Vilà-Guerau de Arellano, J.

English

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15B.3 ROLE OF NOCTURNAL TURBULENCE AND ADVECTION
IN THE FORMATION OF SHALLOW CUMULUS
Jordi Vila`-Guerau de Arellano ∗
Meteorology and Air Quality Section, Wageningen University, The Netherlands
1. MOTIVATION
This extended abstract is a short summary of
the complete research presented at de Arellano
(2007). The formation and further development of
shallow cumulus over land is driven by the daily
variations in surface turbulent fluxes. Their evo-
lution is also dependent on the thermodynamic
characteristics and on the atmospheric stability
within and above the atmospheric boundary layer
(ABL). To date, studies have focused on the role of
the land-atmosphere interaction in controlling shal-
low cumulus formation (Ek and Holtslag, 2004;
van Heerwaarden and de Arellano, 2008) and in
discussing their main dynamic characteristics and
their controlling processes (Zhu and Albrecht,
2002; Brown et al., 2002).
In comparison with the formation of fair-
weather cumuli above maritime conditions, above
land, shallow cumulus are highly dependent on the
diurnal cycle of the atmospheric boundary layer,
external forcings and the horizontal variability of
surface properties. Therefore, and as a result of
their non-steady development, their representation
in large-scale models is more problematic than
above the sea (Neggers et al., 2004), (Lenderink
et al., 2005), (Berg and Stull, 2006).
However, an important condition, the initial
morning characteristics of the thermodynamic ver-
tical structure, has received little attention, partic-
ularly with regard to how the structure of the noc-
turnal boundary layer (NBL) and the layer above
it evolve to establish the optimal stability condi-
tions for triggering shallow cumulus formation dur-
ing the day. As mentioned by (Zhu and Albrecht,
2002), weaker stratification and smaller inversion
potential temperature jump favour cloud formation.
Both are largely dependent on the initial thermo-
dynamic state at dawn. In this paper, we present
observational evidence of these feedbacks mech-
anisms during the transition from nocturnal to diur-
nal boundary layer conditions.
In order to assess the role of nocturnal bound-
ary layer on shallow cumulus over land, two con-
secutive nights were studied by means of de-
tailed surface and upper air observations. These
∗Corresponding author address: Jordi Vila`-Guerau de
Arellano, Meteorlogy and Air Quality Section, Droeven-
daalsesteeg 4, 6700 AA Wageningen, The Netherlands,
e-mail:jordi.vila@wur.nl
measurements were taken on the Southern Great
Plains of the Atmospheric Radiation Measure-
ment (SGP-ARM) site in Oklahoma and Kansas,
USA. The high spatial distribution, more than four
radiosounding launching stations within a radius
of 200 km around the central facility, and the
high temporal frequency, with a radiosonde was
launched every three hours, made the ARM site
particularly suitable for the purpose of this study.
Moreover, the ARM site has been previously used
to study cloudy boundary layers over land (Zhu
and Albrecht, 2002).
The nights under study were the 19-20 June
1997 and the 20-21 June 1997. Although both
nights presented very similar characteristics in syn-
optic and mesoscale terms, differences above the
nocturnal boundary layer and the intense turbu-
lent mixing in the NBL led to the development of
two diurnal boundary layer with very different tur-
bulent characteristics. Hence, on the 20th June
1997, a cloudless boundary layer was formed with
a large entrainment rate driven partly by shear
at the surface and at the inversion zone (Pino
et al., 2003) while on the 21st June, shallow cu-
muli were observed with a coverage of 20%-30%
above the ARM site (Zhu and Albrecht, 2002;
Brown et al., 2002).
The primary goals of this study were: (i) to
analyze the observational evidence of the role of
advection and turbulent mixing during the night in
establishing the conditions required to form daily
shallow cumuli and (ii) to examine, by means of
sensitivity analysis, the dependence of cloud de-
velopment to modifications of the initial morning
values of the atmospheric stability and the prop-
erties within the entrainment region.
2. RESEARCH FINDINGS IN BRIEF
2.1 Temporal evolution
The temporal sequence (23,02,05,08 LT) of the
vertical profile of potential temperature at site C
during the two consecutive nights is shown in fig-
ure 1. In order to distinguish between the evolu-
tion of the profiles within and above the nocturnal
boundary layer (NBL), the boundary layer height
h estimated as the height of the mean wind max-
imum (LLJ) is also shown. The similar evolution
of the longwave radiative cooling and intense ver-
1
FIG. 1: Temporal evolution of the potential tem-
pertaure vertical profiles at site C during the nights
of 19-20 and 20-21 June. The boundary layer
height h is also indicated. At 02 and 05 LT the
boundary layer height was the same on both nights
(500 m).
.
tical mixing within the NBL yields to a very simi-
lar development of the stable stratification during
both nocturnal boundary layers. Above the NBL,
the θ-profile at 23 LT during the first night is slightly
colder. However, at 02 LT, both profiles are almost
identical characterized by the same boundary layer
height and slightly higher stability values for the
second night.
On the 21st June, during the time interval be-
tween 02 and 05 LT, the air mass above the NBL
cooled strongly down. For instance, if we take
1000 m as a reference height, we observed that
during the night 19-20 June, the θ values remain
almost constant (≈ 312 K) whereas during the sec-
ond night they have decreased by 4 K. Closely as-
sociated with this cooling is an increase in the in-
stability of the upper layer.
The temporal evolution of the specific mois-
ture profiles is similar (figure 2). The drier layer
above the BL during the night of 20-21 June com-
pared within the first night is modified by a large
increase in moisture content. During the period
02-05 LT, this increase is from 5 g(Kg)−1 to 15
g(Kg)−1 at 1000 m. At 05 LT of 21st June, and
except for the maximum and minimum values of
the specific humidity at 650 m and 1000 m respec-
tively, we observed higher water vapour contents
up to 1300 m, well above the NBL. It is also worth
pointing out that the strong mechanical turbulent
mixing with the NBL leads to a well mixed-layer on
FIG. 2: Same as figure 1, but showing specific
humidity.
both nights, i.e. the difference between the surface
values and the top NBL is less than 1 g(Kg)−1 due
to the almost constant on time positive value of the
moisture flux. To complete the previous analysis of
figure 2, the specific moisture vertical profile at 08
LT (21 June) once again demonstrate the estab-
lishment of optimal conditions for the formation of
shallow cumulus: a well-mixed profile upto 1500 m
with high water vapour content (≈ 15 g/Kg).
2.2 Dependance of cloud formation on initial
morning conditions
As shown by the previous analysed observations
above, thermodynamic modifications of the layer
above the NBL result in wide variations in the po-
tential temperature and the moisture content verti-
cal distribution. The morning transition conditions
are therefore influenced by these variations, partic-
ularly by the formation of the initial potential tem-
perature (∆θvo) and moisture (∆qo) interface jump
during the early stages of the development of the
convective boundary layer. These nocturnal varia-
tions have a strong influence on the further evolu-
tion of the boundary layer growth and are thus re-
sponsible for setting up the optimal conditions for
the formation of boundary layer clouds once diur-
nal convection begins. Here, a sensitivity analysis
was carried out to study the dependence of bound-
ary layer height and the lifting condensation level
on the initial values of the potential temperature
and moisture jump at the interface and the poten-
tial temperature lapse rate.
A mixed layer model (Lilly, 1968; Tennekes,
1973) was used to calculate the boundary layer
evolution and to estimate wether water vapour sat-
urates (i.e. LCL below h (clouds potentially can
be formed) or above h (absence of clouds). See
de Arellano (2007) for more details on the model
formulation. The mixed layer model reproduces
the evolution of the slab convective boundary layer
variables (sub-cloud layer) observed during the
21st June and it includes the explicit calculation
of the time evolution of the LCL by using a parcel
method couple to the evolution of the thermody-
namic variables solved by the mixed-layer model.
We take a similar approach as Zhu and Albrecht
(2002) who employed the mixed layer model as a
conceptual tool that is able to reproduce accurately
the bulk characteristics of the convective boundary
layer.
FIG. 3: Time evolution of the boundary layer
height h (continuous line) and lifting condensation
level LCL for the following initial values of the mois-
ture jump: -0.5 g/Kg (dashed line) and -5.0 (g/Kg)
(dotted line). (a) Prescribing initial values similar
to the early morning conditions at 08 LT 21st June
(see Table 2); (b) similar to Table 2 but initial ∆θ =
2 K; (c) similar to Table 2 but γθ = 2.10−3 K/m
and (d) similar to Table 2 but β = 0.3.
Our mixed layer model initial conditions are
similar to the ones prescribed by Brown al. (2002)
in order to to reproduce similar convective bound-
ary layer conditions to those observed on 21st
June 1997 at siteC (Zhu and Albrecht 2002, Brown
al. 2002). de Arellano (2007) summarizes
the initial conditions prescribed in the mixed-layer
model. The sensitivity tests focus on the inital
values of ∆θvo, ∆qo, γθ and the ratio of the en-
trainment heat flux to the surface heat flux (βθv).
As previously analyzed, the first three variables
were largely influenced by the arrival of the colder
and moister air mass above the NBL bewteen 02
and 05 LT. Figure 3 presents the time evolution
of the boundary layer height and the lifting con-
densation level calculated for two different cases:
∆qo = −0.5 (g/Kg) (closer to the sunrise condi-
tions of 21st June) and ∆qo = −5.0 (g/Kg) (closer
to the sunrise conditions of 20th June). Figure 3a
shows the model results closer to the initial obser-
vations (for ∆qo = −0.5 (g/Kg)) and the other
figures show the results of the sensitivity tests.
Notice, however, that in all the cases, we found
that the absolute decrease of the moisture jump at
the inversion (from -5.0 to -0.5) leads always to a
moistly boundary layer (lower LCL) due to the less
entrainment of dry into the boundary layer.
In Figure 3a, we show the evolution of h and
LCL using the initial conditions closer to the sit-
uation oberved during the 21st June (see Table
2). The model results using an initial ∆qo =
−0.5 (g/Kg) reaches the condition h = LCL at
around 11 LT. At approximately this time, clouds
were observed over the central facility C and the
ceilometer reported a cloud-base height (similar
to LCL) at around 1000 m between 10 and 11
LT (see figure 5 at Brown et al (2002)). A de-
crease in the initial jump of the specific humidity
(∆qo = −5. (g/Kg)) would lead to a dealy in one
hour of the necessary condition h = LCL due to
the enhancement of the entrainment of dry air and
the subsequent raise of LCL.
Figure 3b shows the same situation but now
increasing the initial jump of the virtual potential
temperature at the interface to 2.0 K. This test is il-
lustrative for the previous day situation (20th June)
characterized by large inversion jumps for ∆θo and
∆qo at the entrainment zone. A shallow boundary
layer with a slow growth rate is the main feature
of this sensitivity test leading to a late onset of the
condition h = LCL (for case ∆qo = −0.5 (g/Kg))
or a cloudless boundary layer because of LCL > h
(initial ∆qo = −5. (g/Kg)).
The increase of atmospheric instability above
the boundary layer is studied by reducing the tem-
perature lapse rate to the value 2.10−3 K/m (see
Figure 3c). In consequence, the boundary layer
deepens faster and the condition h = LCL is at-
tained earlier than in the case presented at figure
3a. It is also important to notice that this rapid
growth leads an increase on the entrainment of
dry air yielding to higher LCL (see case ∆qo =
−5. (g/Kg)). This decrease of the temperature
lapse rate was also used by Brown et al (2002) to
reproduce by means of LES shallow cumulus with
an enhanced vertical development.
Finally, we show at figure 3d a sensitivity test
to the ratio of the entrainment heat flux to the sur-
face heat flux. As studied by Pino et al (2003) dur-
ing the previous day, larger values of this ratio are
expected due to the enhancement of the entrain-
ment flux by the presence of shear at the surface
and at the entrainment zone. The model results
show that in spite of the increase of warm and dry
air brought into the boundary layer by the entrain-
mnet process, the condition h = LCL is reached
slightly before than in the observed case (for case
∆qo = −0.5 (g/Kg)) due to the more rapid growth
of the boundary layer.
In spite of the conceptuality of the study, the
analysis indicate the subtlety in setting up optimal
conditions to the formation of shallow cumulus over
land. Briefly, a decrease on the morning initial po-
tential jump and temperature lapse rate leads to a
higher rate of the boundary layer growth increas-
ing the possibility of cloud formation. In turn, this
higher rate is normally associated with larger en-
trainment events of warmer and drier air into the
boundary layer which can delay reaching the con-
dition h = LCL. A small jump of moisture at the
entrainment region could compensate for this dry-
ing effect of the boundary layer and facilitate cloud
formation.
3. CONCLUSIONS
We investigated the role of the nocturnal bound-
ary layer and its layer aloft in setting up appropri-
ate conditions for the formation of shallow cumuli.
Surface and upper-air observations of two consec-
utive nights were studied and compared in order to
determine the processes involved in producing two
different diurnal boundary layers (one clear and the
other cloudy) under similar driving diurnal surface
conditions. Both nights displayed very similar tur-
bulent structures and evolution, characterized by
an intense turbulent mixing (friction velocity be-
tween 0.5 and 0.6 m/s) driven by the wind shear
generated by the low level jet. However, during
the second night, the air mass above the NBL was
modified by a low-level advection event becoming
colder (a decrease of approximately 4 K) and with
a higher specific humidity content (an approximate
increase 10 g/Kg). As a result, the conditions at the
interface between the NBL and the layer aloft were
greatly modified during the second night. We can
summarize these key observed conditions in the
generation of favourable conditions for cloud for-
mation during the day as follows: (1) reduction in
the potential temperature and moisture jump at the
interface near sunrise, (2) increase in the instability
above the boundary layer during morning transition
and (3) possibility of rapid formation of well-mixed
profiles of potential temperature and specific hu-
midity due to high levels of turbulence at nigh and
in the morning transition.
By means of a mixed layer model, we demon-
strate the sensitivity of the evolution of a diurnal
boundary layer to changes in the initial values of
the inversion potential temperature and moisture
jump (condition 1) and to the temperature lapse
rate (condition 2). The analysis aimed to de-
termine the conditions under which the boundary
layer height becomes higher than the lifting con-
densation level. Higher boundary layer values are
found when the potential temperature jump and the
temperature lapse rate in the free troposphere are
reduced. However, this enhancement is closely as-
sociated with a warmer and drier boundary layer
which leads to higher lifting condensation levels.
The drying effect can be compensated for if the
moisture levels in the free troposphere are fairly
similar to those of the boundary layer (relatively
small jump in moisture content at the entrainment
region).
To conclude, at a similar order of magnitude
and range of variability of the surface turbulent con-
ditions during the night, these three mentioned fac-
tors will lead to vertical profiles of the potential tem-
perature and the specific humidity that are optimal
for triggering shallow cumulus development over
land. This study thus indicates the importance of
accurately modelling vertical thermodynamic vari-
ables at night in order to be able to reproduce cloud
formation and their thermodynamical characteris-
tics during diurnal conditions. The high spatial den-
sity and high frequency of the vertical profiles mea-
surements of U , θ and q and of the measurements
of the surface condition around the ARM site make
this observational data set very appropriate for the
study of how boundary layer processes interact
with large scale horizontal advection in mesoscale
models.
REFERENCES
Berg, L. and R. Stull, 2006: Evaluation of a new pa-
rametertization for fair-weather cumulus. Pro-
ceedings of the 17th Symposium on Boundary
Layers and Turbulence, American Meteorolog-
ical Society, 0, 22–25.
Brown, A. R., R. T. Cederwall, A. Chlond, P. G.
Duynkerke, J. Golaz, J. M. Khairoutdinov,
D. Lewellen, A. Lock, M. K. Macveen, C. Mo-
eng, R. A. J. Neggers, A. P. Siebesma, and
B. Stevens, 2002: Large-eddy simulation of
the diurnal cycle of shallow cumulus convec-
tion over land. Quart. J. Roy. Meteorol. Soc.,
128(B), 1075–1094.
de Arellano, J. V.-G., 2007: Role of nocturnal tur-
bulence and advection in the formation of shal-
low cumulus. Quart. J. Roy. Meteorol. Soc.,
133, 1615–1627.
Ek, M. and A. A. M. Holtslag, 2004: Influence of
soil moisture on boundary layer development.
J. Hydromet., 5, 86–99.
Lenderink, G., A. P. Siebesma, S. Cheinet,
S. Irons, C. G. Jones, P. Marquet, F. Mu¨ller,
D.Olmeda, J. Calvo, E. Sanchez, and P. M. M.
Soares, 2005: The diurnal cycle of shallow
cumulus clouds over land: a single column
model intercomparison study. Quart. J. Roy.
Meteorol. Soc., 123, 223–242.
Lilly, D. K., 1968: Models of cloud-topped mixed
layers under s strong inversion. Quart. J. Roy.
Meteorol. Soc., 94, 292–309.
Neggers, R. A. J., A. P. Siebesma, and
G. Lenderink, 2004: An evaluation of mass
flux closures for diurnal cycles of shallow cu-
mulus. Mon. Wea. Rev., 132, 2525–2538.
Pino, D., J. Vila`-Guerau de Arellano, and P. G.
Duynkerke, 2003: The contribution of shear to
the evolution of a convective boundary layer.
J. Atmos. Sci., 60, 1913–1926.
Tennekes, H., 1973: A model for the dynamics
of the inversion above a convective boundary
layer. J. Atmos. Sci., 30, 558–567.
van Heerwaarden, C. C. and J. V.-G. de Arel-
lano, 2008: Relative humidity as an indicator
for cloud formation over heterogeneous land-
scapes. J. Atmos. Sci., in press, 0.
Zhu, P. and B. Albrecht, 2002: A theoretical and
observational analysis on the formation of fair-
weather cumuli. J. Atmos. Sci., 59, 1983–
2005.


A model for the dynamics of the inversion above a convective boundary layer.

A theoretical and observational analysis on the formation of fairweather cumuli.

An evaluation of mass flux closures for diurnal cycles of shallow cumulus.

Evaluation of a new parametertization for fair-weather cumulus.

Influence of soil moisture on boundary layer development.

Large-eddy simulation of the diurnal cycle of shallow cumulus convection over land.

Models of cloud-topped mixed layers under s strong inversion.

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Role of nocturnal turbulence and advection in the formation of shallow cumulus.

The contribution of shear to the evolution of a convective boundary layer.

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Role of nocturnal turbulence and advection in the formation of shallow cumulus

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Role of nocturnal turbulence and advection in the formation of shallow cumulus

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