Scaling Variances of Scalars in a Convective Boundary Layer Under Different Entrainment Regimes

Turbulent humidity fluctuations in the convective boundary layer (CBL) under clear-sky conditions were investigated by deriving moments up to fourth-order. Highresolution humidity measurements were collected with a water vapour differential absorption lidar system during the HD(CP) 2 Observational Prototype Experiment (HOPE). Two cases, both representing a well-developed CBL around local noon, are discussed. While the first case (from the intensive observation period (IOP) 5 on 20 April 2013) compares well with what is considered typical CBL behaviour, the second case (from IOP 6 on 24 April 2013) shows a number of non-typical characteristics. Both cases show similar capping inversions and wind shear across the CBL top. However, a major difference between both cases is the advection of a humid layer above the CBL top during IOP 6. While the variance profile of IOP 5 shows a maximum at the interfacial layer, two variance peaks are observed near the CBL top for IOP 6. A marked difference can also be seen in the third-order moment and skewness profiles: while both are negative (positive) below (above) the CBL top for IOP 5, the structure is more complex for IOP 6. Kurtosis is about three for IOP 5, whereas for IOP 6, the distribution is slightly platykurtic. We believe that the entrainment of an elevated moist layer into the CBL is responsible for the unusual findings for IOP 6, which suggests that it is important to consider the structure of residual humidity layers entrained into the CBL.

Section: Discussionsupporting

confidence: 71%

Turbulent Humidity Fluctuations in the Convective Boundary Layer: Case Studies Using Water Vapour Differential Absorption Lidar Measurements

Muppa

Behrendt

Späth

et al. 2015

“…Beljaars et al (1983), de Bruin et al (1991, Roth and Oke (1995), Katul et al (1995), Andreas et al (1998), and recently Detto et al (2008) (further referred to as DK08). Vertical differences occur if the relative importance of the surface flux and entrainment flux is different for both scalars: differences in entrainment regime (de Bruin et al 1993;Michels and Jochum 1995;Jochum et al 2004;Moene et al 2006a). Other causes for decorrelation of scalars are the active role of temperature and humidity, modulations of the surface layer by the outer layer (and unsteadiness) and advective conditions (see Katul et al (2008) for an extensive review of literature on this subject).…”

Section: Introductionmentioning

confidence: 99%

The Effect of Surface Heterogeneity on the Temperature–Humidity Correlation and the Relative Transport Efficiency

Moene

Schüttemeyer

2008

Self Cite

We derive a conceptual model of the flow over heterogeneous terrain consisting of patches with contrasting Bowen ratios. Upward moving eddies are assumed to carry heterogeneous properties, whereas downward moving eddies carry homogeneous properties. This results in a decorrelation of temperature and humidity as the contrast between the patches increases. We show that this model is able to reproduce the relationship developed by Lamaud and Irvine (Boundary-Layer Meteorol. 120:87-109, 2006). Some details differ from their expression but are in accordance with data obtained over African savannah. We extend the conceptual model to a combination of any scalars, not necessarily linked through the surface energy balance (as is the case for temperature and humidity). To this end we introduce a new parameter that describes the surface heterogeneity in surface fluxes. The results of the current model can be used to predict the discrepancy between similarity relationships for different scalars over heterogeneous terrain.

“…Moeng and Wyngaard 1984;Moene et al 2006b). Unlike temperature, passive scalars often decrease or increase monotonously with height throughout the boundary layer, making the minimum dependent on the precise height of the layer interacting with a near-surface sensor.…”

Section: Research Questions and Organisation Of The Papermentioning

confidence: 99%

“…If we assume that as demonstrated for potential temperature, the minimum of the turbulent near-surface PDF roughly indicates the minimum of the respective quantity in the whole layer interacting with the surface, then the height where the χ v profile reaches this minimum should roughly indicate the top of direct interaction, including a possible entrainment layer if the absolute value of the entrainment ratio is high (cf. Moeng and Wyngaard 1984;Moene et al 2006b). Two possible estimates of this height can be derived from the sample minimum and the minimum of the fitted beta distribution, respectively, both of which are in close agreement in cases a and b.…”

Section: Relation To the Vertical Profilementioning

confidence: 99%

“…A small degree of mixing as this air moves to the surface might make the average even better related to the near-ground PDF minimum than the minimum-a hypothesis we could not test as the airborne temperature measurements do not have the same short response time as the near-ground sensors. Variance is known to increase again near the top of the boundary layer, and may not decrease with height in cases of a high entrainment ratio of the respective scalar (Moene et al 2006b). With the above response-time restrictions of the airborne temperature measurement, it remains unclear whether this occurs in our dataset, and what is the effect on the PDF minimum at various greater heights.…”

Section: Agreement Among Ground-based Sensorsmentioning

confidence: 99%

See 1 more Smart Citation

Boundedness of Turbulent Temperature Probability Distributions, and their Relation to the Vertical Profile in the Convective Boundary Layer

Graf

Schüttemeyer

Geiss

et al. 2009

Higher-order moments, minima and maxima of turbulent temperature and water vapour mixing ratio probability density functions measured with an eddy-covariance system near the ground were related to each other and to vertical boundary-layer profiles of the same scalars obtained through airborne soundings. The dependence of kurtosis on squared skewness showed a kurtosis intercept below the Gaussian expectation, suggesting a compression of the probability density function by the presence of natural boundaries. This hypothesis was corroborated by comparing actual minima and maxima of turbulent fluctuations to estimates obtained from the first four sample moments by fitting a four-parameter beta distribution. The most sharply defined boundaries were found for the minima of temperature datasets during the day, indicating that negative temperature fluctuations at the sensor are limited by the availability of lower temperatures in the boundary layer. By comparison to vertical profiles, it could be verified that the turbulent minimum of temperature near the ground is close to the minimum of potential temperature in the boundary layer. The turbulent minimum of water vapour mixing ratio was found to be equal to the mixing ratio at a height above the minimum of the temperature profile. This height roughly agrees with the top of the non-local A. Graf (B) · M. Herbst · H. Vereecken Agrosphere (ICG-4), 123 460 A. Graf et al.unstable domain according to bulk Richardson number profiles. We conclude that turbulence statistics measured near the surface cannot be solely explained by local effects, but contain information about the whole boundary layer including the entrainment zone.