Wind Science 101: I. Overview of Wind Patterns Eugene S. Takle Professor Department of Agronomy Department of Geological and Atmospheric Science Director, Climate Science Program Iowa State University Ames, IA WESEP REU Short Course Iowa State University Spring 2011

Outline  Global scale  3-D global circulation patterns and wind energy  Surface and upper-air tropical and mid-latitude weather systems, including prevailing westerlies  Mesoscale  Great Plains Low-Level Jet and nocturnal LLJs  Sea-breeze  Monsoon circulation  Off-shore resources  US wind resource maps  Forecasting wind resources  Atmospheric boundary layer  Structure and diurnal/seasonal evolution  Impact of static and dynamic stability on horizontal wind speeds and vertical profiles  Turbulent flows and interactive wakes

Not to scale! Mean radius of the earth: 6371 km Height of the troposphere: 0-7 km at poles 20 km at Equator 90% of atmosphere is in the lowest 15 miles (24 km) 99% in lowest 30 miles (48 km) Non-rotating Earth heated at its Equator

Global Precipitation Patterns

NOAA NCEP-NCAR CDAS-1 MONTHLY 300 mb [ u, v ] climatology January Wind speed at 12 km

NOAA NCEP-NCAR CDAS-1 MONTHLY 300 mb [ u, v ] climatology July Wind speed at 12 km

NOAA NCEP-NCAR CDAS-1 MONTHLY Diagnostic above_ground [ u, v ] climatology (m/s) January Wind speed near surface

NOAA NCEP-NCAR CDAS-1 MONTHLY Diagnostic above_ground [ u, v ] climatology (m/s) July Wind speed near surface

NOAA NCEP-NCAR CDAS-1 DAILY 300 mb height (m) and winds (m/s) 1 Apr

Continental and Regional influences Continental scale circulation, jet streams Great Plains Low-Level Jet Nocturnal LLJ Coastal Jets Sea breezes Mountain-valley flows Mountain compression of stream lines Off-shore wind

Mechanism of Low-Level Jets: General Principles Great Plains Low-Level Jet (GPLLJ) Nocturnal Low-Level Jet (LLJ) Coastal Jet (CJ)

Mechanism of Low-Level Jets: General Principles Great Plains Low-Level Jet (GPLLJ) Nocturnal Low-Level Jet (LLJ) Coastal Jet (CJ)

HL Pressure Gradient

HL FcFc FpFp F c = -2ΩxV Coriolis Force

HL Pressure Gradient FcFc FpFp

HL FpFp FcFc VgVg Geostrophic Balance

HL FpFp FcFc V FfFf Frictional Force F f = -C d vV

HL FpFp FcFc V At night, friction is eliminated, flow is accelerated, V increases

HL FpFp FcFc V Coriolis force increase, wind vector rotates and speed continues to increase

HL FpFp FcFc V VgVg Wind vector rotates and speed continues to increase and exceeds geostrophic wind

Mechanism of the Nocturnal Low-Level Jet: General Principles Great Plains Low-Level Jet (GPLLJ) Nocturnal Low-Level Jet (LLJ) Coastal Jet (CJ)

Rocky Mountains Missouri River High TempLow Temp Low Press High Press

H Bermuda High creates flow from the south in summer over the central US, which is accelerated at night by a terrain-induced pressure gradient

Wind speed as a function of height during the LLJ peak on March 24, 2009 at 1000 LST from the Lamont, OK wind profiler (Adam Deppe MS thesis, ISU, 2011)

Height above ground Horizontal wind speed Great Plains Low-Level Jet Maximum (~1,000 m above ground) ~1 km

Mechanism of the Nocturnal Low-Level Jet: Great Plains Low-Level Jet (GPLLJ) Nocturnal Low-Level Jet (LLJ) Coastal Jet (CJ)

High TempLow Temp Low PressHigh Press

HL FpFp FcFc V VgVg

Height above ground Horizontal wind speed Nocturnal Low-Level Jet Maximum (~400 m above ground) ~400 m

Mechanism of the Nocturnal Low-Level Jet: Great Plains Low-Level Jet (GPLLJ) Nocturnal Low-Level Jet (LLJ) Coastal Jet (CJ)

High Temp Low Temp Low PressHigh Press

Coastal Mountains High Temp Low Temp Low PressHigh Press

HL FpFp FcFc V FfFf Frictional Force F f = -C d vV Mountains produce an additional pressure force

Height above ground Horizontal wind speed Coastal Jet Maximum (~ m above ocean) ~ m

Note high winds at mountain ridges

100 km

Musial, W., and B. Ram, 2010: Large-scale Offshore Wind Power in the United States. Assessment of Opportunities and Barriers. NREL/TP pp. [Available online at

Take Home Messages Winds are created by horizontal temperature difference (which create density differences and hence pressure differences) Rotation of the Earth creates bands of high winds (prevailing westerlies) at mid-latitudes Interactions with the day-night heating and cooling of the earth’s surface create changes in the vertical structure of the horizontal wind Orographic feature (coastal regions, mountains, etc) create local circulations that enhance or decrease wind speeds

Wind Science 101 II. Atmospheric Boundary Layer Eugene S. Takle Professor Department of Agronomy Department of Geological and Atmospheric Science Director, Climate Science Program Iowa State University Ames, IA Honors Wind Seminar Iowa State University Spring 2011

High Interannual Variability: Number of Wind Speed Reports per Month ≤ 5 kts at Mason City, IA 1 Oct 2001 – 30 Sep Jan – 31 Dec 1998 Data by Adam Deppe

1 knot = mph = m/s

Number of OccurrencesWinspeed (m/s)

Height (z) Windspeed Power Law Logarithmic Dependence U * = friction velocity k = von Karman’s constant (0.40) z o = roughness length

High Interannual Variability: Number of Wind Speed Reports per Month ≤ 5 kts at Mason City, IA 1 Oct 2001 – 30 Sep Jan – 31 Dec 1998 Data by Adam Deppe

Modeling the Atmospheric Boundary Layer

In Tensor Notation: K = constant One-and-a-half order: Turbulence options:

ε = dissipation Turbulence Kinetic Energy: Third Order: ε = q 3 /Λ

Conceptual Model of Turbine-Crop Interaction via Mean Wind and Turbulence Fields __ ___________________________________ Speed recovery CO 2 H2OH2O Heat day night A conceptual model of turbulence generated by turbines suggests enhancement of near-surface mixing both day and night, which will… reduce daytime maximum temperature in the crop (good) increase night-time temperature in the crop (???) reduce dew-duration in crops (good) enhance downward CO2 flux into the canopy during daytime photosynthesis (good) enhance CO2 flux out of the canopy at night (???) suppress early killing frost (good) help dry down the crop before harvest (good)