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Clear sky Net Surface - PowerPoint Presentation

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Clear sky Net Surface - PPT Presentation

Radiative Fluxes over Rugged Terrain from Satellite Measurements Tianxing Wang txwangrsgmailcom Guangjian Yan gjyanbnueducn Xihan Mu muxihan163com Ling Chen ID: 393244

terrain fluxes surface net fluxes terrain net surface radiative topographic models ann radiation flux rugged shading model effect sky

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Slide1

Clear sky Net Surface Radiative Fluxes over Rugged Terrain from Satellite Measurements

Tianxing Wang (txwang.rs@gmail.com)Guangjian Yan (gjyan@bnu.edu.cn)Xihan Mu (muxihan@163.com)Ling Chen (chenling8247@126.com)Beijing Normal UniversitySlide2

Outline of the presentationBackgroundMethodsResults and discussionSummarySlide3

BackgroundNet surface radiative fluxes, including both net shortwave (0.3~3m) and longwave (3~50m) radiative fluxes, are the driving force for the surface energy balance at the interface between the surface and the atmosphereNet radiative fluxes are key input parameters for most land models, such as GCM, hydrology

and energy models, etc.http://serc.carleton.edu/eslabs/index.htmlThus, currently estimation of net surface fluxes is one of the hottest research issues in the field of global climate change.Slide4

LimitationsMost work focuses on the derivation of downwelling SW and upwelling LW radiation, the effective methods for directly estimating land surface net radiation are highly needed, so that the error propagation can be avoided

Almost all current researches ignore the topographic effect over the rugged terrain areas which account for about 2/3 of the global landTo date, the available radiative fluxes products (e.g., ISCCP, GEWEX,CERES) from remotely sensed data are spatially too coarse

to meet the requirements of land applicationsSlide5

MethodologiesFluxes over horizontal surfaces using Artificial Neuron Network

Short wave topographic radiation modelFluxes over rugged terrainMODIS L1B/MOD03/MOD07/ MOD35/MOD11/MOD04

Fluxes over horizontal surfaces (SW & LW)

Topographic modeling

Fluxes over rugged terrain (SW & LW)

Longwave

topographic radiation model

Correcting

t

errain shadingSlide6

Reasons for using ANNThe ANN model can accept more input variables and output more desired quantitiesIt has been attempted by many researches in the field of radiative flux budget proving its feasibility in such topicIt’s convenient to couple the multi-output of ANN with the topographic model for retrieving fluxes over rugged areaSlide7

Topology and training over 50,000 samples simulated using MODTRAN4 single-hidden layer feed-forward network BP training algorithmInputs

of ANNOutputs of ANN

Altitude

Downward

radiative

flux

Direct solar flux

Net SW

radiative

flux

Solar

zenith angle

Viewing

zenith angle

Aerosol optical depth

Water vapor

index

MODIS

radiances of

band

1~7

Moisture profile

Shortwave

ANN model

Longwave

ANN model

Inputs

of

ANN

Outputs

of

ANN

Altitude

LW downward

radiative

flux

Surface

emitted

radiative

flux

Net

LW

radiative

flux

Viewing zenith

angle

3

water vapor

indices

MODIS

radiances of band

20

,

22

,

23

,

27~29

and

31~33

Temperature profiles

Moisture profilesSlide8

Validation of the ANN modelsTwo years of 2008~2009 in situ data are collected as reference from seven U.S. SurfRad sites under clear skyValidation results of the ANN models

SurfRad sitesFrom :http://www.srrb.noaa.gov/surfrad/The maximum root mean square errors (RMSE) of ANN models are less than 45W/m2 (watts per square meter) and 25W/m2 for net SW and LW radiative fluxes, with the average biases are less than 15 W/m2 and 5W/m2, respectively. These accuracies are better than the existing algorithms showing the effectiveness of the ANN models.Slide9

Topographic radiation modelsTerrain shading (a), resulting in :Flux contribution from the around sloped terrain (b)Sky-view ratio (c)

Key factors affecting radiative fluxes over rugged terrain zero solar direct radiation lower LST due to shadowsDubayah, R. and S. Loechel (1997)

Shadow

Terrain

means the overlaying sphere may be obstructed by terrain, in this situation the sky-view-ratio is less than 1Slide10

Topographic radiation models (SW)

Solar direct flux:Sky diffused flux:Reflected from around terrain:Net flux over rugged terrain:Slide11

Topographic radiation models (LW)Surface emitted flux:Sky emitted flux:Emitted from around terrain:

Net flux over rugged terrain:

Similarly, by considering the three factors, a LW topographic radiation model is also suggested

This model is more complex compared to the SW model, since it relate to LST in shadow area and the broadband emissivity etc.Slide12

Correction for terrain shadingSeven shading situationsIt should be noted that, all inputs in the SW and LW topographic radiation models are the unobstructed fluxes, thus, the terrain shading of the outputs of ANN models need to be removed before incorporated in these models. This figure shows the seven shading situations,including A B C DSlide13

Correction for terrain shading

Correction for terrain shading in Shortwave band:

Correction for terrain shading in

Longwave

band:

These are the correcting formulas for those seven situations, the details of these variables can be found in the paper. Slide14

Results and discussion MODIS data collected on November 4, 2009 over Tibet Plateau, a typical region for terrain undulation and climate change research, are selected as our case studyNet SW surface radiative

fluxes for considering (left) and neglecting (right) the topographic effect the terrain texture of the topographically corrected map is rather obvious the variation of the fluxes due to the terrain undulation is much wider than that of fluxes neglecting the topographic effectfluxes estimated by assuming a horizontal surface are difficult to reflect the true status of surface radiation budget in terms of both spatial distribution and specific flux valuesSlide15

Results and discussionNet LW surface radiative fluxes for considering (left) and neglecting (right) the topographic effect although the terrain texture of the topographically corrected map is not as obvious as that of SW , they can also be visually felled

the variation of the fluxes due to the terrain undulation is much wider than that of fluxes neglecting the topographic effectThe white spots in the left image correspond to the shadowed area, where the LST is low, thus the net LW fluxes are relatively highSlide16

Total net surface radiative fluxes for considering (left) and neglecting (right) the topographic effectResults and discussion Because the net SW fluxes poses a larger magnitude of total fluxes, thus the spatial distribution of the total net fluxes are highly in line with the distribution of net SW fluxes it will attribute great errors to the estimated fluxes if the terrain undulation effect is not taken into account over rugged terrain area. The errors can reach up to 100 W/m

2, and even larger for SW fluxes.Slide17

SummaryTwo ANN models have been developed in this paper, with which the net surface SW and LW radiative fluxes over horizontal surface can be directly retrieved with better accuracyCoupling the outputs of ANN models, the corresponding SW and LW topographic radiation models are also proposedThe results show that the ANN models suggested here are rather effective, and the topographic effect on the net surface fluxes is so significant that the assumption of horizontal surface is not applicable over rugged terrainSlide18

Thanks for your attention!