Seasonal and Diurnal Fluctuations

1
Seasonal and Diurnal Fluctuations
Geography 520 Climatology
Lecture 6
2
The Geographic Grid
Latitude is the angle between a point on a
parallel and the centre of the Earth and a point
on the equator Longitude is the angle between a
point on a meridian and the centre of the Earth
and a point on the Prime Meridian
Figure 1.5, p. 29
3
Sun Incidence Angles

• Sun Angle
• Angle of sun above horizon
• Solar Zenith Angle
• Angle between sun and zenith

4
The Earths Revolution around the Sun

• Revolution
• one complete circling around the Sun
• from above north pole in counterclockwise
  direction
• the path is not circular but is elliptical
• orbits on the plane of the solar system

E
5
The Earths Revolution around the Sun
the Earth and Moon both rotate and revolve in a
counterclockwise direction (when viewed from a
point over the Earths north pole)
6
The Earths Revolution around the Sun
Aphelion - the Earth furthest away from Sun
(July 4)
Perihelion - the Earth closest to Sun (January
3)
152 million km
147 million km

• sun is not in the middle of the plane of the
  ecliptic
• variation in distance of 3
• 7 difference in intensity of solar radiation
  between perihelion and aphelion

7
Oliver and Hidore (2002)
8
The Earths Revolution around the Sun
at equinox, the circle of illumination passes
through both poles the subsolar point is the
equator each location on Earth experiences 12
hours of sunlight and 12 hours of darkness
Figure 1.18, p. 41
9
The Earths Revolution around the Sun
Solstice (sun stands still) On June 22, the
subsolar point is 23½N (Tropic of Cancer) On
Dec. 22, the subsolar point is 23½S (Tropic of
Capricorn)
Figure 1.19, p. 41
10
The Earths Revolution around the Sun

• The latitude of the subsolar point marks the
  suns declination
• Solar declination changes throughout the year

Figure 1.20, p. 42
11
Latitude 40 N
12
Latitude 0
13
Latitude ?
14
Example Day and Night Surface Energy Flow
no clouds in both cases. Clouds can give a
positive RN at night
15
Energy Balance of a Dry Surface
Arya (2001)
Nighttime Temperature inversion promotes sensible
heating of surface
Surface heats atmosphere at day, negative sign to
balance with net radiative flux
Sun heats ground at day, negative sign to balance
with net radiative flux
Nighttime radiative loss by heat stored in ground
16
Energy Budget Experiment over a Vegetated
Surface at The OSU AirportAutumn 2005
17
Radiation Components
S?
S?
L?
L?
QR
18
Sensible Heat Flux
Review

• A direct flux in W m-2
• It is directly related to temperature differences
  between the surface and the adjacent atmosphere.
• Flux follows the temperature gradient
• Energy flows from higher concentrations to lower
  concentrations
• Usually negative (away from the surface) during
  the day
• Temperatures at the surface are higher than air
  aloft
• Surface loses sensible heat to the air
• Usually positive (toward the surface) at night
• Temperatures at the surface are cooler than air
  aloft

19
Sign Convention in this Study

• Sign convention used is with respect to the
  surface plane.
• Interface of air and soil
• Positive fluxes are those which flow toward the
  surface plane.
• Negative fluxes are those which flow away from
  the surface plane.

P 13
(Arya 2001)
20
Sensible Heat Flux at OSU Airport in Autumn
21
Latent Heat Flux
Review

• An indirect flux in W m-2
• A function of the latent energy associated with
  phase changes of H2O, i.e., evaporation and
  condensation
• Negative (away from the surface, toward the
  atmosphere) whenever evaporation or transpiration
  is occurring at the surface
• Evapo-transpiration cools the surface
• Occurring most often during daytime
• Positive (heating the surface) whenever
  condensation is occurring
• Occurs most often during the early morning as dew
  forms
• Occurs during precipitation

22
Latent Heat Flux at OSU Airport in Autumn
23
Cumulative Evaporation
24

• Evaporation fueled by solar power
• during the day, QE reaches its most negative
  value (strongest evaporative cooling) when
  downward shortwave radiation is greatest
• at night, QE reaches its most positive value when
  downward shortwave radiation is zero

25
Ground Heat Flux W m-2

• Due to the conduction of energy up and downward
  in the ground
• Thermal conductivity is a function of soil
  properties
• Usually downward during the daytime
• air-ground interface is warmer than the soil
  beneath
• Usually upward during the nighttime
• air-ground interface is cooler than the soil
  beneath

26
Evaluating Soil Thermal Conductivity
Temperature sensors -0.04 m -0.12 m -0.20 m 2
Heat Flux Plates at -0.08 m
Dan Steinhoff
If we know the heat flux (W m-2) from the heat
flux plates, we can solve for the thermal
conductivity, given
The soil pit
27
Surface Energy Budget Components at OSU Airport
in Autumn
S?
S?
L?
L?
QR
net radiation (black), sensible heat flux (red),
latent heat flux (blue), and ground heat flux
(green).
28
Surface Energy Budget Components at OSU Airport
in Autumn
QH
QE
QG
QR
net radiation (black), sensible heat flux (red),
latent heat flux (blue), and ground heat flux
(green).
Changes in one EB component cause changes in
others.
29
Frontal Situations for OSU Airport Experiment
November 6, 2005
November 7, 2005
Surface Weather Maps and Station Weather 700 EST
(NOAA/NCEP)
30
Frontal Situations for OSU Airport Experiment
November 10, 2005
November 9, 2005
Surface Weather Maps and Station Weather 700 EST
(NOAA/NCEP)
31
Effect of Fronts on the Microclimate
32
Effect of Fronts on the Microclimate
33
Effect of Fronts on the Microclimate
34
Fronts in the Data
Average Temperature C
Average Pressure hPa
Range of Wind Speed kts
Precip in
35
Component Changes
36
Main Conclusions of OSU Airport Energy Budget
Experiment

• The effects of frontal passages on energy budget
  components are clearly observed in changes in
  daily mean values.
• At this time of year, H heats the surface, on
  average, except when cold air masses invade
• Net evaporation loss is evident in the
  measurements
• Net solar irradiance is the driver for
  evaporation
• Most of the residual in energy budget closure can
  be explained by changes in ground stored heat
  energy.

37
Review of Concepts

• Earths Orbit
• Seasons
• Diurnal fluctuations
• Air temperature and wind speed
• Energy budget components