Instability and Convective Storms

1. Instability and Convective Storms AOS 101 Section 301 - March 30, 2009

2. When Do Severe Convective Storms Happen? • During the spring and summer, certain atmospheric conditions can be set up which drive severe convective storms: • Conditionally instable atmosphere • Moisture • “Capping” inversion • Upward vertical motion (“Lifting”) • Wind shear

3. Conditional Instability • Recall from last week – the atmosphere is conditional instability if the environmental lapse rate is less than the dry adiabatic lapse rate and greater than the moist adiabatic lapse rate

4. Conditional Instability DALR = 10 C/km MALR = 6 C/km Environment Height (km) Temp (C)

5. Conditional Instability A “dry” (unsaturated) air parcel will cool at the DALR when lifted. It will always be colder than its environment, and sink back to its original level - STABLE DALR = 10 C/km MALR = 6 C/km Environment Height (km) Temp (C)

6. Conditional Instability A “dry” (unsaturated) air parcel will cool at the DALR when lifted. It will always be colder than its environment, and sink back to its original level – STABLE A “moist” (saturated) air parcel will cool at the MALR when lifted. It will always be warmer than its environment, and continue to rise - UNSTABLE DALR = 10 C/km MALR = 6 C/km Environment Height (km) Temp (C)

7. Conditional Instability A “dry” (unsaturated) air parcel will cool at the DALR when lifted. It will always be colder than its environment, and sink back to its original level – STABLE A “moist” (saturated) air parcel will cool at the MALR when lifted. It will always be warmer than its environment, and continue to rise – UNSTABLE This situation can occur often in the atmosphere, since the atmosphere is ONLY unstable for saturated air DALR = 10 C/km MALR = 6 C/km Environment Height (km) Temp (C)

8. Moisture • Since a conditionally unstable atmosphere is only unstable with respect to an saturated air parcel, some moisture source is required to create severe weather

10. Cyclone advects warm, moist air from the Gulf of Mexico into the central plains

12. Region of negative Lifted Index indicates air is conditionally unstable for an air parcel lifted from the surface to 500 hPa

13. Capping Inversion • For severe weather to form, the air near the surface has to be nearly saturated • It is difficult to reach this level of saturation because of the tendency for the atmosphere to mix moist and dry air • A capping inversion prevents this mixing, allowing the surface to become moist, while the air above remains dry

14. When no capping inversion is present… N S

15. When no capping inversion is present… 1) Surface winds advect moisture from the Gulf of Mexico into the central plains N S

16. When no capping inversion is present… • Surface winds advect moisture from the Gulf of Mexico into the central plains • Small convective cells mix the moist air at the surface with the dry air above N S

17. When no capping inversion is present… • Surface winds advect moisture from the Gulf of Mexico into the central plains • Small convective cells mix the moist air at the surface with the dry air above • Air over the central plains remains relatively dry N S

18. Now when a capping inversion is present… N S

19. Now when a capping inversion is present… 1) Surface winds advect moisture from the Gulf of Mexico into the central plains N S

20. Now when a capping inversion is present… • Surface winds advect moisture from the Gulf of Mexico into the central plains • Capping inversion suppresses convective cells, allowing the air to stay very moist near the surface, and very dry aloft N S

21. Now when a capping inversion is present… • Surface winds advect moisture from the Gulf of Mexico into the central plains • Capping inversion suppresses convective cells, allowing the air to stay very moist near the surface, and very dry aloft • A strong enough upward push will allow moist air to “break” the capping inversion, creating an unstable situation and leading to severe weather N S

22. Vertical Lift • An unstable atmosphere will only generate severe weather when given a push – the initial perturbation is allowed to accelerate out of control in the unstable environment • Since a capping inversion prevents vertical lift, this push has to be relatively strong in order to “break” the capping inversion

23. Vertical Lift • Sources Include: • Strong Surface Heating • Passage of a Warm/Cold Front • Low-Level Convergence or Upper-Level Divergence

24. Wind Shear • Wind shear is a term which describes how the speed and direction of the wind change with height • It is critically important for the formation of severe weather, because wind shear will keep upward motion and downward motion in the storm separated, allowing the storm to survive for a long time

25. NO WIND SHEAR Thunderstorm cloud is created and maintained because of upward motion

26. NO WIND SHEAR Thunderstorm cloud is created and maintained because of upward motion Thunderstorm produces rain, which falls downward and drags air downward, disrupting upward motion that feeds the storm Storm dissipates

27. STRONG WIND SHEAR Thunderstorm cloud is created and maintained because of upward motion

28. STRONG WIND SHEAR Thunderstorm cloud is created and maintained because of upward motion Shear produces rain shaft and downward motion away from region of upward motion Storm is sustainable

29. Elements of Severe Storms

30. Mammatus Clouds

31. Mammatus Clouds

32. Mesocyclone

33. Wall Cloud

34. Shelf Cloud