Remote Sensing of the Urban Heat Island Effect Across Biomes in the Continental USA

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1 Remote Sensing of the Urban Heat Island Effect Across Biomes in the Continental USA Marc Imhoff, Lahouari Bounoua, Ping Zhang, and Robert Wolfe NASA s Goddard Space Flight Center Land Cover Land Use Change

2 Studying the UHI Phenomenon Focus on surface temperature - influenced by albedo, emissivity, and surface fluxes. Bioclimatic context - setting is important as UHI is a relative measure (Temp of urban core compared to nearby nonurban cover). - Important to climate change as urbanization of specific biomes represents a departure from previous flux characteristics. - Impact may be disproportionate to simple area based assessments. 2

3 Percent of Soil Area Covered by Lights Urban Occupation of Fertile Soils 8 CHINA Percent of Soils (UN/FAO) Covered by Lights Grouped by Number of FCC Limiting Factors Number of Factors Limiting Agriculture

4 Consequences of Urbanization NPP Lost or Gained (annually) going from a pre-urban to a post urban world Disproportionate impact: Urbanization (3% surface area) has reduced Continental NPP by 41.5 Million tons C/year Roughly equal to the gains made by agriculture (40% surface area). Annual loss of food web energy 400 Trillion kilocalories (roughly equal to food energy requirement for 448 million people). Reduction of actual food products for 16.5 million persons annually. Imhoff et al. 2004, RSE, Vol. 89, Issue 4, pp

5 Consequences of Urbanization on NPP Satellite Observations DMSP/OLS Urban Map Urban, Peri-urban, Non-urban NPP and Local Climate: Urban Heating Extends Length of growing season locally in cold climates. North East South West AVHRR Monthly NPP (g Cm -2 ) In semi-arid regions cities enhance NPP relative to surrounding areas Mid-Atlantic South East Winter NPP gain negated in peak season by reduced vegetation and heat stress. Seasonal Offset diminishes in tropics

6 Surface Temperature and Albedo 120 o F 49 o C 83 o F 28 o C Atlanta Urban Heat Island and Air Quality Modeling Study D. Quattrochi, MSFC

7 Surface composition (parameters) in and outside urban area 7

8 More than Albedo Biome Level Surface Parameters Influence Heat Radiation S L SiB2 Transfer pathway for Latent Heat P U m e m T m Precipitation +W leaf W c r a Interception Loss eci l E c etc r c T c 2r b e a l E c + l E g Canopy Air Space z 2 R n Surface Layer Root Zone Recharge Zone + W thru W drip -W run W g W 1 W 2 W 3 -W drain interception egi egs r d Ground evaporation Transpiration l E g r soil z 1 d 1 d 2 d 3 Schematic functioning of the Simple Biosphere Model (SiB2) showing the pathway for the latent heat flux calculation. 8

9 Domains: cities in U.S. Datasets and compilations Scales: cities with area size from 10km 2 to more than 4000 km 2 Datasets: - Impervious surface area (ISA) from the Landsat TMbased NLCD 2001 dataset [Yang et al., 2002 ] - Land surface temperature (LST) from MODIS [Wan et al., 2004] - Terrestrial ecoregion map developed by Olson et al [2001] - Topographic data from SRTM30 [Farr and Kobrick, 2000] - NDVI from MODIS [Huete et al., 1997] 9

10 Geospatial urban area definition using Landsat Impervious Surface Area Identifies boundary between urban and low intensity residential area Provide spatially coherent urban groups 10

11 Classification of urban density 11

12 Classification of urban density Four broad biomes: forest, grass/shrub, arid (desert and xeric shrubland), and semi-arid (Mediterranean) 323 settlements are evenly distributed over USA ranging from around 10 km 2 to more than 4,000 km 2 Five urban zones are defined based on ISA density - Urban core pixels: 75% > ISA 50% - Medium density urban pixels: 50% > ISA 25% - Suburban pixels: 0 to 5 km outside the 25% ISA contour - Rural pixels: 15 to 20 km outside the 25% ISA contour Sampling is constrained by biome and elevation UHI=LST urban core - LST Rural 12

13 UHI (degree) UHI and ecological context Average UHI for US cities grouped by biomes Forest Grass Mediterranean Desert -2 UHI=7.1 o C in cities built in forest UHI=4.2 o C in cities built in grass/shrubs UHI=6.0 o C in cities built in Mediterranean UHI=0.25 o C in cities built in desert The average city size of each group is 159/206/1055/160 km 2 13

14 Average LST (C) Average ISA (%) Average NDVI UHI Baltimore vs. Las Vegas 80 Fractional ISA 0.8 Jun-Aug NDVI A Baltimore Las Vegas 0.0 B Baltimore Las Vegas 55 Jun-Aug LST Urban Core (ISA>=75%) Urban1 (75%>ISA>=50%) Urban2 (50%>ISA>=25%) Suburban (0-5Km buffer) Rural (45-50Km buffer) C Baltimore Las Vegas Baltimore, Maryland, in Notheastern temperate broadleaf and mixed forest Las Vegas, Nevada, in desert and xeric shurbland 14

15 Average summer daytime LST(C) UHI and ecological context: Baltimore vs. Las Vegas 60 The composite summer daytime LST profile Las Vegas Baltimore km 20km 15km 10km 5km 25% core 25% 5km 10km 15km 20km 50km Composite averaged LST during summer (Jun-Jul-Aug) day time across all urban zones at Baltimore and Las Vegas. 15

16 A Biome-Centric View of Remotely Sensed Surface Parameters in US cities Imhoff, Zhang, Wolfe and Bounoua, 2009, RSE 114 (2010)

17 Urban Heat Islands and Ecological Context MODIS - Aqua LST ( ) FE FA GN DE MS GS GT FW Summer = Jun-Aug (average LST) Winter = Dec - Feb (average LST) LST at 1:30 pm local time Day = 1:30PM Local time Night = 1:30AM Local time Imhoff, Zhang, Wolfe and Bounoua, 2009, RSE 114 (2010)

18 Summer day LST anomaly ( o C) Summer day LST anomaly ( o C) Quantifying the relationship between LST and ISA 8 6 A: Cities in forests 8 6 B: Cities in desert LST = X ISA R2 = NLCD2001 ISA anomaly (%) LST = X ISA X ISA R 2 = NLCD2001 ISA anomaly (%) Variations in ISA explain 88% of the variation in LST for urban areas in forested biomes The rate of change in LST as a function of ISA is 11.5% for urban areas in forests, while only 6.9% for those characterized by short vegetations. In desert environments, the LST s response to ISA presents is a U-shaped horizontal gradient decreasing from the urban core to the outskirts of the city and then increasing again in the suburban to the rural zones 18

19 UHI (degree) The magnitude of UHI and size of urban area Average UHI for US cities grouped by size 10 Forest Grass >1000 (km 2 ) US urban settlements are grouped based on the total contiguous area for each urban polygon defined by 25% ISA threshold Average UHI during summer day time is affected by urban size 19

20 UHI and urban extent for discrete urban centers in temperate forest (continental US) Urban extent defined by contiguous area with 25%+ ISA 20

21 Conclusions and future directions This research highlights significant positive relationships between the UHI magnitude, the ISA, and ecological setting estimated exclusively from remotely sensed observations The use of ISA as an estimator of the extent and intensity of urbanization is more objective than population density based methods and can be consistently applied across large areas for inter-comparison of impacts on biophysical processes. We will use a combination of satellite and ecological map data to extend the characterization of the UHI response to global urban settlements. The uncertainties in ISA outside USA are challenges for future studies as the concept of ISA in the USA does not necessarily transfer to many developing country cities where infrastructure characteristics and properties are often different 21