Fine-scale Characteristics Of Low-level Meteorological Elements In Beijing Urban Area

The temporal and spatial characteristics of temperature, specific humidity, wind speedand direction and summer precipitation in Beijing urban area and the urban effects havebeen investigated by use of hourly automatic weather station data during2008-12. Studieson Beijing urban boundary layer are carried out though the control and sensitivity numericalstudies of clear day during6thand7thAug.,2010over Beijing. Results from data analyzingshow are as follows.(1) Urban Heat Island (UHI) exists in Beijing through the all year. Theseasonal average value of UHI is biggest in winter and smallest in summer. The diurnalvariation of UHI shows that it is stronger during night than day. The strongest nighttime UHIis in winter while the strongest daytime UHI is during summer. The UHI in Beijing ischaracterized by multi-center.(2) The values of specific humidity (q) in urban area are lowerthan those in rural area during summer daytime and early night in Beijing, which is known as“urban dry island”(UDI). Values show a multi-center distribution, due to the non-uniformdistribution of non-evaporating urban impervious-surfaces, which decreaseevapotranspiration, increase run-off, and thus lower urban specific humidity levels. Air iswetter in urban than rural in late summer night because of rural dewfall which not happensin urban areas due to urban heat island. In winter, the urban values of q are bigger than ruralones at most hours which is affected by anthropogenic emissions. As vegetation andprecipitation in winter are much less than summer, anthropogenic emissions is the mainfactor that affects the q difference between urban and rural areas at low levels. Diurnalvariations of q in urban area are different with rural ones because of urban effects. The ruralq continues to decrease during summer night, while urban q increases gently during earlynight and then changes a little during late night. In winter, rural q fluctuates withtemperature changes while urban q increases gently during08-19LST, and decreases during20-07LST. That because human activities are more frequent during daytime than nighttime.(3) Studies on10-m wind directions show that they are affected by seasonal prevailing winds,topography and urban effects. Valley-breeze time is longest in summer and shortest in winter.Mountain-breeze duration time is longest in winter and shortest in summer. During summervalley-breeze time, the southerlies bypass Beijing urban area because of buildings, while airflows converge into city during summer mountain-breeze time due to the combined effects of topography, urban effects and seasonal prevailing wind. During winter breeze-time, aconvergence line is formed in northwest-southeast direction in urban area. Spring andautumn are the transition seasons, and bifurcation is observed in both valley-and mountain-breeze time.(4) Wind speeds are reduced by the large Beijing surface-roughness. Alow-speed region is observed in more urbanized areas between Second Ring Road and ThirdRing Road due to its high z0. The difference between urban and rural average speed is biggestin spring, and is bigger during daytime than nighttime. The speed differences are differentbetween loops and rural areas. Areas within the Third Ring Road have the maximumdifference, and areas between Fifth and Sixth Ring Roads have the minimum.(5) Summerthunderstorms bifurcated and bypassed the urban center, due to a building-barrier effectduring both daytime and nighttime weak-UHI (<1.250C) periods. This produced aregional-normalized rainfall (NR) min in the urban-center and directly-downwind of theurban area (of up to-35%), with max values along its downwind lateral edges (of>15%).Strong-UHIs (>1.250C), however, induced thunderstorm-formation (again day and night)which produced an NR max in the most urbanized area of up75%.The results from WRF/Noah/UCM for6thAugust,2010show that:(1) The surfaceradiation and thermal fluxes balance are affected by urban underlying surface. Urban surfacehas smaller albedo so that it absorbs more radiation, and has bigger residual heat.(2) Urbanareas have bigger temperature than non-urban areas. The height of UHI is about1500mduring daytime, and between1500m and2000km, the urban air temperature is lower thannon-urban areas. During night, the UHI height is about300m.(3) The10wind speeds aredecelerated by the high Z0in urban.(4) The low level flow is affected by topography andurban effects. Air converges to urban areas under200m with the affection of UHI.This paper shows that temperature, humidity, air flow and summer precipitation arestrongly affected by urban effects besides the much studied “urban heat island” in Beijing.The local urban effects have to be taken into account in fine weather forecasting. In addition,this research can provide support to urban atmospheric environmental governance and cityplanning and construction.