpected low level turbulence without any previous warning in the form of a PIREP.
This jet is caused by a nighttime cooling of higher terrain causing a nocturnal inversion and this helps the pressure difference at the top of the boundary layer. A high pressure area over land during nighttime will assist in forming the jet. After sunrise convective heating of the land will eventually break down the jet.
Some jets move from a high pressure toward a developing low and if moving over a body of water, they can bring in lots of moist warm air and as a result some severe weather may develop.
Before sunrise the jet will reach maximum speeds to around 25 to 40 kts at low altitudes of 500 to 1500 feet AGL. You may expect significant windshear when passing through these altitudes, after sunrise the jet will dissolve due to heating and convection of the ground by the sun.
Found mainly in coastal areas and they have their origin in land-water temperature differences which will cause air to flow to the warmer area, thereby creating a sea-land breeze and in the right conditions together with an inversion a low level coastal jet. Some coastal areas are more prone to this phenomenon than others.
This jet may cause up to windspeeds of around 50 kts with large vertical windshear vectors. Pilots operating at airports along the coast should be vigilant for this phenomenon, even more so when there are mountains and valleys running parallel with the coast situated nearby.
When cool air mass moves inland and a line of mountains or hills runs parallel with the coast, colder air moving south to the low pressure area can get trapped under the cool air mass moving over those hills. Thus creating a jet just below the frontal zone near the top of the hills. Windshear and temperature differences can be felt by the pilot crossing this area.
Flying through this area the pilot may see OAT differences between 5 to 10°C indicating the passage through the frontal zone. An abrupt change in windspeed and direction is also on the list of events together with turbulence.
Talk to pilots and instructors local to the area about these phenomena and find out if there are any. See under what circumstances they experienced this and how they reacted. This will prepare you mentally so that when it happens you will know how to react and keep the flight safe or be able to avoid these.