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Desertification Vulnerability Assessment
Published in Ajai, Rimjhim Bhatnagar, Desertification and Land Degradation, 2022
In this direction, Singh and Ajai (2019) developed a composite method to identify desertification ‘hotspots' and ‘bright spots' using Bowen ratio, Land Surface Temperature (LST), Extra-terrestrial solar radiation (Ra) and NDVI. Microclimate of the region was analysed using Bagnouls-Gaussen bioclimatic aridity index (Bagnouls and Gaussen 1953). Three classes were formed using BGI values, indicating low, moderate and high vulnerability to DLD. The second step involved analysis for vegetation characteristics w.r.t. its role in DLD, such as erosion protection, fire resistance and drought tolerance. Next, analysis of LST, Ra and NDVI for three cropping seasons (winter, summer and rain) was done. In places where Ra is large, solar insolation will be large and thus, higher will be the rate of evaporation (Burgess 2009). In order to limit moisture loss, plants shed their leaves to reduce evapotranspiration. Hence, persistently high Ra can give an indication of the stressed ecosystem condition. This effect may be observed through NDVI data. Together with LST, which is required for a variety of climatic, hydrological, ecological and biogeochemical studies (Wan and Li 1997), NDVI can serve as an indicator of underlying ecosystem's activity, thereby signifying desertification conditions (Sivakumar 2007). In addition to this information, Bowen ratio analysis was also done for the three seasons (winter, summer, and rain). Bowen ratio is the ratio of sensible heat flux to latent heat flux in energy terms. It is expected to increase along the land degradation gradient. In simplified terms, it can be calculated as in Equation 10.21. β=γTs−Taes−ea
Climate effects
Published in Bernardo Caicedo, Geotechnics of Roads: Fundamentals, 2018
For conditions with abundant water content, the Bowen ratio ranges from 0.1 to 0.3. These values indicate that there is one to three times more energy involved in evaporation than in heating. In contrast, dry surfaces have high Bowen ratios.
Ocean–Atmosphere Interactions
Published in Yeqiao Wang, Atmosphere and Climate, 2020
Direct measurements of air-sea flux are few, limited both in time and in space. These measurements of air-sea fluxes are important, however, for developing, calibrating, and verifying the estimated air- sea flux from parameterization schemes.[8,9] The parameterization schemes for air-sea fluxes use state variables of the atmosphere and the ocean (e.g., wind speed, temperatures, humidity) to estimate the fluxes. A commonly used parameterization scheme for air-sea fluxes is the "bulk-aerodynamic" formula. This is based on the premise that wind stress is proportional to the mean wind shear computed between surface and 10 m above surface, and sensible heat flux and latent heat flux are proportional to the vertical temperature and moisture gradients computed between surface and 2 m above surface. As a result, air-sea fluxes have been computed globally and regionally from a variety of analyzed and regionally observed or analyzed atmospheric and oceanic states leading to a number of air-sea flux intercomparison studies.[10-12] These intercomparisons provide insight into the uncertainty of estimating air-sea fluxes as well as revealing salient differences in the state variables used in the parameterization scheme. For example, Smith et al.[10] found that in many regions of the planet the differences in surface air temperature and humidity amongst nine different products of air-sea fluxes had a more significant impact than the differences in the surface air wind speed (at 10 m) on the differences in the air-sea fluxes. Climatologically, large values of sensible heat flux are observed in the winter along the western boundary currents of the middle-latitude oceans, when cold continental air passes over the warm ocean currents (e.g., Gulf Stream). In the tropics and in the sub-tropical eastern oceans, the sensible heat flux is usually small. In the former region, climatologically, the wind speeds and the vertical temperature gradients between the surface and 2 m above the surface are weak. In the sub-tropical eastern oceans, with prevalence of upwelling, the SSTs are relatively cold leading to generally smaller sensible heat flux. The latent heat flux is observed climatologically to be large everywhere in the global oceans relative to the sensible heat flux, with exceptions over polar oceans in the winter season. The ratio of sensible to latent heat flux, called the Bowen ratio, has a latitudinal gradient with a higher (smaller) ratio displayed
Evaluating the impacts of land use and land cover changes on surface air temperature using the WRF-mosaic approach
Published in Atmospheric and Oceanic Science Letters, 2018
In southeastern China, the LULC changes showed increasing leaf area index values, sensible heat fluxes, Bowen ratio values, soil moisture, and roughness lengths, as well as decreasing albedos, latent heat fluxes, upward moisture flux at the surface, and total cloud fraction (Table 1). With the decreases in albedo, the SWUB was weakened. This effect, due to changes in albedo, was stronger than the increased SWDB and resulted in an increased SWB. The skin temperature increased, which induced increases in the sensible heat flux and LWUB. The Bowen ratio increased with decreased latent heat flux and moisture fluxes from the land surface to the atmosphere (also due to the increased roughness length, which induced weakened near-surface wind speeds). Soil moisture weakly increased, with a relative value of 0.4% for surface soil moisture at 0–10 cm. Meanwhile, the increases in sensible heat flux (2.09 W m−2) and the decreases in latent heat flux (−1.32 W m−2), which induced more heat flux release to the atmosphere and heated near-surface atmosphere, contributed to the increased SAT.
Evaluation and spatio-temporal analysis of surface energy flux in permafrost regions over the Qinghai-Tibet Plateau and Arctic using CMIP6 models
Published in International Journal of Digital Earth, 2022
Junjie Ma, Ren Li, Zhongwei Huang, Tonghua Wu, Xiaodong Wu, Lin Zhao, Hongchao Liu, Guojie Hu, Yao Xiao, Yizhen Du, Shuhua Yang, Wenhao Liu, Yongliang Jiao, Shenning Wang
The Bowen ratio is the ratio of sensible to latent heat flux. It summarizes how the energy budget is partitioned between H and LE. The available energy shifts more towards the H with an increasing Bowen ratio (Eugster et al. 2000; Hu et al. 2019; Gu et al. 2015). In the Arctic, spring is more appropriately called light winter, in which shortwave radiation increases gradually. The increment is not significant due to the high snow albedo. At this stage, the energy budget is similar to that in winter. The average negative sensible heat flux is balanced with the long-wave radiation. The latent heat flux occupies an insignificant position in the energy budget (Figure 6a), Furthermore, the Bowen ratio is negative for most of the Arctic (Figure 10a). The snow and active layer start to melt during this period. The latent heat flux also increases with increasing surface soil moisture content. The summer period has strong shortwave radiation, the snow cover is the least (Serreze et al. 2007). The net shortwave radiation is balanced with the net longwave radiation, H, LE, and ground heat flux, leading to the thawing of the active layer in permafrost regions. The average Bowen ratio is close to one. However, it varies widely from 0.5–2 in different regions (Figure 10b). This is primarily related to the local surface soil moisture content (Westermann et al. 2009). The shortwave radiation decreases sharply in autumn, and the snow cover for a longer period of time has not yet formed. During this period, the H shifts towards negative values (Figure 8c). However, the freezing of active layer soil does not begin completely. Shortwave radiation in winter is approximately zero owing to the arrival of the polar night. At this time, the longwave radiation dominates the system. Longwave radiation is primarily balanced by negative H, which heats the land surface and cools the atmosphere. The LE has only minor importance in the energy balance during this period.