By characterizing the dynamics of a convective boundary layer above a relatively
sparse and uniform orchard canopy, we investigated the impact of the
roughness-sublayer (RSL) representation on the predicted diurnal variability
of surface fluxes and state variables. Our approach combined numerical
experiments, using an atmospheric mixed-layer model including a land-surface-vegetation representation, and measurements from the Canopy
Horizontal Array Turbulence Study (CHATS) field experiment near Dixon,
California. The RSL is parameterized using an additional factor in the
standard Monin–Obukhov similarity theory flux-profile relationships that
takes into account the canopy influence on the atmospheric flow. We selected
a representative case characterized by southerly wind conditions to ensure
well-developed RSL over the orchard canopy. We then investigated the
sensitivity of the diurnal variability of the boundary-layer dynamics to the
changes in the RSL key scales, the canopy adjustment length scale,

The atmospheric boundary layer (ABL), as a component of the global climate
system, is characterized by the turbulent exchange of energy, momentum and
matter between the Earth's surface and the lower atmosphere, as well as by
the influence of larger-scale atmospheric processes

A number of observational studies have demonstrated the importance of canopy
effects on the turbulent exchange of energy, mass and momentum within the RSL
for different canopy types

The flux-profile relationships are commonly used within the surface scheme of
the atmospheric models. There have been efforts to incorporate the effect of
RSL turbulence, by using the above-mentioned RSL-adapted flux-profile
relationship in the surface schemes of numerical atmospheric models

Extending these previous works, our study aimed to elucidate the ABL system
for real conditions, taking the representation of the RSL into account. In
order to consider all the relevant physical processes needed to represent the
diurnal variability of the state variables above the canopy, we implemented
the RSL formulation proposed by

Our research is thus an exploratory study of the potential alterations to the
boundary-layer dynamics as calculated by large-scale models

The CHATS experiment took place in the spring of 2007 in one of
Cilker Orchard's walnut blocks in Dixon, California, USA. A detailed
description of the site, instrumentation and data treatment has been provided
by Patton et al. (

The observations analysed in this study were made on a 30 m mast located near
the northernmost border of the orchard site in order to ensure a fetch of
about 1.5 km for the predominantly southerly winds

The CHATS data set is used in our study to initialize and constrain our
soil–vegetation–atmosphere modelling system. The model evaluation of the
diurnal variability of the state variables in and above the roughness
sublayer makes use of diurnal observations of the mean and turbulent
variables at the same heights (at the canopy top (10 m) and at 19 m above the
canopy) as for the selected study cases (Sect.

An atmospheric boundary-layer model with a zero-order jump
approach, based on mixed-layer theory

Schematic overview of the coupled land–vegetation–atmospheric system
and its representation in the mixed-layer model. The vertical origin of the
coordinate system is placed at the displacement height

Based on the mixed-layer model, the diurnal variability of the mean
thermodynamic variables and atmospheric constituents reads as follows:

The influenced

The functions

The displacement height,

Finally, the RSL functions

To initialize and validate our modelling system, we selected
observations from a representative day from the second phase of the CHATS
campaign (from 13 May to 12 June) focusing on the walnut trees after leaf out
(fully vegetated canopy). The representative case is based on two
requirements that the data satisfied: (i) well-mixed conditions and (ii) well-developed
RSL. Our assumption of a well-mixed boundary layer is
justified for sunny (cloudless) days characterized by convective conditions.
Moreover, the lidar data (see figures in the Supplement) showed
quite a homogeneous signal, which in the absence of radiosoundings implies
well-mixed conditions of up to 500 m height at noon (12:00 LT, local time). In order to
ensure the maximum influence (fetch) of the canopy on the atmospheric flow,
leading to a potentially well-developed RSL, we selected data with predominantly southerly winds, since the measurement tower was placed at the northernmost
part of the orchard field

Several systematic experiments were performed, in which the representation of
the drag coefficient and the impact of the RSL on mean gradients (Eqs.

Numerical model runs, description and abbreviations.

The numerical experiment which does not take subsidence into account has
prescribed zero subsidence (no divergence of the mean horizontal wind), while
the numerical experiments with subsidence have imposed constant divergence of
the mean horizontal wind (Appendix

The numerical experiments started at 08:00 LT, which is
equivalent to 15:00 coordinated universal time (UTC), and lasted for 9 h. In the absence of initial measurements at the residual layer (roughly
350 m), we imposed the upper boundary conditions of the model to optimize the
representation of the temporal evolution of the potential temperature,
specific humidity, wind direction and boundary-layer height (Tables

Furthermore, we put special emphasis on validating the modelled quantities at
the canopy top (

Finally, the initial value of

We start our analysis by evaluating the modelling system to
represent the observations of the selected study cases. Figure

Figure

Our explanation of this overestimation is the frequently observed imbalance
of the observed surface-energy system

Observed and modelled radiation and surface-energy balance
components:

The comparison presented here confirms that our modelling system is capable
of reproducing the diurnal variations in radiation with sufficient accuracy.
As in many other studies

Observed non-closure of the surface-energy balance on 27 and 31 May 2007 during the CHATS experiment.

Figure

It is important to mention that

Temporal evolution of the observed versus modelled mixed-layer
quantities on 27 May 2007:

The role of the large-scale advective cooling on the CBL dynamics was also
recorded through the diurnal evolution of the potential temperature (Fig.

Similar behaviour of the diurnal evolution of the specific humidity at 29 m
above the ground surface was observed (Fig.

The analysis presented in Fig.

Temporal evolution of the observed versus modelled boundary-layer
dynamics at 29 m above the ground surface:

The observed diurnal variability of the wind enables us to further verify the
role of the large-scale forcing and the local canopy. Here, we compare the
observed and modelled temporal evolution of the wind direction, individual
wind speed components and absolute wind velocity (Fig.

The results of the case study of 27 May 2007 are corroborated by those of the case study of 31 May 2007 (not shown), showing similar patterns and structure of the CBL dynamics in both cases.

In summary, our modelling system is capable of reproducing the land–canopy–atmosphere characteristics of the case studies with satisfactory accuracy at a height well above the canopy. In the following section, we study the impact of the canopy on the boundary-layer state variables within the roughness sublayer near the canopy top.

Calculated mean absolute error (MAE) of MXL

Figure

Observed versus modelled modulus of the wind speed

Both the MXL

Table

In order to extend and generalize our results, we performed a parameter-space
sensitivity analysis on two stability-dependent scales in the RSL
formulation:

Budget of the mixed-layer wind speed components

We further extend our analysis of the impact of the canopy-related parameters
on the atmospheric flow by studying their relative contribution to the
momentum budget, compared to other contributions, e.g. entrainment or
geostrophic forcing (Appendix

Figure

In summary, although the variation of the RSL scale

The impact of the RSL on the potential temperature and specific
humidity at canopy-top level and their respective surface heat fluxes is
presented in Figs.

We find a slightly larger disagreement in the results for observed and
modelled specific humidity at canopy-top level (up to 0.5 g kg

Finally, in the range of

The interpretation of the CHATS height-dependent observations, employing a numerical model that integrates various spatial-temporal scales relevant within the CBL, reveals that the diurnal variability of the state variables above the orchard canopy is highly dependent on the contributions of local and non-local effects. Local effects are related to the land–canopy–atmosphere exchange of momentum and energy, while the non-local effects are either driven by boundary-layer dynamics, such as entrainment, or by mesoscale phenomena, such as subsidence and/or horizontal advection.

At mesoscales, as described by Hayes et al (

In the absence of detailed observations of the temporal evolution at the
entrainment zone, we are able to provide only first-order estimates of the
large-scale effects relevant to our cases and discuss their impacts on the
budgets of potential temperature and specific humidity (Fig.

Focusing now on the surface conditions, and on canopy scales, the
representation of the RSL has a large impact on the drag coefficients and
mean gradients of the thermodynamic variables within the RSL, and to a lesser
extent to the surface fluxes. Our findings are in agreement with those of
Physick and Garratt (

By combining observations, collected at different heights above a walnut
orchard canopy during the Canopy Horizontal Array Study (CHATS), with model
experiments incorporating a land–vegetation–atmosphere model, we
investigated the contributions of canopy and large-scale atmospheric forcings
on the diurnal variability of boundary-layer height, the evolution of
mixed-layer properties and of canopy–atmosphere exchange of momentum,
potential temperature and specific humidity. We selected a representative day
with southerly wind conditions for our study to maximize the effects of the
canopy fetch and compared it with another day (wind veering from northerly to
southerly) characterized by less fetch influence. We pay particular attention
to determining the sensitivity of the surface fluxes and the boundary-layer
evolution to changes in the canopy adjustment length scale,

On the basis of our findings, we reach the following conclusions.

The investigated CHATS convective boundary layers are strongly affected by large-scale processes such as advective cooling, subsidence and entrainment of dry and warm air from the free troposphere. Quantifying these large-scale forcings by using the observations, the coupled soil–vegetation–atmosphere modelling system satisfactorily represents the surface fluxes and convective boundary-layer dynamics at the CHATS site.

In our modelling framework, and in general in the coupled land–atmosphere models,
the representation of the surface fluxes is locked and controlled by the boundary
conditions. The sensible and latent heat fluxes are bounded by the surface available
energy, and the momentum flux is constrained by the pressure gradient and the
entrainment of momentum, the latter dependent on the boundary-layer growth.
As a consequence, adding a roughness-sublayer representation in the surface scheme
of the model alters the partitioning of the surface fluxes (e.g. sensible and
latent heat) through the altered roughness length and displacement height.
Specifically for our case studies, the canopy's impact on convective boundary-layer
dynamics is relatively minor due to its small effect on modelled surface fluxes
and the bulk boundary-layer properties well above the canopy (

The sensitivity analysis on roughness-sublayer scales, analysed through
changes in

Changes in

When interpreting the CHATS measurements above the canopy, the mesoscale advective processes or subsidence play an important role in determining the convective boundary-layer dynamics. Analysis of the bulk potential temperature and specific humidity budgets showed that the influence of the advection can be around one-fourth of the total potential temperature budgets.

The model input and output data is available at

The model source code, can be available upon request.

Any interested party can access the CHATS data set via

Initial and boundary conditions for model runs of 27 May 2007 (147 DOY) for the CHATS experiment.

Initial and boundary conditions for model runs of 31 May 2007
(151 DOY) for the CHATS experiment
(similar to Table

Assuming that in the free troposphere the wind is in balance (equilibrium)
between the pressure gradients and Coriolis force, the budgets of the
mixed-layer wind components are expressed by the following equations:

Combining the Eqs. (

The authors declare that they have no conflict of interest.

We would like to thank Ian Harman (CSIRO – Commonwealth Scientific and Industrial Research Organisation, Canberra, Australia) for providing us with the roughness-sublayer model code, as well as Edward G. Patton (NCAR – National Center for Atmospheric Research, Boulder, Colorado) for giving us access to the CHATS data set and for the comments on the boundary-layer height evaluation. The article processing charges for this open-access publication were covered by the Max Planck Society. Edited by: S. Galmarini Reviewed by: three anonymous referees