Important Recent Studies By Mildrexler et al 2011a and 2011b On Monitoring the Hottest Land Surface Temperatures On Earth

[Figure 3 from Mildrexler et al 2011]

I have been alerted to two papers that appeared in 2011 that are very significant contributions to our monitoring of surface temperatures for land areas of the globes. These papers have built on the article in EOS by by D. Mildrexler, M. Zhao and S.W. Running titled “What Are the Hottest Spots on Earth?” (subscription required).

that I disucssed in my post

EOS Paper On The Hottest Spots on Earth Illustrates The Major Role of Landscape on Surface Temperatures

The first 2011 papers is

Mildrexler, D. J., M. Zhao, and S. W. Running (2011), A global comparison between station air temperatures and
MODIS land surface temperatures reveals the cooling role of forests
, J. Geophys. Res., 116, G03025, doi:10.1029/2010JG001486

with the abstract [highlight added]

Most global temperature analyses are based on station air temperatures. This study presents a global analysis of the relationship between remotely sensed annual maximum LST (LSTmax) from the Aqua/Moderate Resolution Imaging Spectroradiometer (MODIS) sensor and the corresponding site‐based maximum air temperature (Tamax) for every World Meteorological Organization station on Earth. The relationship is analyzed for different land cover types. We observed a strong positive correlation between LSTmax and Tamax. As temperature increases, LSTmax increases faster than Tamax and captures additional information on the concentration of thermal energy at the Earth’s surface, and biophysical controls on surface temperature, such as surface roughness and transpirational cooling. For hot conditions and in nonforested cover types, LST is more closely coupled to the radiative and thermodynamic characteristics of the Earth than the air temperature (Tair). Barren areas, shrublands, grasslands, savannas, and croplands have LSTmax values between 10°C and 20°C hotter than the corresponding Tamax at higher temperatures. Forest cover types are the exception with a near 1:1 relationship between LSTmax and Tamax across the temperature range and 38°C as the approximate upper limit of LSTmax with the exception of subtropical deciduous forest types where LSTmax occurs after canopy senescence. The study shows a complex interaction between land cover and surface energy balances. This global, semiautomated annual analysis could provide a new, unique, monitoring metric for integrating land cover change and energy balance changes.

The conclusions includes the text

Because LST is more tightly coupled to the radiative and thermodynamic characteristics of the Earth’s surface, it may be an improvement to substitute LST for Tair in calculations of the global average surface temperature in the radiative‐convective equilibrium concept equation [Pielke et al., 2007].

We found the strength of the LSTmax/Tamax relationship to be land‐cover‐dependent. At low temperatures, LSTmax and Tamax are well coupled for all land cover types. Forests are the only cover type that maintains a strongly coupled LSTmax/Tamax relationship at highest temperatures and are distinct from the other land cover types because both LSTmax and Tamax tend to range between the same values. The transpiration of forest ecosystems through the growing season dissipates more energy and lowers the Bowen ratio, and is the key driver for the stronger coupling of LSTmax and Tamax. Forests cover over 21% of the Earth’s surface and span a very large latitudinal gradient. The global regulation of surface temperature highlights the important role of forests in local, regional and global climate.

Humans continue to dramatically influence global land cover through habitation, forest clearing, agriculture, and increasingly through anthropogenic driven climate change. This study reinforces the need to include land use and land cover change in holistic climate change studies and the important role that forests have in the global energy balance……

The second paper is

David J. Mildrexler, Maosheng Zhao, Steven W. Running, 2011. Satellite Finds Highest Land Skin Temperatures on Earth.  Bulletin of the American Meteorological Society Volume 92, Issue 7 (July 2011) pp. 855-860. doi: http://dx.doi.org/10.1175/2011BAMS3067.1

The introduction of this paper includes

“…the hottest spot on Earth based on scattered site-based air temperature measurements is a limited approach due to the poor spatial coverage of the instruments where measurements are taken compared with Earth’s expansive barren deserts where the hottest conditions occur. The World Meteorological Organization (WMO) has approximately 11,119 weather stations on Earth’s land surface collecting surface temperature observations (ftp://ftp.ncdc.noaa.gov/pub/data/gsod/2010). When compared to the 144.68 million km2 of land surface, that’s one station every 13,012 km2.

The conclusion of this paper contains the text

As more years of data accumulate, the dataset presented in Fig. 3 could become a new type of integrative global change measurement. The annual maximum LST histogram merges into a single metric important biophysical and biogeographical factors of the Earth system that are usually measured individually. These contributing factors can include 1) intensification of extreme maximum surface temperatures; 2) changes in land cover; 3) changes in albedo; 4) surface–atmosphere energy fluxes; 5) changes in ecosystem disturbance regimes; 6) air temperature; and 7) atmospheric aerosol concentrations. However the potential for change in each of these factors varies tremendously and unpredictably in time and space. This integrative measurement is strongly influenced by the biogeographic patterns of Earth’s ecosystems, providing a unique comparative view of the planet every year such that changes in any component of the trimodal distribution are detectable. For example, changes that result in the lower tail of the distribution shifting from -30° to -20°C would indicate a major change in the cryosphere. Likewise, if we see the central spike for forests becoming progressively smaller and the spike that represents deserts becoming larger, further research could focus on determining the causality. This potentially valuable annual maximum LST metric is automated and easy to produce for continued global monitoring of the highest temperatures on Earth.

These are two very seminal papers as they open up another (and more spatially representative) approach to monitor variability and longer term trends in surface temperatures of the land.

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