ARM data from 19 different deployments, including fixed and mobile sites, provide a comprehensive set of observations to analyze the relationship between in situ and satellite measurements.
Near-surface air temperature is one of the most important variables in weather and climate science. It is widely used for assessing climate change and evaluating weather and climate models and also has relevance for human health and agriculture. Near-surface air temperature (also known as 2m air temperature or T2m based on the height it is typically measured at) is routinely observed at meteorological stations. However, there are still large regions of the world that are poorly observed. Global measurements of this important variable from satellites are limited by the fact that satellite instruments are more sensitive to land surface temperature (LST) than to air temperature. To improve the ability to estimate near-surface air temperature globally from satellites, scientists need to understand how these two quantities are related in different regions and under different meteorological conditions. The Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility has deployed in situ and remote-sensing instruments for atmospheric measurements to over 19 locations around the globe, providing a comprehensive dataset for such an analysis.
Previous studies of relationships between T2m and LST have typically focused on one site and used a gridded climatology or used stations from one geographic region. By using detailed data from 19 different ARM deployments around the globe, this study characterizes how the relationships vary over a wide range of geographical locations and meteorological conditions such as wind speed and amount of incoming sunlight. The high frequency (1-minute) and comprehensive observations from the ARM site also enable insights into how the relationships change over the course of the day and in response to rapid changes in cloud cover. The results provide important information for improving global measurement of near-surface air temperature from satellites.
This study describes the analysis of coincident in situ observations of LST and screen-level air temperatures (T2m) acquired from 19 ARM Climate Research Facility deployments. Site locations range over Alaska, North America, Europe, West Africa, China, India, and the Tropical Western Pacific, enabling the LST-T2m relationship to be characterized over a wide range of geographical locations and environmental conditions. The diurnal cycles of both LST and T2m are resolved through the use of 1-minute temperature observations. The comprehensive observations at the ARM sites enable analysis of how these relationships change with environmental conditions such as wind speed and cloud cover. The results indicate that under cloud-free, low wind speed conditions, daytime LST is often several degrees Celsius higher than T2m at low-to-middle latitudes and at high latitudes during the summer months. In contrast, LST and T2m are often close (e.g., < 2°C) under cloudy and moderate-to-high wind speed conditions, or when solar insolation is low or absent (e.g., at night), or at high latitudes during winter, spring, and autumn. LST is found to exhibit a virtually instantaneous drop of up to several degrees when a cloud passes over during an otherwise mostly cloud-free day, while the T2m response to clouds is more muted, resulting in differences of several degrees during such an event. A particular focus of this study is on the relationship between daily extremes of LST and T2m, which has received little attention in previous studies. The daily LST minimum is typically less than the minimum in T2m, although the daily minimums are generally well correlated. Notably, the correlations at high latitudes during winter, spring, and autumn are very close to unity. For these situations, LST may provide a reasonable proxy for T2m. In contrast, the difference in the daily maximums is often quite large (a few degrees Celsius or more, quite frequently exceeding 10°C) and is found to increase with decreasing latitude (increasing sunlight). The largest differences occur during the spring or summer seasons. Results from the Amazonia site are suggestive of the influence of vegetation on the LST-T2m difference. Results from the Steamboat Springs and Nainital sites, both of which are at about 2,000 m above sea level, suggest that elevation and aspect also influence the LST-T2m relationship, as the results from these sites do not always conform to the general pattern seen at other, more low-lying sites. This study provides a reference for those who are interested in LST-T2m differences over a variety of different geographical regimes and investigates the factors that influence those differences. The results presented here will inform users of T2m and satellite LST datasets about the relationship between these two temperatures and aid those working on methods to predict T2m from satellite LST.
Contacts (BER PM)
Atmospheric Radiation Measurement Climate Research Facility Program Manager
Met Office Hadley Center
Exeter, United Kingdom
This study was carried out within the framework of the European Union Surface Temperature for All Corners of Earth (EUSTACE) project. EUSTACE has received funding from the European Union’s Horizon 2020 Programme for Research and Innovation, under grant agreement 64017. The author would like to thank the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Climate Research Facility for providing the in situ data used in this study (https://www.arm.gov/).
Good, E. J. 2016. "An In Situ-Based Analysis of the Relationship between Land Surface ‘Skin’ and Screen-Level Air Temperatures," Journal of Geophysical Research-Atmospheres 121(15), 8801-819. DOI: 10.1002/2016jd025318. (Reference link)
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