MODEL CONVECTIVE AND STRATIFORM PRECIPITATION PARTITION DEPENDENCE

ON HORIZONTAL RESOLUTION

Gomes, Jorge Luís 1, Chou, Sin Chan 1

1. Centro de Previsão de Tempo e Estudos Climáticos – Instituto Nacional de Pesquisas

Espaciais - Cachoeira Paulista - Brazil

gomes@cptec.inpe.br, chou@cptec.inpe.br

ABSTRACT

Model precipitation can be produced

explicitly or through convective

parameterization schemes. Different types

of precipitation produce distinct vertical

profiles of latent heat to the atmosphere,

however, to estimate the effect of these

different profiles of heating it is important

to know accurately the partition between

convective and stratiform precipitation.

Models with resolution coarser than 20 km

are able to reproduce the cumulus

convection with some skill through

parameterization schemes. On the other

hand, models with grid-size resolution

smaller than 1 km should solve the

convection explicitly. Within the range of

these two resolutions hybrid solutions are

suggested, with cumulus convection

acting together with the explicit form of

representation. In the present work, the

Eta model was used to simulate a

precipitation event associated with the

South Atlantic Convergence Zone (SACZ).

This type of system exhibits a large band

of stratiform cloudiness with embedded

convective cells. The Eta Model uses the

Kain-Fritsch cumulus parameterization

scheme. Cloud microphysics was treated

by Ferrier scheme. The convective

scheme has a parameter that controls the

fraction of condensate that goes into rain

and snow in each layer. A change is

proposed to the parameter to include

resolution dependence. The convective

scheme converts less condensed water

into precipitation, part of the condensed

water is made available to cloud

microphysics scheme and another part

evaporates. In grid resolution higher than

1 km, convective scheme still acts in

removing convective instability and all

precipitation is produced by cloud

microphysics. Simulations with different

horizontal resolutions, 20, 10 and 5 km,

have been carried out up to 5-day

integrations which is the average duration

of SACZ events. The changes produced

two major impacts: the position of

maximum precipitation area was better

simulated and the amount of total

precipitation became closer to the

observations. The simulations showed that

the increase of horizontal resolution

changed the distribution of stratiform and

convective precipitation.

1. INTRODUCTION.

The precipitation of the NWP models may

be generated implicit by though convective

parameterization scheme and explicitly

though excess of water vapor over a

prescribed threshold. Convective

parameterization schemes work in a

model grid where convective instability is

found. The cloud microphysics schemes,

such as Lin et al. (1983), Zhao (1997),

Eta-Ferrier (2002), etc. produce

precipitation explicitly. According with the

model resolution it is expected greater or

lesser activity of each of these two

schemes. According Molinare (1993)

models with resolution greater than 20 km

the precipitation would be simulated by

implicit convective schemes with

reasonable skill. Models with resolutions

lesser than 3 km, where the model grid

approaches the scale of cloud

development, the precipitation would be

simulated by explicit schemes. Adequate

representations of implicit and explicit

precipitation partition by NWP are

essential to get a better representation of

precipitation. With increasing the

resolution the explicit scheme becomes

more important. In this case, the clouds

processes become more sensitive with

respect to the model grid scale. In this

paper the objective is to evaluate the

precipitation production and partition at

different horizontal resolution for a case of

SACZ.

2. THE ETA MODEL.

The Eta Model was first developed in

Belgrade University by Mesinger (1988)

and a comprehensive physical package

has been incorporated into the model by

Janjic (1990, 1994). It is a hydrostatic/no

hydrostatic model with an accurate

treatment of complex topography using

eta vertical coordinate system (Mesinger,

1984). The model topography is

represented as discrete steps whose tops

coincide exactly with one of the model

vertical layer interfaces (BLACK, 1994).

The model uses a semistaggered

Arakawa E grid as a horizontal grid. The

radiation package used in the model is

one developed at GFDL. Planetary

boundary layer (PBL) uses a modified

Mellor-Yamada Level 2.5 scheme (Black

1994). The prognostics variables of the

model are: temperature, specific humidity,

zonal and meridional components of the

wind, surface pressure, turbulent kinetic

energy and cloud hidrometeores. The

explicit precipitation is generated by cloud

microphysics Eta-Ferrier scheme Ferrier

(2002), hereafter referred to FR, and

implicit precipitation generated by Kain-

Fritsch cumulus parameterization scheme

(Kain and Fritsch, 1990 and 1993; Kain,

2004), hereafter referred as KF. Further

details of the model can be found at

Mesinger et al. 1988 and Black, 1994.

2.1. PRECIPITATION SCHEMES

In this section a brief review of the

precipitation schemes used in this work

are show and a new modification in Kain-

Fritsh scheme is proposed.

- The Eta-Ferrier scheme (FR). The

scheme predicts changes in water

vapor and condensate in the forms

of cloud water, rain, cloud ice, and

precipitation ice

(snow/graupel/sleet). The

individual hydrometeors are

combined into total condensate.

The water vapor and total

condensate that are advected in

the model.

- Kain-Fritsch scheme (KF): uses a

Lagrangian Parcel method along

with vertical momentum dynamics

to estimate the properties of

cumulus convection, It

incorporates a trigger function, a

mass flux formulation and closure

assumption. The trigger function

identifies the potential updraft

source layers associated with

convection, whereas the mass flux

formulation calculates the updraft,

downdraft and associated

environmental mass flux. The

scheme assumes conservation of

mass, thermal energy, total

moisture and momentum. The

efficiency Productions of rain and

snow are controlled by a

parameter that specifies the

fraction of the precipitation mass to

be transferred from KF to the host

model.

3. METHODOLOGY

The Eta model was configured with 20, 10

and 5 km of horizontal resolutions. In the

first and second cases the vertical

resolution used was 38 layers and the

model run in a hydrostatic mode. The time

steps were 40 and 20 seconds

respectively. With 5 km resolution the

model was run in non-hydrostatic mode 50

layers, with time step set to 10 seconds.

The domain was centered at -23.5S, -

48.0W. The domain of the 3 resolution

was the same to avoid any possible

differences in the simulations. Initial and

lateral boundary conditions were taken

from NCEP (National Centers for

Environmental Prediction) global analyses

with T126L28 resolution. The lateral

conditions were updated every 6 hours.

The domains for all experiments are

shown in Figure 2. The model was

integrated up to 132 hours. The resolution

dependence was introduced into the

scheme through the F parameter. This

parameter varies between 0 and 1. When

the model resolution is 3 km or higher the

parameter is set to 1; between 3 and 40

km the parameter follows a function given

by Figure 1; and when the model grid size

is 40km or smaller, the parameter is set to

0. The F parameter included the horizontal

resolutions dependence into the

precipitation efficiency and the convective

adjustment through temperature and

moisture profiles. In the 20 km experiment

where F parameter was set to 0.4

cloud water is converted to

case, the convective activity was reduced

by 40%. For the resolutions

the convective efficiency and th

convective activity were reduced by 6

and 80%.

Figura 1: F parameter (non dimensional).

20 km

a)

10 km

b) Figure 2: Topography and domains of the

experiments for: a) 20 km; b) 10 km and c)

5 km

3.1. CASE ANALYSIS

The case chosen for this study was the

SACZ event from January 24

The JAN24 cloud band

meridional orientation and

rainfall over southeast Brazil, mainly in the

southern part of São Paulo State, where

the maximum of the precipitation

200-300 mm was observed (Figure 3

resolutions dependence into the

cy and the convective

through temperature and

n the 20 km experiment

set to 0.4, 60% of

cloud water is converted to rain. In this

he convective activity was reduced

10 and 5km

the convective efficiency and the

reduced by 60%

non dimensional).

5 km

c)

Topography and domains of the

experiments for: a) 20 km; b) 10 km and c)

The case chosen for this study was the

event from January 24-29, 2004.

exhibited a

and significant

all over southeast Brazil, mainly in the

southern part of São Paulo State, where

of the precipitation between

was observed (Figure 3 a

and b). The Figure 2b shows the

PERSIANN (Precipitation Estimation from

Remotely Sensed Informatio

Artificial Neural Networks) accumulated

precipitation for JAN24 period. The

precipitation band exhibited

orientation, with axis that

Triângulo Mineiro through the extreme

south of the São Paulo State and followed

by the Atlantic Ocean. The sequence of IR

images from GOES-8 shows that the

system was acting over the region

between January 24

streamlines at high levels showed a

cyclonic vortex positioned closer

coastline of Northeast o

the SACZ over the southern and

southeast region. The cold

ocean was maintained by

trough.

a)

Figure 3: Observed precipitat

accumulated for SACZ event

observations and b) PERSIANN data

4. RESULTS

4.1. CONTROL RUN

In the 20 km simulated

precipitation was positioned

(Figure 5a) when compared with the

The Figure 2b shows the

PERSIANN (Precipitation Estimation from

Remotely Sensed Information using

Artificial Neural Networks) accumulated

precipitation for JAN24 period. The

exhibited southern

that extends from

Triângulo Mineiro through the extreme

São Paulo State and followed

The sequence of IR

8 shows that the

system was acting over the region

n January 24-29, 2004. The

streamlines at high levels showed a

cyclonic vortex positioned closer to the

coastline of Northeast of Brazil which kept

the SACZ over the southern and

The cold front over the

ocean was maintained by upper level

b)

Observed precipitation

accumulated for SACZ event: a) surface

and b) PERSIANN data

imulated maximum

positioned too south

a) when compared with the

observations (Figure 5a and 5b). In the

C20, the precipitation band was positioned

over Santa Catarina and Paraná States,

whereas the observations showed the

cloud band to the north. In this case, the

maximum accumulated precipitation was

400 mm over Santa Catarina State,

whereas in the observations the amounts

were smaller and the cloud band

positioned to the north, over São Paulo

State. The comparison between the

Figures 5g and 5n shows that most part of

precipitation was generated by the implicit

scheme. The runs with different

resolutions showed the same distribution

of implicit and explicit precipitation

(Figures 5 c, j and p; Figures 5e, l and r).

The differences between C10-C20 and

C05-C20 where showed a little shift in the

precipitation band toward north but a

significant increase in the maximum

precipitation when increased the

horizontal resolution was increased.

However, the increase in the total

precipitation is associated with an

increase of convective scheme (Figures

5g, j and l), although it would be expected

that the explicit scheme acted more

strongly with resolution increase.

4.2. F PARAMETER

The comparison between the 20-km runs,

control and F experiment showed that the

position of the precipitation band is further

north, closer to the observation (Figure 3a

and b). One can note in Figure 5b and h

that there was a reduction in the amount

of precipitation produced by the implicit

scheme. The greater availability of liquid

water for the explicit scheme contributed

to an increase in the amount and area of

precipitation produced by the explicit

scheme. The explicit scheme became

more active and contributed to positioni

the precipitation maximum to the north

(Figures 5n and o). With the increase of

resolution, the liquid water produced by

the convective scheme and the

temperature and humidity tendencies were

reduced by 60% and 80% in the 10-km

and 5-km runs, respectively. Comparison

among the Figures 5b, d and f against the

Figures 5a and b one can note that the

resolution increase improved the

simulated position of precipitation band.

5. CONCLUSIONS

Experiments carried out with the Eta

Model at 20, 10 and 5-km resolutions with

original KF and FR setups ) showed that

the model produced heavy precipitation

and had some position error in the

precipitation band associated with the

SACZ event. The KF scheme produced

the largest contribution to the total

precipitation in the control runs. Despite

the resolution increase, the precipitation

maximum was intensified due to greater

activity of the KF convective scheme.

The inclusion of resolution dependence to

the F parameter caused the reduction of

the convective activity, the reduction in the

precipitation amount generated by th

scheme and an increase of the explicit

precipitation. This new precipitation

partition reduced the overprediction

resolution insensitive of the schemes in

20km

C20 E0.420

a) b)

g) h)

n) o)

Figure 5: 5-day accumulated p

The different resolutions are

second row is the implicit precipitation and the third row is explicit

6. REFERENCES

BLACK, T. L., 1994: The new NMC

mesoscale Eta model: Description ad

forecast examples. Wea. Forecasting

265-278.

FERRIER, B. S., Y. LIN, T. BLACK, E.

ROGERS, AND G. DiMEGO, 2002:

Implementation of a new grid

and precipitation scheme in the NCEP Eta

model. Preprints, 15th Conference on

generated by the

of the explicit

precipitation. This new precipitation

partition reduced the overprediction and

sensitive of the schemes in

the control runs. As the horizontal

resolution increases the position

precipitation band associated with the

SACZ event was also

10km 5km

C10 E0.610 C05

c)

d) e)

j)

k) l)

p)

q) r)

accumulated precipitation for the control and F-parameter

shown in columns. The first row is the total precipitation, the

second row is the implicit precipitation and the third row is explicit

BLACK, T. L., 1994: The new NMC

mesoscale Eta model: Description ad

Wea. Forecasting, 9,

FERRIER, B. S., Y. LIN, T. BLACK, E.

ROGERS, AND G. DiMEGO, 2002:

Implementation of a new grid-scale cloud

in the NCEP Eta

model. Preprints, 15th Conference on

Numerical Weather Prediction, San

Antonio, TX, Amer. Meteor. Soc

KAIN, J. S. 2004: The Kain

convective parameterization: An update. J.

Appl. Meteor., 43, 170-181.

___, and J. M. FRITSCH, 1993:

Convective parameterization for

mesoscale models: The Kain

scheme. The Representation o

Convection in Numerical Models, Meteor.

Monogr., No. 46, Amer. Meteor. Soc., 165

170.

. As the horizontal

lution increases the position of the

on band associated with the

also better positioned.

5km

E0.805

f)

m)

s)

parameter experiment runs.

The first row is the total precipitation, the

second row is the implicit precipitation and the third row is explicit precipitation.

Numerical Weather Prediction, San

Amer. Meteor. Soc., 280-283.

KAIN, J. S. 2004: The Kain-Fritsch

convective parameterization: An update. J.

181.

___, and J. M. FRITSCH, 1993:

Convective parameterization for

mesoscale models: The Kain-Fritsch

scheme. The Representation of Cumulus

Convection in Numerical Models, Meteor.

Monogr., No. 46, Amer. Meteor. Soc., 165-

___, AND ___, 1990: A one-dimensional

entraining/detraining plume model and its

application in convective parameterization.

J. Atmos. Sci., 47, 2784-2802.

MESINGER, F., 1984: A blocking

technique for representation of mountains

in atmospheric models. Riv. Meteor.

Aeronaut., 44, 195-202.

___, JANJIC, Z. I., NICKOVIC, S.,

GAVRILOV, D. AND DEAVEN, D. G.

1988: The step-mountain coordinate:

Model description description and

performance for cases of Alpine lee

cyclogenesis and for a case of

Appalachian redevelopment. Mon. Wea.

Rev., 116, 1493-1518.

MOLINARI, J., 1993: Na overview of

cumulus parameterization in mesoescale

models. The Representation of Cumulus

Convection In Numerical Models. Amer.

Meteor. Soc., Vol. 24, 46, 155-158.

ZHAO, Q., T. BLACK AND M. E. Baldwin,

1997: Cloud prediction scheme in the Eta

model at NCEP. Wea. Forecasting, 12,

697-712.

Acknowlegments: This work is partially

funded by FAPESP under grant no.

04/09649-0 and partially by CNPq under

grant no 308725/2007-7