Chapter 10 Desert Boundary Layers

1. Chapter 10Desert Boundary Layers

2. Boundary Layer Review • BL - the region of the troposphere immediately above Earth’s surface where vertical turbulent transfers of heat, moisture and momentum are large compared to the troposphere above. • Daytime temperature lapse rate is nearly dry adiabatic (9.8 C/km). • Nighttime temperature lapse rate is stable (inversion).

3. Remember Energy Budget Components

4. Why Study Boundary Layers • We live there. • Some of our meteorological conditions at the surface originate higher in the troposphere (precip, large scale T changes, etc.), but many develop within the BL. • Air pollution (public health) –most originates at surface and stays within BL • Dust storms • Diurnal temperature fluctuations • Mesoscale wind circulations

6. Turbulence

7. Two Types of Air Flow • Turbulent flow – eddies mix the air between layers, causing an exchange of properties between the layers (mixing) • Laminar flow – there is no turbulence, and the layers of air “feel” each other only through molecular mixing.

8. Two Sources of Turbulence • Vertical shear of the horizontal wind – night time and daytime • Buoyancy – daytime, mostly

9. Turbulent Flow Dry, high speed, low temperature TURBULENT MIXING FrictionalStress Moist, slow speed, high temperature

10. Laminar Flow Dry, low temperature ONLY MOLECULAR MIXING FrictionalStress Moist, high temperature

11. Wind Direction Variation – A Measure of Turbulence

12. Potential Temperature (q) • The temperature that a parcel of air would have if it was moved dry adiabatically to 1000 mb. • Within a well-mixed layer of air, qdoes not change with height.

13. Boundary Layer Structure

14. El Paso, Texas sounding - 5PM local time(northern Chihuahuan Desert) Daytimeboundary layer

15. Jacksonville, FL sounding - 7PM local time(Non-arid location) Daytimeboundary layer

16. El Paso, Texas sounding - 5AM local time(northern Chihuahuan Desert) Nighttimeboundary layer

17. Jacksonville, FL sounding - 7PM local time(Non-arid location) Nighttimeboundary layer

18. Denver, CO sounding - 6PM local time Daytimeboundary layer

19. Denver, CO sounding - 6AM local time Residual Mixed layer Nighttimeboundary layer

20. Norman, OK sounding - 7PM local time Elevated Mixed layer Daytimeboundary layer

21. What Controls Desert Daytime Boundary Layer Depth? • Amount of heating during the day – the more heating, the deeper the mixing. - length of day and sun angle - land-surface physics (surface moisture, sand versus rock surface) • Subsidence limits the growth (subtropical deserts).

22. What Controls Desert Night-time Boundary Layer Depth? • Large vertical shear (change) in the horizontal wind creates turbulence near the surface at night – so the larger this shear, the deeper the boundary layer. • The amount of heat that was stored in the substrate during the day, and that is released to the atmosphere at night.

23. Unique Aspects of Desert Boundary Layers • Deeper • Hotter • Dustier • Drier

24. Great Basin Desert

25. Boundary Layer Structure Near a Coast – The Atacama Desert 0800 LT 1400 LT 130 km Inland 60 km Inland At the Coast

26. Season Variation – Arabian Peninsula 0300 LT 1500 LT

27. Regional Variability - Sahara

28. Summary • BL structure depends on the type of desert (coastal vs. hot) • Hot desert BLs • Deeper • Hotter • Drier • Dustier