Effect of albedo dependence Solar Zenith Angle (SZA) on energy budget modelling over Desert and Gobi Surfac

e 考虑太阳天顶角的地表反照率参数化对沙漠、

戈壁地表能量平衡的模拟效应

BaoYan

byan@lzb.ac.cn

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05

101520253035

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2003.JJA

Possible Factor: Feedback of too much modelling rainfall? － convection parameterization-Better rainfall simulation has a

little impact on air temperature

Giorgi,F(1999), J. G. R.RegCM2&CCM3—negative bias of air temperature

Modify surface energy balance modelling to improve performance of surface temperature

Large area Desert&Gobi in NW-China, Search a good albedo parameterization to improve …offline LSM…

Research Status 1:

• Most land models assume that the bare soil albedo is a function of soil texture (color) and moisture, but independent of solar zenith angle (SZA), which means that surface albedo doesn’t change with geographical location and time (Sahara and Australian desert).

Research Status 2:

• Remote sensing data from the satallite and aircraft platforms as well as field measurements have shown the anisotropy of bare soil surface(Kimes,1983);

• Satellite data have convincingly shown the significant geographic variation of desert (Tsvetsinskaya,

2002,Wang,2004);• The albedo obtained for the bare soil also increases fo

r sun angles away from solar noon (Ranson 1991)

Albedo parameterizations dependence on SZA

• Paltridge(1981) derived the relation between surface albedo and SZA.

• Wu(1993) developed a simple formulation using HEIFE observations.

• Zhang et al.(2002) parameterized the relation between albedo and SZA using data in 2001 of DHEX.

• Wang et al.(2005), bare soil albedo normalized by its value at 60°SZA respectively with a one-parameter and a two-parameters formulations based on Modis/ BRDF and albedo data over thirty desert location.

Study purpose

• Evaluate these formulations in offline LSM to improve the performance of model in surface energy and heat budget;

• Apply to regional modelling in arid area to improApply to regional modelling in arid area to improve simulation of surface heat condition ve simulation of surface heat condition (Omitte(Omitted)d)

Bare soil surface albedo parameterization

• BATS(Dickinsion1993): surface albedo is function of soil texture and soil wetness moisture ,Only direct solar radiation were considered in energy balance. This scheme was widely used in LSMs (LSM, CLM)

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Data and Methodology

JTEXSOLAR,UR,DLR,ULR(1.5 m) Heat flux (3.2 m)ground heat flux(5cm,20cm)wind, air temperature, air densityspecific humility , atmospheric pressure(10 m). -10min Fig.1 Location information

• a. Adding a program to calculate the real-time SZA

• b.Substituding with the measured albedo at 60° SZA dark sand (sand

95.6%)39 °57.930′N, 98° 51.258′E

Desert

sand and gravel (sand 93.4%)

39° 58.535′N, 98° 51.518′EGobi

Soil textureLocationSite

Tab.1 Location information

r

r

Four EXP.:BATS(CTL)/BRDF/B/ZQDesert:Jun.19th –Jul.8th,2005Gobi: Jun.19th –Jun.28th,2005

Fig.2 Atmospheric forcing data

BATS :almost a constant , Gobi 0.29 Desert 0.27> Observations and values in similar latitude belts;B&BRDF: Better simulation in diural variation with value of 0.15 to 0.3;Larger at noontime and smaller in the early morning and late afternoon. BRDF is more sensitive to the larger SZA. ZQ: Slightly diural change.

Good agreement between modelling and observation.Higher at noon when the model failed to capture the smaller albedo (the invalid data of reflected radiation at very higher SZA in early morning had been deleted). RMSE:70-75W.m-2 (30%) in BATS and ZQ, while 8.4W.m-2 and 8.6 W.m-2 ,or 3.2% and 3.6 ％ relative error in BRDF and B at noon.

Field Exp. Station Site Location Altitude(m) Surface Type Albedo

HEIFE

Huayin39°09'N， 10

0°05'E 1454 Gobi 0.228

39°26'N， 100°12'E 1378 Desert 0.246

DHEX Dunhuang40°10'N， 94

°31'E 1150 Gobi 0.255

JTEX Jinta

39°58'N， 98°52'E 3500 Gobi 0.23

39°58'N， 98°51'E 3500 Desert 0.185

Tab.2 Albedo of typical Gobi and Desert surface.

With a correlation coefficient of R2≥0.98 with slope ＝ 1.5BATS &ZQ with a small SZA far overestimate the reflected radiation(>=100W.m-2) ZQ is not sensitive to smaller SZA. BRDF &B :slightly overestimate.

Fig.4 Overall regression relationship between the modelling and measured reflected radiation over Gobi and Desert in first 10 days ( Jun.19th-Jun.28)

(a)BATS(b)B(c)BRDF(d)ZQ

Fig.5 Difference between CTL or Observations and modelling in first ten days. Panels on the left are the difference of CTL.

and panels on the right are difference of observations.

B:67.16;

BRDF:67.97

ZQ:3.75

BATS:-67.75

B:-0.59

BRDF:0.22

ZQ:-64

BATS:-74.52

B:-33.76

BRDF:-33.56

ZQ:-72.19

B:40.76;

BRDF:40.95

ZQ:2.33

B:4.56

BRDF:4.58

ZQ:0.29

BATS:-4.93

B:-0.37

BRDF:-0.35

ZQ:-4.67

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Ground heat flux

Tab.3 Bias of surface heat conditions at noon

BiasAbsorbed

radiation(W.m-2)Sensitive heat flux(W.m-2)

Ground temperature(K)

Observation BATS Observation BATS Observation BATS

BATS -67.75 　 -74.52 　 -4.93 　B -0.59 67.16 -33.76 40.76 -0.37 4.56

BRDF 0.22 67.97 -33.56 40.95 -0.35 4.58

ZQ -64 3.75 -72.19 2.33 -4.67 0.26

Conclusions: Several SZA dependence formulations were incorporated into the BATS land surf

ace model to evaluate and improve the performance of surface heat condition of model. The major findings of the study are :

a. SZA is a necessary factor when calculate bare soil surface albeo, and the scheme dependence of SZA in surface albedo and reflected radiation modelling are better than schemes independence of SZA although with the higher albedo at noon and lower albedo in the early morning and late afternoon. Scheme of BRDFand B have better modelling than scheme of ZQ, for the former two are more sensitive to variation of SZA;

• The higher albedo of the four schemes at noon cause the lower absorbed radiation, the smaller sensitive heat flux and the lower ground temperature in modelling. Compared with original BATS, BRDF,B and ZQ improve the bias in different degree with 67.16 W.m-2 and 67.59W.m-2 and 3.75W.m-2 in absorbed radiation, and 40.76W.m-2 and 40.95W.m-2 and 2.33 W.m-2 increase in absorbed radiation. About 62％ of the extra solar energy goes into an increase in sensible heat while another 15 ％ is transfort into soil. The remaining energy is primarily used to increase the ground temperature, and the increased ground temperature of BRDF,B,ZQ is respectively 4.56,4.58,0.26;

• Compare to observation, the great negative bias between BATS modelling and the measured decreased to -0.59 W.m-2 of B and 0.22 W.m-2 of BRDF, and scheme of ZQ slightly decrease the bias. Bias of sensitive heat flux decreased 50% compared with BATS and ZQ scheme, which cause the negative bias of ground temperature to -0.37 of B and -0.35 of BRDF at noon.

Discussions

• Better modelling of albedo in clear day than in cloudy day;

• In dry bare soil , importance of soil wetness versus SZA to surface albedo

THQNK

YOU