L5

Question 1

Land-surface models/ vegetation models

Answer 1

• Computer based model
  • Used to simulate land processes
  • Numerical models that solves the coupled fluxes of water, energy and carbon between the land-surface and atmosphere, within a context of direct and indirect human forcings and ecological dynamics
  • They are also known as terrestrial carbon models, dynamic global vegetation model (DGVM)
  • Names come from configurations they have

Question 2

Land surface models work for simplified configurations eg for:

Answer 2

• Meteorology
• Soil ecology
• Vegetation dynamics
• Hydrology

Question 3

Land surface models also work for full-complexity configurations eg:

Answer 3

• These resolve many cycles/ patterns etc

Question 4

How can we use land surface models?

Answer 4

independently or within an Earth system model

Question 5

We can use land-surface models independently or within an Earth System Model

Answer 5

• An earth system model is a set of models that can be run independently or together to simulate the Earth global climate
  • Land model is an important component of Earth System models
  • Land- surface models can involve complex code all centred by the coupler which connects eg land ice to ocean
  • These models are ran in high performance computing systems (HPC)

Question 6

Examples of terrestrial feedbacks within the Earth system (components of land-surface model)

Answer 6

• Snow cover and climate
  
  • Soil moisture- evapotranspiration- precipitation
  • Land use and land cover change
  • Carbon cycle
  • Reactive nitrogen
  • Chemistry-climate (BVOCs, O3, CH4, aerosols)
  • Biomass burning (fires)

Question 7

Land is a critical … of CO2 emissions

Answer 7

Sink

• Trees involved in the model produce CO2 etc
• These models can tell us implications of future emissions etc

Question 8

Many land-surface models available

Answer 8

• Often ran in conjunction with one another
  • 16 land-models participated in the 2022 carbon budget
  • They vary in what they feature eg fires vs no fires
  • Often have different configurations but are made by the same companies

Question 9

How do land-surface models work in the global carbon budget?

Answer 9

• Take an average from the land models results
  • Provide certain elements towards the carbon budget others eg gas reserve etc which are not in land-surface models

Question 10

How are land models interdisciplinary?

Answer 10

• Started off very simple then added more ecosystem functions to the land model
  • Evolved from land as a lower boundary to the atmosphere to land as an integral component of the Earth system
• Developed along with computer capability and research questions

Question 11

What is the role of a land model in an Earth system model?

Answer 11

Land-atmosphere exchanges
- Energy, water vapour, CO2, dust, trace gases

Land surface states
- Soil moisture, soil temperature, canopy temperature, snow water equivalent, C and N stocks

Land surface characteristics
- Soil texture, surface roughness, albedo, emissivity, vegetation type, LAI index

• When using a land model all these fluxes are passed from land to atmosphere whilst conserving each other

Question 12

What are the main components of a land model?

Answer 12

• Surface energy fluxes
  
  • Hydrology
  • Biogeochemical cycles

The land surface model solves surface energy balance , surface water balance and carbon balance equations at each model timestep

Question 13

surface energy balance inputs or forcings

Answer 13

• Direct solar and Incoming long wave radiation

Question 14

surface energy balance outgoings

Answer 14

Response fluxes

- Reflected solar

- Outgoing longwave

Question 15

The land surface affects the energy balance through what three properties?

Answer 15

Albedo
Surface Roughness
Evapotranspiration

Question 16

Land models can look at the … effect

Answer 16

temperature effect eg converting forest to cropland

Question 17

albedo directly affects…

Answer 17

net radiation

trees are darker than grasses and have lower albedo

Question 18

surface roughness affects…

Answer 18

sensible and latent heat fluxes

trees are tall and aerodynamically rough

grasses are warmer and drier

Question 19

latent heat flux…

Answer 19

evaporatively cools

trees transpire more water than grasses

Question 20

influence of deforestation on climate - if we took all forest and converted it to grassland (on the three properties)

Answer 20

• Albedo only shows a decrease in temperature
  
  • Trees absorb more radiation as they are darker
  • Evapotranspiration shows a warming effect
• Forests evapotranspire more than grass and cool

Surface roughness shows a warming effect

Question 21

net response (annual mean)

Answer 21

• Deforestation both increases and decreases temperature depending on location
• Deforestation cools higher latitudes but warms tropical latitudes

Question 22

surface water balance

Answer 22

• Also needs to resolve other water balances such as snow and soil water
• Precipitation comes in
• Various evaporation and runoff methods
  • Models also needs to resolve change in soil moisture over time

Question 23

surface carbon balance

Answer 23

• Model computes net ecosystem exchange (NEE)
  • Depends on GPP
  • GPP = gross primary productivity (photosynthesis)
  • Losses out of the system from respiration, carbon lost through fire and land use change
• Resolves plant and soil carbon pools

Question 24

land use change … total C storage

Answer 24

decreases

- When cropland is replaced

- Forest stores more carbon than grasslands

Question 25

Land modelling challenges: Land surface heterogeneity

Answer 25

• Models are ran at 100 x 100km resolution (low)
• Take into account heterogeneity by the process of subgrid tiling

Question 26

Subgrid tilling

Answer 26

• Grid cell is divided by land unit
• Land units involve eg vegetated, lake, urban, glacier, crop
• Soil column is then placed into natural land units where soil occurs
• Patches are then defined in vegetated column into plant functional types

Question 27

Urban land unit

Answer 27

• Features configurations eg roofs, sun walls, shade walls,

Question 28

Glacier

Answer 28

• Elevation classes

Question 29

Crops

Answer 29

Unrrig, irrig

Question 30

How does land unit work?

Answer 30

• Each grid cell has a percentage of the type of land unit
• We don’t know eg where lakes will be, but we know the percentage of it lakes shall make up
• Information comes from satellite data
• Resolve energy etc for each land unit and provides an output per grid cell weighted depending on land unit percentage
• Land surface models are parameter heavy

Question 31

Where do parametrisations in land models come from?

Answer 31

1. Laboratory understanding
   
   • Of plant physiological processes
   2. Empirical relationships
      - From as large a sample of the real world as possible, from field and satellite observations
   3. Optimality theory
      - Plants are rational actors, on average

Question 32

Sources of uncertainty in land-surface modelling

Answer 32

1. Forcing (scenario) uncertainty, GHG emissions scenarios, land use etc
   
   2. Response (model) uncertainty from parameterisations, resolution etc
      - Using multiple models corrects for this
   3. Internal (natural or unforced) variability; initial value problems
      - Running several temperatures eg air temperature
      - Several experiments

Question 33

Paths to reducing model uncertainty

Answer 33

model intercomparisons
model intracomparison
model benchmarking
model data-fusion
comparison to real- world manipulative experimens
model hierarchy

Question 34

conclusions

Answer 34

• Land models enable scientists and policymakers to better understand and manage the Earth’s natural resources, and to address some of the most pressing environmental challenges for our time
  • Land-surface models are a starting point for the science, but not the science itself
  • Land surface models are very complex and multidisciplinary, however are easy to run, but hard to interpret