Radiative Fluxes over Rugged Terrain from Satellite Measurements Tianxing Wang txwangrsgmailcom Guangjian Yan gjyanbnueducn Xihan Mu muxihan163com Ling Chen ID: 393244
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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 (0.3~3m) and longwave (3~50m) 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!