Differences of Atmospheric Boundary Layer

Characteristics between pre-monsoon and

monsoon period over the Erhai Lake

Xu Lujun, Liu Huizhi, Du Qun, Wang Lei, Liu Yang, Sun Jihua

State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC)

Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences

The 11th edition of the International Symposium on Tropospheric Profiling

Outline

Motivation

The WRF_LAKE model calibration

Subgrid-scale orography parameterization

Differences of the PBL Characteristics in different seasons

Conclusion

Motivation

Lake-air interactions have significant impacts on local climate, acting as

indicators of climate changes（Adrian et al.,2009）

The global extent of natural lakes(<1 km2) is twice as large as previously

known（Downing et al.,2006）

34 million lakes; 4.2 million km2 in area

(Ma et al.,2006)

(Gerken et al.,2013)

(Feng et al.,2015)

In China: 2693 lakes; 81414.6 km2 in area

Motivation

Cangshan

Mountain Erhai

Lake

Study area

The WRF_LAKE model calibration

Lake surface temperature of initial simulation (red line) and observation (black line)

The simulated lake surface temperature was lower than the observation during the day.

It was partly due to the surface layer absorbed less radiation to heat surface water and

transferred more radiation into deeper water.

At night, the simulated lake surface temperature was higher than observations. The

turbulent mixing is strong for Erhai Lake during night, leading to disappearance of the

thermal stratification.

The WRF_LAKE model calibration default Dry season

2012.12.05-2012.12.15

Wet season

2012.8.8-2012.8.19

Absorption

coefficient (β)

β=0.4 η=1.1 (Zhang et al.,2012) η=2.4 (Zhang et al.,2012)

Extinction

coefficient (η)

η=0.45

β=1-e-zη (Deng et al.,2012)

β=1-e-zη (Deng et al.,2012)

Eddy diffusion

coefficient (ke) ke 0.05ke(Gu et al.,2013) 0.02ke

Surface roughness

(Z0)

0.001

𝐳𝟎𝐦 = 𝐦𝐚𝐱(𝛂𝛎

𝐮∗,𝐜𝐮∗

𝟐

𝐠)

𝐳𝟎𝐡 = 𝐳𝟎𝐦𝐞𝐱𝐩{−𝛋

𝐏𝐫(𝟒 𝐑𝟎 − 𝟑. 𝟐)}

𝐳𝟎𝐪 = 𝐳𝟎𝐦𝐞𝐱𝐩{−𝛋

𝐒𝐜(𝟒 𝐑𝟎 − 𝟒. 𝟐)}

𝐑𝟎 = 𝐦𝐚𝐱(𝟎. 𝟏,𝐳𝟎𝐦𝐮∗

𝛎)

(Subin et al.,2012)

β&η ke z0

Case a

Case b √

Case c √

Case d √ √

Case 1 √

Case 2 √ √

Case 3 √ √

Case 4 √ √ √

Lake surface temperature β&η ke z0

Case a

Case b √

Case c √

Case d √ √

Case 1 √

Case 2 √ √

Case 3 √ √

Case 4 √ √ √

Dry season

Wet season

β&η 0.4(0.45)→0.6(1.1) Ts↑0.5℃

Z0 default → parameterization Ts↑2.3℃

ke → 0.05ke Ts diurnal range ↑ 4.1℃

β&η 0.4(0.45)→0.76(2.4) Ts↑0.8℃

Z0 default → parameterization Ts↑1.5℃

ke → 0.05ke Ts diurnal range ↑5.9 ℃

2.3℃

Sensible heat flux β&η ke z0

Case a

Case b √

Case c √

Case d √ √

Case 1 √

Case 2 √ √

Case 3 √ √

Case 4 √ √ √

Without calibration of z0,

the simulated lake surface

temperature was colder than the

air, resulting in negative sensible

heat flux

Dry season

Wet season

Latent heat flux β&η ke z0

Case a

Case b √

Case c √

Case d √ √

Case 1 √

Case 2 √ √

Case 3 √ √

Case 4 √ √ √

The latent heat flux is too large

1. strong mechanical mixing with

default z0

2. large wind speed over

complex terrain

Dry season

Wet season

Validation of lake-atmosphere interaction processes

Changes in the lake surface temperature followed and lagged behind its

upper atmosphere due to large water thermal capacity.

24.5℃

9.2℃

Average: 16.5℃

Average: 17℃

1.6℃ 29.2℃

Lake surface temperature

Air temperature

Monthly averaged diurnal cycle of lake surface temperature and air temperature

observation (black) simulation (red)

Validation of lake-atmosphere interaction processes

Monthly averaged diurnal cycle of energy fluxes

observation (black) simulation (red)

Sensible heat flux

Latent heat flux

Lake storage heat flux

Net radiation

Wet season Dry season Dry season

600W m-2

794W m-2

𝜕𝑢

𝜕𝑡= ⋯ − 𝑐𝑡

𝑢∗2

∆𝑧

𝑢

𝑉

𝑐𝑡 =

1 𝑖𝑓 ∆2ℎ > −20 𝑎𝑛𝑑 𝜎𝑠𝑠𝑜 < 𝑒

𝑙𝑛𝜎𝑠𝑠𝑜 𝑖𝑓 ∆2ℎ > −10 𝑎𝑛𝑑 𝜎𝑠𝑠𝑜 > 𝑒

𝛼𝑙𝑛𝜎𝑠𝑠𝑜 + 1 − 𝛼 𝑖𝑓 −10 > ∆2 ℎ > −20 𝜎𝑠𝑠𝑜 > 𝑒

∆2ℎ + 30

10 𝑖𝑓 −20 > ∆2 ℎ > −30

0 𝑖𝑓 − 30 > ∆2ℎ

Subgrid-scale orography parameterization In order to include the effects of real orography, Jimenez and Dudhia ( 2012) introduced

a sink term into the equation of conservation of momentum

test subgrid-scale orography

parameterization

NOTOPO ×

TOPO √

Subgrid-scale orography parameterization

Observation NOTOPO TOPO

Mean bias

0.8 m/s →0.3 m/s

east-southeast wind southeast wind east-southeast wind

Differences of the PBL Characteristics in different seasons

The Erhai Lake is located in the subtropical monsoon climate zone. It has two distinct

periods all the year around, which are warm-wet season (May to October) and cold-dry

season (November to April of the next year).

Dry season Wet season

time 2012.03.31~

2012.4.30

2012.6.30~

2012.7.31

resolusion 27 km 9 km 3 km 1 km

Boundary

condition NCEP （0.5°×0.5°）reanalysis data

landuse MODIS

Test name Landuse

LAKE lake

NOLAKE crop

Temperature difference between lake and land

-19.6℃

-2.5℃ 2.4℃

4℃

Tlake-Tland before monsoon onset

Weather condition:

April There was no synoptic process during the selected pre-monsoon period.

July During monsoon period, the northwest Pacific Subtropical High (STH) controls

most of synoptic processes in Erhai area. The selected monsoon period in wet season

covered a westward extension process of the STH, bringing precipitation at Erhai area.

-9.5℃

-1.7℃

1.5℃

2.7℃

Tlake-Tland after monsoon onset

T2 during pre-monsoon period

14:00 02:00 Lake test

No lake test

Thermal

contrast induced

by the Erhai

Lake is negative

during daytime

and positive

during night.

Lake effect on

regional

temperature is

related to lake

depth.

T2 during monsoon period Cooling effect

and warming

effect of the

Erhai Lake is

weaker than

that during pre-

monsoon

period. 02:00

Lake test

No lake test

14:00

The isotherm

trend in the

region

overlapped

with the terrain

height contour.

Horizontal wind at 10 m during pre-monsoon period

East

wind

The lake

adjusts local

thermal

difference

and

circulation

intensity.

02:00 Lake test

No lake test

14:00

Southeast

wind

cyclonic circulation

divergence area

02:00 Lake test

No lake test

14:00

Temperature

difference

between lake

and land

became

smaller than

that during

pre-monsoon

period,

resulting in

weaker lake

breeze and

land breeze

circulations.

Horizontal wind at 10 m during monsoon period

Cross section for zonal wind(25.7°N)

14:00 02:00

LAKE West wind

700m

The superposition of valley breeze and lake breeze causes unstable convection, leading to

diverse local weather (Lv et al.，2008)

The strength of wind was correlated to temperature difference between lake and land and

topographic variations around the lake (Bennington et al. 2010).

400m

500m 200m

Thermal contrast↓

mountain wind ↓

Circulation height↓

NOLAKE

pre-monsoon

East wind

600m

300m

200m

Cross section for zonal wind(25.7°N)

14:00 02:00

LAKE

West wind

NOLAKE

monsoon

Influenced by shielding effect of the Cangshan Mountain, lake breeze circulation

was established on the west shore. While on the east shore, it was not.

The height of convective boundary layer ranged from 1900m to 2300m, which was higher

than that in the plain area (1000m～1600m) (Wang et al. 2016)

At night, the lake made the stable boundary layer get warmer on the order of 1 K, and its

height get higher on the order of 400 m.

1900m

1400m

1 K↓

Cross section for potential temperature(25.7°N)

14:00 02:00

LAKE

NOLAKE

pre-monsoon

1100m

The Erhai Lake increased temperature difference in the boundary layer, leading to a

300 m lower boundary layer during daytime and a 200 m higher boundary layer during

nighttime.

800m

1300m

400m

200m

Cross section for potential temperature(25.7°N)

14:00 02:00

LAKE

NOLAKE

monsoon

1300m

The lake increased specific humidity during daytime,

while at night, the lake decreased specific humidity

Cross section for relative humidity(25.7°N)

14:00 02:00

LAKE

NOLAKE

pre-monsoon

Specific humidity in the boundary layer: 6→13.2 g kg-1

Moisture contrast between lake and land could run up to 1400 m, reaching as large as 2 g kg-1. At night, vertical distribution of moisture field was mainly affected by advection.

Cross section for relative humidity(25.7°N)

14:00 02:00

LAKE

NOLAKE

monsoon

Conclusion

The WRF_LAKE model need to be calibrated:

The default lake model shows negative bias of lake surface

temperature. By the combination of the added absorption and

extinction coefficients, the parameterization of surface roughness

length and the reduced eddy diffusion coefficient, the model

reproduces the correct diurnal cycle of lake surface temperature.

Topographic correction over complex terrain help to correct positive

wind bias and confine overmuch latent heat exchange.

Conclusion

The Erhai Lake has great impact on local

circulation and boundary layer:

The Erhai Lake enlarges thermal contrast between valley and mountain

slope at the Dali basin. The lake reduces air temperature by 2~3 ℃

during daytime, and increases air temperature by nearly 2 ℃ in the

evening.

Due to its small roughness and large thermal capacity, the Erhai Lake

enlarges local wind speed. A cyclonic circulation is maintained by the

combination of mountain breeze and land breeze in the south of the

lake.

The lake decreases air temperature, increases specific humidity and

reduces boundary layer height during daytime, while at night, the lake

increases air temperature, decreases specific humidity and increases

boundary layer height.

Conclusion

Local circulation and boundary layer structure are

influenced by monsoon:

After monsoon onset, temperature difference between land

and lake becomes smaller, resulting in a weaker local

circulation. The height of circulation reduces by 500 m.

Specific humidity of the boundary layer increases by 8.8 g

kg-1 after monsoon onset.

Thank you for your attention