Characteristics of vertical windobserved within and over the convective boundary layeron the Loess Plateau, China*Masanori NISHIKAWA1, Tetsuya HIYAMA2, Atsuhiro TAKAHASHI3,Wei LI1, Atsushi HIGUCHI4, Wenzhao LIU5, Yoshihiro FUKUSHIMA3
1
Grauduate school of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan
2
Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya 464-8601, Japan
3
Research Institute for Humanity and Nature (RIHN), Kyoto 602-0878, Japan
4
Center for Environmental Remote Sensing, Chiba University, Chiba 263-8522, Japan
5
Institute of Soil and Water Conservation, Chinese Academy of Sciences, Shaanxi 712100, China*E-Mail: s040110d@mbox.nagoya-u.ac.jp, TEL: +81-52-789-3439, FAX: +81-52-789-3436
1. Introduction
Wind Profiler Radar (WPR) enables us toobtain turbulent data within the Atmospheric
Boundary Layer (ABL) due to its high resolutionsin the measurements of refractive index in time andspace. Recently, daytime Convective Boundary
Layer (CBL) was strongly investigated using WPR.
The previous researches were carried out at flat andhomogeneous surfaces of North America andEurope, where WPR networks have been alreadydeveloped. It seems that it is necessary to fill datain blank regions and at various land surfaces. Basedon these research backgrounds, we have carried out
temporary continuous ABL observations over theunique topography of the Loess Plateau.
Based on the observation data obtained in
2004, we showed a possibility that CBL processes
over the tableland of the Loess Plateau wereaffected by its unique topography. We implied that
CBL processes were characterized by the strong
vertical mixing, extended within and over the CBL.
In this study, we mainly analyzed the data of theintensive observations, which was carried out from
May to July 2005. We investigated the local
circulations caused by the unique topography of theLoess Plateau, and the characteristics of verticalwind observed during CBL developments.
2. Study site and methodology
The study site is located at the research field of
the Changwu Agro-Ecological Experimental
Station on the Loess Plateau (35˚12N, 107˚40E),
one of the field stations of Chinese Academy ofScience (CAS). According to the brochure of thestation, the elevation is approximately 1200 m, themean annual precipitation is 584 mm and the mean
temperature is 9.1 ˚C. The study site is located on aflat tableland of the Loess Plateau. The flat terrain
extended from northwest to east directions andsteep gullies are existed from east to northwest
directions. The main crop species around the station
are winter wheat, corns and apples. Most parts ofthe tableland are randomly occupied by these
agricultural fields.
ABL observations were carried out using 1290
MHz-WPR (L-28, Sumitomo Electric Industries,Japan). To obtain meteorological data within theatmospheric surface layer, we installed Flux &Radiation Observation System (FROS) including
turbulent and radiation sensors. We conducted
intensive observations from 11 May to 13 July
(totally 64days). In the observation period, wecarried out several kinds of manual observations(weather conditions, type and amount of clouds,
and soil temperatures etc.) We also used theobservational data carried out from May to June of2004, for the comparisons.
3. Results and discussion
We firstly checked diurnal changes of winddirection within the ABL, as the indicator of localcirculations peculiar to Loess Plateau. Based on
this analysis, local circulations occurred in 53 days
within the total 64 observation days. Local
circulations didn’t occur in the rest of the 11 days.
We divided cases whether local circulations
occurred or not, and investigated CBL processes inthe two cases. As the result, when local circulations

didn’t occur, synoptic wind dominated and strong
vertical wind appeared only within the CBL.
Figure 1 shows the typical diurnal variationsof the wind direction and wind velocity on the 30 mstage of FROS, time-height sections of echo
intensity, and the vertical wind velocity using WPR,
when local circulations apparently occurred. FromFig. 1c, strong upward and downward winds
occurred and repeated within and over the CBL.
Especially these vertical winds sometimes
penetrated the CBL top. Echo intensity was also
developed with the strong upward and downward
winds (Fig. 1b). Vertical wind penetrated the CBL
top were only observed under the unstable
atmospheric condition accompanied with cumulus
developments in the afternoon from mid to lateJune. This period corresponded to the time of theharvest of winter wheat. The surface was so drythat sensible heat flux was larger than latent heat
flux. Therefore it might be suggested that verticalwind penetrated the CBL top were caused by thecombination of local circulations and theconvective land-CBL interactions.
Acknowledgements
This research is funded by the Yellow River
Studies (YRiS) project, RIHN. We would like tothank the Changwu Agro-Ecological Experimental
Station on the Loess Plateau, and the Institute ofSoil and Water Conservation, CAS for their kindcooperation during observations.
Reference
Angevine, W. M., A. B. White, and S. K. Avery,
Boundary-layer depth and entrainment zone
characterization with a boundary-layer profiler,
Boundary-Layer Meteorol., 68, 375-385, 1994.

a)

b)

c)

Fig.1 Typical diurnal variations of a) wind direction(circles) and wind velocity (line withtriangles), b) time-height section of echo
intensity corrected with range (circles: CBL
height determined from the median filtering
method (Angevine et al., 1994)), c)
time-height section of vertical wind velocity,
observed when local circulations appeared(19 June 2005).