UMEP/ Tutorials/ Footprint
Each meteorological instrument has a ‘source area’ (sometimes referred to as footprint), the area that influences the measurement. The shape and location of that area is a function of the meteorological variable the sensor measures and the method of operation of the sensor.
For turbulent heat fluxes measured with a sonic anemometer, extensive effort has been directed to try and model the ‘probable source area location’ (Leclerc and Foken 2014). Numerous models exist, but the Kormann and Meixner (2001) and Kljun et al. (2015) models are used in UMEP. Both models require input of information about the wind direction, stability, turbulence characteristics (friction velocity, variance of the lateral or crosswind wind velocity) and roughness parameters. Kljun et al. (2015) requires the boundary layer height.
Initial Practical steps
- Start the QGIS software
- If not visible on the desktop use the Start button to find the software (i.e. Find QGIS under your applications)
- Select QGIS 2.16.3 Desktop (or the latest version installed)
When you open it on the top toolbar you will see UMEP.
- If UMEP is not on your machine, download and install the UMEP plugin
- Read through the section in the online manual BEFORE using the model, so you are familiar with it’s operation and terminology used.
Data for Tutorial
Use the appropriate data
a) Reading -- Download BB - week 5 or here
b) London - download
Prior to Starting
- Read through the section in the online manual BEFORE using the model, so you are familiar with it’s operation and terminology used:
- Download the Data needed for the Tutorial - you will be told which the appropriate data are:
- Load the Raster data (DEM, DSM, CDSM) files – DOES A CDSM EXIST? Yes for London, No for Reading
- Go to: Layer > Add layer > Add Raster Layer > Locate downloaded files
- Have a look at the layers (see lower left) - if you untick the box filenames from the top you can see the different layers.
Source Area Modelling
- To access the Source area model or Footprint model:
- UMEP -> Pre-processor > Urban Morphology > Source Area Model (Point) this appears like this
The output is a source area grid showing the cumulative percentage of source area influencing the flux at the point of interest.
It is possible to input a text file to generate a source area based on morphometric parameters (e.g. an hourly dataset). However, for now you can moodify the input variables set in the interface. Format of file is given in the manual.
To work with a site with no value known a priori.
- Use the Image Morphometric Parameters Calculator (Point) tool in the UMEP plugin to select a point to get the initial parameter values:
- UMEP-> Pre-Processor -> Urban Morphology -> Image Morphometric Calculator
- Open the output files
- Anisotropic file – has the values in, e.g., 5 degree sectors – i.e. what you selected. This is appropriate if the area is very inhomogeneous.
- Isotropic file - has the average value for the area
- Use these values to populate the source area model window.
In the UMEP plugin the roughness length and zero plane displacement length can be calculated with a morphometric method based on the Rule of Thumb (Grimmond and Oke 1999) as the default. There are other methods available: Bottema (1995), Kanda et al. (2013), Macdonald et al. (1998), Millward-Hopkins et al. (2011) and Raupach (1994, 1995). Many of these have been developed for urban roughness elements. The Raupach method was originally intended for forested areas but has also been found to perform well for urban areas.
With wind profile and eddy covariance anemometric data and the source area model, appropriate parameters can be determined and morphometric methods assessed (e.g. Kent et al. 2017).
Questions for you to explore with UMEP: Source Area
- What is the impact of the atmospheric and surface characteristics on the source area dimensions?
- How do the source area characteristics vary for different sensor levels for the wind profile?
- How do the morphometric roughness methods compare with values obtained in the observatory? What is the influence of vegetation state?
- Does wind direction impact the choice of the most appropriate method?
- What is the difference in source area with models?
- What inputs are the respective models most sensitive to?
- Bottema M 1995: Parameterisation of aerodynamic roughness parameters in relation to air pollutant removal efﬁciency of streets. Air Pollution Engineering and Management, H. Power et al., Eds., Computational Mechanics, 235–242.
- Grimmond CSB and TR Oke 1999: Aerodynamic properties of urban areas derived, from analysis of surface form. Journal of Applied Climatology 38:9, 1262-1292
- Kanda M, Inagaki A, Miyamoto T, Gryschka M, Raasch S 2013: A new aerodynamic parameterization for real urban surfaces. Boundary- Layer Meteorol 148:357–377. doi:10.1007/s10546-013-9818-x
- Kent CW, Grimmond CSB, Barlow J, Gatey D, Kotthaus S, Lindberg F, Halios CH 2017: Evaluation of Urban Local-Scale Aerodynamic Parameters: Implications for the Vertical Profile of Wind Speed and for Source Areas. Boundary-Layer Meteorol 164:183-213.
- Kljun N, Calanca P, Rotach MW, Schmid HP 2015: A simple two-dimensional parameterisation for Flux Footprint Prediction (FFP). Geoscientific Model Development.8(11):3695-713.
- Kormann R, Meixner FX 2001: An analytical footprint model for non-neutral stratification. Bound.-Layer Meteorol., 99, 207–224 http://www.sciencedirect.com/science/article/pii/S2212095513000497#b0145
- Kotthaus S and Grimmond CSB 2014: Energy exchange in a dense urban environment – Part II: Impact of spatial heterogeneity of the surface. Urban Climate 10, 281–307 http://www.sciencedirect.com/science/article/pii/S2212095513000497
- Leclerc MY and Foken TK 2014: Footprints in Micrometeorology and Ecology. Springer, xix, 239 p. E-book
- Macdonald, R. W., R. F. Griffiths, and D. J. Hall, 1998: An improved method for estimation of surface roughness of obstacle arrays. Atmos. Environ., 32, 1857–1864
- Millward-Hopkins JT, Tomlin AS, Ma L, Ingham D, Pourkashanian M 2011: Estimating aerodynamic parameters of urban-like surfaces with heterogeneous building heights. Boundary-Layer Meteorol 141:443–465. doi:10.1007/s10546-011-9640-2
- Raupach MR 1994: Simpliﬁed expressions for vegetation roughness length and zero-plane displacement as functions of canopy height and area index. Bound.-Layer Meteor., 71, 211–216. doi:10.1007/BF0070922
- Raupach MR 1995: Corrigenda. Bound.-Layer Meteor., 76, 303–304.
Contributors to the material covered
University of Reading: Christoph Kent, Simone Kotthaus, Sue Grimmond University of Gothenburg: Fredrik Lindberg Background work also comes from: UBC (Andreas Christen); Germany: Kormann and Meixner (2001); Japan: Kanda et al. (2013); UK: Millward-Hopkins et al. (2011), Macdonald et al. (1998); Australia: Raupach (1994, 1995); Netherlands: Bottema (1995)
Authors of this document: Kent, Grimmond (2016). Lindberg