Development of a convective mixing layer
and generation of shallow convective clouds
Taro Shinoda
Hydrospheric Atmospheric Research Center, Nagoya University

Introduction

2

Development process of convective mixing layer

First we show the importance of the development process of convective mixing layer and its expression in
the numerical simulation. Figure 1 shows time seriesof the echo top height observed by the HUML 3 radarfrom 1000 to 1800 BST 4 on July 14,1998 and thatsimulated by the numerical model with 1 km horizontal grid resolution. Initial shallow convective clouds
are observed at about 1050 BST by the Doppler radarobservation. On the other hand, initial shallow convective clouds are generated at about 1500 BST in the
1 GEWEX (Global Energy and Water Cycle Experiment)
Asian Monsoon Experiment (GAME) / Huaihe River Basin
Experiment (HUBEX), Intensive Observation Period (IOP)
2 Lower Atmosphere and Precipitation Study (LAPS) - Core
Research for Evolutional Science and Technology (CREST)
3 Hokkaido University Meteorological Laboratory
4 Beijing Standard Time: 8 hours ahead of UTC)(a) HUML Radar Observation Result

Echo-top Height (km)The Meiyu front develop over eastern China during the summer monsoon season and is characterized
by meridional moisture gradient in the lower troposphere. A southern region far from the Meiyu front
is wet. This region is generally covered by a Paciﬁc
subtropical high and there is less large-scale convergence; namely a convective forcing is weak. However, deep convective clouds frequently develop in
the evening over this region suggested by Kato et al.
(1995). During the latter half of the GAME/HUBEX
IOP 1 , these deep convective clouds and their diurnal
variation were observed by successive Doppler radarobservations. Shinoda (2001) supposed that there
are three eﬀective factors to develop deep convective
clouds over this region by using the results of numerical simulation. These factors are the latent heat ﬂux
from the surface, the development process of the convective mixing layer, and the humidity in the middle
troposphere. In this LAPS-CREST 2 project, we will
be able to conﬁrm the last two factors by observing
the meteorological conditions and their time variationin the lower and middle troposphere by utilizing theobservational instruments.

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(b) Model Result

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Fig. 1 Time series of echo top height observed by
the HUML radar at 20 dbZe (a) and that of thesimulated (b) from 1000 to 1800 BST on July 14,
1998.

numerical simulation. There is a signiﬁcant disagreement on the times between the observed and simulated initial shallow convective clouds. These diﬀerence times might be attributed to an imperfection ina development process of the convective mixing layer
in the numerical simulation.
It is generally considered that the vertical proﬁles ofthe potential temperature and the mixing ratio of water vapor are uniform in the convective mixing layer.
However, Warner and Telford (1967) showed that theconvective mixing layer is characterized by a population of turbulent (buoyant) plumes rising from thewarm surface by using ground and aircraft observation results. The development of the convective mixing layer is maintained by the growing plumes. The
temperature excess in the plumes are about 1 K compared with the environment. Their horizontal scale isabout 200 to 500 meters, and their upward velocity is
about 1 m/s. The convective mixing layer are developed by the entrainment around the top of this layer,
and initial shallow convective clouds are generated at
the top of these turbulent plumes. In this numericalsimulation, the horizontal grid resolution is 1 km, so
turbulent plumes cannot resolve explicitly. The ef-

Height : 500 m

1100 BST

1.0
0.5

Height (km)

2.0

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(b) Potential Temperature (K)
dum
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(c) Mixing Ratio of Vapor (g/kg)
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Distance (km)Fig. 2 Cross sections of vertical wind speed (a),potential temperature (b), and mixing ratio of water vapor (c) with wind vectors at 1100 BST in thesimulation.

fect of turbulent plumes is dealt with a sub-grid scale
process.
We perform a simulation to resolve the turbulent
plumes explicitly by utilizing the ﬁne grid resolution(100 m). In the ﬁne grid resolution, we can simulate
the development of turbulent plumes in the convective mixing layer after 1040 BST. Figure 2 shows crosssections of vertical wind speed, potential temperature, and mixing ratio of water vapor at 1100 BST.
From this ﬁgure, we can see the structure of turbulent plumes. In this simulation, the horizontal scaleof turbulent plumes is about 500 m. Potential temperature and water vapor mixing ratio excesses in the
turbulent plumes are about 1 K and 1 g/kg, respectively. Entrainment from the middle troposphere areseen in the downdraft region. Initial shallow convective clouds are generated over these turbulent plumes.
Figure 3 shows time series of heating rate at theheight of 500 m in the convective mixing layer. This
heating rate means the vertical heat transport at thislevel. “TADVZ” shows the vertical advection of heat,and it means that heat is transported by the resolvable scale. On the other hand, “TMIXZ” shows thevertical mixing of heat, and it means that heat istransported by the eﬀect of sub-grid scale. When thehorizontal grid resolution is 1 km, the vertical heattransport is only performed by the mixing. On theother hand, when the horizontal grid resolution is 100
m, the vertical heat transport is performed by the

TADVZ-60 <---> -20 km
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TMIXZ
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(a) Horizontal Grid : 1 km

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(b) Horizontal Grid : 100 m

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Fig. 3 Time series of heating rate at the height of
500 m. These rates are averaged over the horizontal domain from −60 to −20 km. The heating rate
simulated in the 1 km grid resolution (a), and thatin the 100 m grid resolution (b) are shown. Notations “TADVZ” (solid lines) and “TMIXZ” (broken lines) depict the contribution of vertical advection and vertical mixing of heat, respectively.

advection after 1040 BST. This time corresponds to
the generation of turbulent plumes. These turbulent
plumes explicitly transport heat (and also water vapor) by the vertical advection. The amount of heattransport by the vertical advection in the 100 m grid
resolution is larger than that by vertical mixing in 1
km grid resolution. This heating means that the vertical transport of heat is eﬀectively carried out by the
turbulent plumes.
Figure 4 shows time series of the cloud top heightfrom 1000 to 1800 BST in the simulation. When thehorizontal grid resolution is 1 km, initial shallow convective clouds are generated at 1430 BST. On theother hand, when the horizontal grid resolution is 100
m, initial shallow convective clouds are generated at1050 BST, and this time corresponds to the observed
time. Therefore, to express the vertical transport ofheat (water vapor) by turbulent plumes explicitly isimportant for the development of the convective mixing layer and the generation of initial shallow convective clouds. However, it is diﬃcult to simulate theﬁne grid resolution by the computational restriction.
We have to use some parameterizations to express theeﬀect of turbulent plumes on the development of theconvective mixing layer not only in the cloud resolving

Cloud Bottom

Cloud Top

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Cloud-top/bottom Height (km)140Cloud-top/bottom Height (km)12(b) Horizontal Grid : 100 m

Time (BST)

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(c) 800-500 hPa RH : 70%
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(d) 800-500 hPa RH : 60%
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(a) 800-500 hPa RH : 90%
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(b) Control Run --- RH : 80%
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Cloud-top/bottom Height (km)160(a) Horizontal Grid : 1 km

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Cloud Bottom

Cloud Area (8 km) : Gray Area

Cloud Area (Grid Number)

Cloud Top

Cloud Area (8 km) : Gray Area

Fig. 4 Time series of cloud top/bottom height andcloud area at the height of 8 km from 1000 to 1800
BST simulated in the 1 km horizontal grid resolution (a), and that in the 100 m horizontal grid
resolution (b). Cloud top height (solid line) is denoted by the height of the highest grid, whose mixing ratio of cloud water exceeds 0.1 g/kg. Cloudbottom height (broken line) is denoted by theheight of the lowest grid, whose mixing ratio ofcloud water exceeds 0.1 g/kg. Cloud area (bargraphs) at the height of 8 km is denoted by the
number of grids, whose sum of mixing ratio of liquid/ice water substances exceeds 0.1 g/kg.

Fig. 5 Time series of cloud top/bottom height andcloud area at the height of 8 km from 1000 to 2200
BST in the simulation. Relative humidity in the
middle troposphere (from 800 to 500 hP a) is (a)90%, (b) 80% (the control run), (c) 70%, and (d)60%. Explanations of this figure are the same as
Fig. 4.

model, but also in the regional and climate model.

3

In order to clarify the development process of theconvective mixing layer, we will perform a ﬁeld observation in LAPS-CREST project. We will be able toobserve the vertical proﬁle and its time series of windﬁeld in the convective mixing layer by utilizing thewind-proﬁler radar. We will be able to observe thoseof temperature and mixing ratio of water vapor ﬁeld
by utilizing the microwave radiometer, too. Additionally, we have a possibility to observe the wind ﬁeld in
the turbulent plumes by utilizing the wind-proﬁler
radar. One of the purposes of the LAPS-CREST
project is to clarify the development process of theconvective mixing layer by comparing the successive
observation results by utilizing the wind-proﬁler radarand microwave radiometer with the numerical simulation results by utilizing the large-eddy-simulation(LES) and/or cloud-resolving model with ﬁne grid
resolution.

In order to study an eﬀect of humidity in the middle
troposphere for the development of deep convective
clouds, we perform sensitivity tests. Figure 5 showstime series of cloud top/bottom height and cloud areaat the height of 8 km, when relative humidity (RH) in
the middle troposphere is changed. In the control run,relative humidity in the middle troposphere is about
80% from the sounding data, deep convective clouds
are generated (Fig. 5(b) ). When the environment in
the middle troposphere is humid (RH = 90%), deep
convective clouds develop and stratiform cloud area inthe upper troposphere is widely extended (Fig. 5(a)). On the other hand, when the environment in the
middle troposphere is relatively dry (RH = 70% and
60 %), deep convective clouds and stratiform region
do not develop (Fig. 5(c) and (d) ). Therefore, theTime (BST)The humidity in the middle
troposphereEvaporation Coolingand Buoyancy Lost

Dry

Moist

vective clouds, we will perform a ﬁeld observation in
LAPS-CREST project by utilizing the microwave radiometer. We will be able to observe the vertical proﬁle and its time series of temperature and mixing ratio of water vapor ﬁeld by utilizing the microwave
radiometer. By analyzing the time series of relative
humidity in the middle troposphere and the generation of deep convective cloud, we can clarify the relationship of them.

4
Moistening
in the Middle
Troposhere

Develop to
Deep Convection

Shallow Convection
Disappear

Fig. 6 Schematic illustration of the relation between the development of deep convective cloud
and the humidity in the middle troposphere.

moist environment in the middle troposphere is suitable condition for the development of deep convective
clouds.
Figure 6 shows a schematic illustration of a relation between the development of a deep convective
cloud and the humidity in the middle troposphere.
When the environment in the middle troposphere ishumid, negative buoyancy by evaporation cooling isrelatively small; therefore, a shallow convective cloud
generated around the top of convective mixing layer
does not lose its positive buoyancy and develops intoa deep convective cloud. On the other hand, whenthe environment in the middle troposphere is relatively dry, negative buoyancy by evaporation coolingis large; therefore, a shallow convective cloud cannot
maintain its positive buoyancy and cannot develop
into a deep convective cloud. But the environment in
the middle troposphere is moistened by evaporationof the shallow convective cloud, when it disappears.
This convective activity generates a moist environment in the middle troposphere by the transport ofwater vapor from the lower troposphere.
In order to clarify the eﬀect of humidity in the
middle troposphere on the development of deep con-Concluding Remarks

Shinoda (2001) supposed three eﬀective factors to develop deep convective clouds over the southern region far from the Meiyu front (wet region) during
the summer monsoon season. First is the latent heatﬂux from the lower surface, especially from the paddy
ﬁelds which widely distributed around the Yangtze
and Huaihe River Valleys. Second is the development
process of the convective mixing layer and third is thehumidity in the middle troposphere. In the LAPSCREST project, we will perform a ﬁeld observation
by utilizing the wind-proﬁler radar and the microwave
radiometer. These instruments can successively observe the vertical proﬁles of wind, temperature andwater vapor ﬁelds in the lower layer (boundary layer)
and in the middle troposphere. We expect to estimate the eﬀective factors in the development of theconvective mixing layer and the generation of shallow
convective clouds quantitatively by comparing the observation results and the numerical simulation resultswith the ﬁne grid resolution. Finally, we want to produce the boundary layer parameterization including
the eﬀect of the turbulent plumes and shallow convective clouds on the regional and climate model.

References
Kato, K., J. Matsumoto and H. Iwasaki, 1995: Diurnal variation of c b -clusters over China and its
relation to large-scale conditions in the summerof 1979. J. Meteor. Soc. Japan, 73, 1219–1234.
Shinoda, T., 2001: Studies on generation of deep convection over eastern China during the summer
monsoon. Doctor paper of Dep. Geophysics, Faculty of Sci., Hokkaido Univ., Japan, 124pp.
Warner, J. and J. W. Telford, 1967: Convection below
cloud base. J. Atmos. Sci., 24, 374–382.