Abstract
Abstract. In the spring of 2013, extensive measurements with multiple Doppler lidar systems were performed. The instruments were arranged in a triangle with edge lengths of about 3 km in a moderately flat, agriculturally used terrain in northwestern Germany. For 6 mostly cloud-free convective days, vertical velocity variance profiles were calculated. Weighted-averaged surface fluxes proved to be more appropriate than data from individual sites for scaling the variance profiles; but even then, the scatter of profiles was mostly larger than the statistical error. The scatter could not be explained by mean wind speed or stability, whereas time periods with significantly increased variance contained broader thermals. Periods with an elevated maximum of the variance profiles could also be related to broad thermals. Moreover, statistically significant spatial differences of variance were found. They were not influenced by the existing surface heterogeneity. Instead, thermals were preserved between two sites when the travel time was shorter than the large-eddy turnover time. At the same time, no thermals passed for more than 2 h at a third site that was located perpendicular to the mean wind direction in relation to the first two sites. Organized structures of turbulence with subsidence prevailing in the surroundings of thermals can thus partly explain significant spatial variance differences existing for several hours. Therefore, the representativeness of individual variance profiles derived from measurements at a single site cannot be assumed.
Highlights
The vertical velocity variance, w 2, is one of the relevant parameters describing the turbulent structure of the convective boundary layer (CBL)
Even if it is assumed that temporal and spatial integration are comparable, i.e., if time can be transformed into space via the mean wind speed (Taylor’s hypothesis; Taylor, 1938), lidar measurements are representative of a restricted region only
Hogan et al (2009), e.g., found that scaled variance profiles derived from lidar measurements at one particular site displayed a case-to-case variability that was about as large as the scatter of the fit functions given by Lenschow et al (1980) and Sorbjan (1986), which had been derived from aircraft measurements
Summary
The vertical velocity variance, w 2, is one of the relevant parameters describing the turbulent structure of the convective boundary layer (CBL). Hogan et al (2009), e.g., found that scaled variance profiles derived from lidar measurements at one particular site displayed a case-to-case variability that was about as large as the scatter of the fit functions given by Lenschow et al (1980) and Sorbjan (1986), which had been derived from aircraft measurements. The locations had to be close enough to be situated within the area of the given surface heterogeneity For this configuration, the turbulence characteristics derived from the lidar measurements at the three sites should be similar within the range of statistical errors according to Lenschow et al (1994). By investigating spatial differences of vertical velocity variance, the representativeness of point measurements of vertical turbulence profiles can be assessed.
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