| The sensible heat flux is an important part of land-atmosphere exchanges.Accurate measurement of sensible heat flux is important to understand atmospheric motions and local climate characteristics.In recent years,the continuously accelerating urbanization process has significantly changed the underlying surface type,which has resulted in a significant change in the water and heat balance between surface and air in town.Observation and research of sensible heat flux in town is necessary.The data observed by Large Aperture Scintillometer?LAS?from April,2018 to April,2019 in Pancheng town were used to analyze the spatial-temporal characteristics and influencing factors of sensible heat flux.The results show that:?1?The sensible heat flux exhibits a unimodal diurnal variation in Nanjing town,which is significantly larger during the daytime than that during the nighttime.In the daytime,the sensible heat flux on clear days is greater than that on partly cloudy days and cloudy days.At night,the sensible heat flux on clear days is slightly less than that on partly cloudy days and cloudy days.The average annual sensible heat flux on clear,partly cloudy and cloudy days are60.82±71.04 W·m-2?55.70±54.91 W·m-2?48.31±42.01 W·m-2.The sensible heat flux during the daytime is close to that in spring and summer,which is significantly larger than that in winter;the difference between the four seasons at night is small.The daytime and nighttime values in August are 112.19 and 23.54 W·m-2,and in February are 35.57 and 11.57 W·m-2.?2?During clear daytime,the sensible heat flux with different wind directions varies significantly in town.The values of sensible heat flux larger than 150 W·m-2 are mostly distributed in buildings and anthropogenic activity high-density wind direction areas,while the values of sensible heat flux less than 50 W·m-2 is mostly distributed in wind direction areas with a large proportion of vegetation.That is,as the proportion of the impervious layer in the underlying surface of the source area increases,the proportion of net radiation allocated to the sensible heat flux increases significantly.When the proportion of impervious layers is greater than 60%,the increase trend is not obvious.?3?Compared with EC,during the observation period,the sensible heat flux observed by LAS during the daytime and nighttime in town are 25.85 and 18.16 W·m-2 larger,and in August and January are 35.45?15.03W·m-2 larger on average.The difference in the source area is the main reason for the difference in sensible heat flux.As the source area overlap increases,the consistency of the observation results becomes better.The fitting slope between the LAS observations and the remote sensing data inversion results in the same source region is much larger than the EC,and the correlation is obviously preferred.This shows that LAS has a clear advantage over EC in observing sensible heat flux in large-scale,non-uniform underlying surface in town.?4?The differences between the five methods for determining atmospheric stability used to calculate sensible heat flux are mainly the inconsistent conversion time of atmospheric stability and the difference in sensible heat flux at night in town.If taking the Monin-Obukhov length as the criterion for judging the stability of the atmosphere,the structure parameter of the refractive index of air method has a low misjudgment rate among the five methods and the data is derived from LAS.It is a more suitable method for determining the atmospheric stability at night in town.?5?Among the factors that affect the sensible heat flux in town,changes in effective height have the greatest impact.The effect of changes in wind speed is larger,especially in autumn and winter.The change of Bowen ratio has a small effect on the sensible heat flux in town with a large Bowen ratio.The influence of temperature,the structure parameter of the refractive index of air,roughness length and displacement height can be neglected. |
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