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Evidence for the Influence of Agriculture On Weather & Climate Through the Transformation & Management of Vegetation : illustrated by examples from the Canadian Prairies. R. L. Raddatz Hydrometeorology and Arctic Laboratory Meteorological Service of Canada.

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slide1

Evidence for the Influence of Agriculture

On Weather & Climate

Through the

Transformation & Management of Vegetation

: illustrated by examples from the Canadian Prairies

R. L. Raddatz

Hydrometeorology and Arctic Laboratory

Meteorological Service of Canada

slide2

Land Surface – Atmosphere Interaction

Moisture (mass) balance of surface layer of the land:

P = I +E+T+ R + D +∆Sw

Surface energy balance:

Q* = QG + QH + QE

Moisture (mass) balance of the atmosphere:

P =E+T+ ( F+ - F- ) + ∆Sv

slide3

Main Properties of Vegetation

(that influence the transfer of heat, moisture

and momentum from land surface to overlying air)

Physiological

- leaf area

- stomatal resistance

- rooting depth

Physical

- albedo

- roughness length

+ Impact of vegetation on soil moisture

Including Irrigation

(Arora, 2002)

slide4

Vegetation Transformation & Management

by Agriculture

15-18 million km2

Cropland (12%)

34 million km2

Pasture & Range Land (22%)

(Leff et al., 2004)

slide5

Illustrative examples from:

Cropped Grassland Eco-climatic Region

Canadian Prairies Provinces

50% of area in

annual field crops

Annual field crops

are a primary source

of Evapotranspiration

slide6

1st Generation Prairie Crop Phenology & Water Use Model

Vegetation Weather Soil

- spring wheat - daily precipitation - available water

- Perennial grasses - maximum temperature holding capacity

- minimum temperature

- incidental solar radiation

- photoperiod

Planting Date

Initial Soil Moisture

Heat Units

Precipitation

Potential

Evapotranspiration

Vegetative Cover

Water Balance

Model

Water-Demand

Consumptive Use

Rooting Depth

Available Soil Moisture

Water-Use

slide7

Planting Dates

Initial Soil

Moisture

Soil Moisture Module

PBL Module

Growth Module

Vapour Deficit

& Aerodynamic

Resistance

Top-Zone

Root-Zone

Crop Stage

Crop

Water-Demand

Leaf-Area

Soil Resistance

&

Skin Humidity

Canopy

Resistance

Rooting

Depth

Crop Water Use

2nd Generation Prairie Crop Phenology & Water Use Model

  • Crop
  • Wheat
  • Canola
  • Potatoes
  • Weather
  • Daily Precipitation
  • Daily Temperature Extremes
  • Gridded upper-air data
  • Photoperiod
  • Soil
  • Soil Textural Class
  • Terrain Heights
  • Drag coefficients

Column

Model

slide8

Land Surface – Atmosphere Coupling

Often via the Vegetation(Basara & Crawford, 2002)

  • strong linear relationship between root-zonesoil moisture
  • and - Evaporative fraction ETf = QL / ( QH + QL )
  • - Near surface
  • - maximum temperatures
  • - afternoon mixing ratios
  • - Boundary layer
  • - mean potential temperatures
  • - mean mixing ratios
  • correlation with top zone soil moisture was weak and non-linear
slide9

Impact of

Annual Crops

(and green-up of

Deciduous Trees)

on the

Evaporative Fraction

July 10

Cropped Transitional Grassland

Canadian Prairies

Lower mean afternoon mixing-layer

depths in June & July than in May due

to increase QL and reduced QH

due to transpiration from annual field

crops and from aspen groves

July 10

slide10

1135 CST

Initiation

1005 CST

No cloud

Initiation of Convection

With Weak Synoptic Forcing

1335 CST

Max area

slide11

Time of convective initiation more highly correlated

with root-zone than with top-zone soil moisture

R = 0.77

Atmospheric Boundary Layer – Soil Moisture Coupling

  • Occurs, primarily, via the vegetation.
slide12

Cumulus Convection – Soil Moisture Feedback

  • Where annual crops dominate the vegetation, the primary

cumulus convection – soil moisture feedback occurs on the

seasonal time scale.

slide13

Agriculture & Weather and Climate

Through agriculture (land clearing, cultivation, and the grazing of

domesticated animals), man has transformed, and now manages to

varying degrees, the vegetation (i.e., the physiological and physical

properties of the land cover), and directly (via irrigation) or indirectly

(via the vegetation) the soil moisture over large tracks of land.

By altering the properties of the vegetation, agriculture influences the

magnitude of the net radiation (via surface albedo), and how this energy

is partitioned into sensible and latent heat fluxes (via stomatal

resistance). It may also influence the vertical flux of momentum

(via the roughness length).

Agriculture also has an impact upon the aerodynamic coupling between

the land surface and the atmosphere (via aerodynamic resistance),

and, thus, it has a further impact on the surface fluxes.

slide14

Evidence for the Influence of Agriculture on Weather & Climate

Tables:(Extensive but not comprehensive)

Region | Ag-Impact | Wx Element | Obs or Mdl | Author

Framework for GroupingStudies

1. Agriculture’s Influence on Near Surface Weather Elements.

2. Agriculture’s Influence on the Regional Hydrologic Cycle**

3. Agriculture - Tele-connections & Inter-seasonal Influence.

slide16

Incremental Change

Mean Daily Maximum Temperatures & Afternoon Mixing Ratios

More Humid

Drier

Drier

slide17

Plains-to-Mountains Circulation

(Influence adjacent areas - Local effects become regional effects)

Stohlgren et al., 1998

Chase et al, 1999

Strong, 2000

Influence on

Foothills Weather

Impact upon

Plain’s Vegetation

slide18

Agriculture’s Influence upon the Regional Hydrologic Cycle

  • (1) Affects the availability of convective energy (CAPE).
  • (2) Affects the availability of water vapour (Recycling Ratio).
  • Spatial discontinuities in vegetation and/or soil moisture
  • can induce mesoscale thermal circulations (land-land breezes)
  • that initiate moist deep convection.
slide19

Moist deepconvection

- Results from the release of CAPE

when boundary-layer air parcels

lifted to level of free convection

by a dynamic or thermal

mechanism.

Severe Weather

from Thunderstorms

- Flooding

- Hail

- Tornadoes

- etc.

Convective Rainfall

Pielke et al, 2001

slide20

Cropped Grassland Eco-climatic Region – Canadian PrairiesET = f (Weather, Vegetation Phenology & Soil Moisture)

Sensible

Heat

Latent

Heat

Bowen Ratio = Sensible Heat

Latent Heat

where

Bo > 1.0 (Sensible > Latent )

Bo < 1.0 (Latent > Sensible)

Net

Radiation

Q* = QG + QH + QE

Evapotranspiration from the annual field crops controls the seasonal pattern of the partitioning of the surface net radiation.

slide21

Lifted Index : LI = T50 - Tparcel

A widely used measure of the amount of CAPE

for the development of moist deep convection

LI > 0

LI = 0 to -3

LI < -3

slide22

Reduction in Lifted Index due to Regional Evapotranspiration

Reduction LI = Increase CAPE

(Segal et al, 1995)

slide24

Example:CAPE highly sensitive to low-level moisture

1400 to 3200 J kg-1

Convective Available Potential Energy (CAPE)

11 to 15 g kg-1

Winnipeg $60 million

Hail Storm

Specific Humidity of CBL

4 mm d-1

Evapotranspitation

attributed to Agro-ecosystem

July 16, 1996

regional atmospheric water balance
Regional Atmospheric Water Balance

F+

F-

Re

Ri

ET

F+ = horizontal influx of water vapour

F- = horizontal efflux of water vapour

Re = areal average rainfall from external moisture

ET = areal average or regional evapotranspiration

Ri = areal average rainfall from internal moisture

(Budyko, 1982, Brubaker, 1993, Trenberth, 1999)

slide29

Summer Recycling Ratio for Region

  • Ri / R = 1 / ( 1+ ( 2F+ / (ET*A ))
  • where ( R = Re + Ri )
  • Relative contribution of water vapour from

regional evapotranspiration to total rain

  • Measure of importance of evapotranspiration

to the regional hydrologic cycle.

slide30

Advection or Horizontal Flux ( F + and F- )

*

*

*

*

*

*

“q” is liquid equivalent of the water vapour in the atmospheric column

( 100 to 25 kpa ) .

“u” & “v” are vertical mean, with mixing ratio weighting, wind components.

slide31

Wheat Modeling Sites

Rainfall & Land-use weighted Evapotranspiration

slide32

Summer Recycling Ratio for Region

SummerRecycling Ratio

1997 24%

1998 35%

1999 25%

Regional ET (i.e., recycled regional moisture)

is a significant source of water vapour mass for

summer rainfall.

(Bosilovich & Schubert, 2002

Summer recycling ratio for western Canada (1990-1995) is 29%.

slide33

June – July - August

  • Cropped Grassland
  • Eco-climatic Region
  • Canadian Prairies
  • poor correlation between
  • horizontal influx (advection)
  • of moisture and summer rain.
  • good correlation between
  • regional moistening efficiency
  • and summer rainfall, where
  • M = ET*L / F, and
  • ET = f (crop stage,
  • soil moisture)

June – July - August

M = ET*L / F

Trenberth, 1999

slide34

Agriculture’s Influence on Mesoscale Thermal Circulations

Spatial discontinuities in vegetation and/or soil moisture can induce

mesoscale thermal circulations (land-land breezes) that may initiate

moist deep convection.

(Segal & Arritt, 1992; Lee & Kimura, 2001).

slide35

Mixing-Layer Depth

Sensible Heat Flux

Land – Land Breeze

Meso-scale circulation induced by ET discontinuity

slide36

Mixing-Layer Depth

Sensible Heat Flux

Land – Land Breeze

Wet | Dry

Meso-scale circulation induced by ET discontinuity

slide37

1135 CST

Initiation

1005 CST

No cloud

1335 CST

Max area

slide39

Tele-connections

Thunderstorms are conduits for heat & moisture

from lower to higher altitudes. Thus, spatially

coherent and persistent patterns of moist deep

convection, in the tropics and during mid-latitude

summers, may influence the ridge and trough

positions in the polar jet stream.

Agriculture, by having an impact upon

deep convection, particularly in Tropics,

can affect the weather on a global scale.

(Chase et al., 1996; Chase et al., 2000; Zhao et al., 2001)

slide40

Inter-Seasonal Influence

A high level of root-zone

soil moisture in the spring,

and vegetation to transfer

that moisture to the

atmospheric boundary layer

during the growing season,

are necessary, though not

sufficient, conditions for a

convectively active summer.

May 31, 2002

( Shukla & Mintz, 1982;

Timbal et al., 2002;

Koster and Suarz, 2004;

GLACE Team, 2004)

May 30, 2004

slide41

Soil Moisture Hot Spots

(Global Land Atmosphere Coupling Experiment)

- Regions where soil moisture

anomalies have a substantial

impact on summer rainfall.

- Transition zones between

wet & dry areas where

adding moisture to the

boundary layer can lead

to moist deep convection

and where ET is relatively

high but still sensitive

to soil moisture.

-Through agriculture (land clearing, cultivation, and the grazing of domesticated animals),

man has transformed, and now manages to varying degrees, the vegetation and directly

(via irrigation) or indirectly (via the vegetation) the soil moisture over large tracks of land.

slide42

Cropped Grassland

  • Eco-climatic Region
  • Canadian Prairies
  • good correlation between

regional moistening efficiency

and summer rainfall, where

ET = f (crop stage,

soil moisture)

  • poor correlation between

spring soil moisture and

summer rainfall.

Thus, agriculture influences

the current season’s

convective rainfall, but

the inter-seasonal influence

is weak.

M = ET*L / F

slide43

Evidence for the Influence of Agriculture on Weather & Climate

Tables:(Extensive but not comprehensive)

Region | Ag-Impact | Wx Element | Obs or Mdl | Author

Framework for GroupingStudies

1. Agriculture’s Influence on Near Surface Weather Elements.

2. Agriculture’s Influence on the Regional Hydrologic Cycle**

2.1 Convective Available Potential Energy (CAPE)

2.2 Regional Moisture Recycling

2.3 Mesoscale Thermal Circulations

3. Agriculture - Tele-connections & Inter-seasonal Influence.