| Autor: |
Gao, Zhongming, Liu, Heping, Li, Dan, Katul, Gabriel G., Blanken, Peter D. |
| Zdroj: |
Journal of Geophysical Research - Atmospheres; April 2018, Vol. 123 Issue: 8 p4110-4121, 12p |
| Abstrakt: |
A number of studies already suggested that entrainment of warm/dry air from aloft degrades the similarity between air temperature (T) and specific humidity (q) in the atmospheric surface layer (ASL). Less is known about entrainment of cool/dry air on the ASL T‐qsimilarity. Using eddy covariance measurements over a large inland water surface, enhanced T‐qsimilarity with increased friction velocity (u*) was measured during daytime unstable conditions. Explaining this enhancement is the main goal of this work. Cool/dry air masses originating and advecting from land set the upper boundary condition on a quasi‐equilibrated internal boundary layer embedding the warm/moist ASL over water. With increased u*, the entrainment velocity (dhI/dt∝ u*) increases, thereby enhancing entrainment of cool/dry air originating from land, where tis time and hIis the internal boundary layer depth. By analyzing the scale‐dependent (spectral) correlation coefficients, the enhanced T‐qsimilarity with increased u*was shown to originate from spectral correlation coefficients in the middle‐ to low‐frequency ranges (large eddies ≫ z, the distance from the surface). With further increases in u*, similarity between Tand qwithin large eddies was further enhanced and phase differences between Tand qwas reduced. Quadrant analysis showed large increases in flux and time fractions of downward large cool/dry eddies with increasing u*. These results provide evidence that enhanced entrainment of synchronized, large cool/dry eddies was a primary cause for the increased T‐qsimilarity with increasing u*. Similarity between temperature and humidity improved with increasing wind shearTemperature and humidity of large eddies became more synchronized with increased wind shearEnhanced temperature‐humidity similarity was conjectured to be due to intensified entrainment of more synchronized, large cool/dry eddies at higher wind shear conditions |
| Databáze: |
Supplemental Index |
| Externí odkaz: |
|
Plný text