INSTITUTE FOR ATMOSPHERIC AND CLIMATE SCIENCE ETH ZURICH, Switzerland
Solar Absorption in the Atmosphere
Towards estimates from surface based measurements and collocated satellite products M. Z. Hakuba, M. Wild, D. Folini, Arturo Sanchez-Lorenzo, G. Schaepman-Strub
Motivation
The mean state and temporal evolution of the global energy budgets' components have been studied for more than a century. Still, some components, such as the absorbed solar energy and its partitioning between the surface and the atmosphere are afflicted with large uncertainties. We combine surface solar radiation (SSR) measurements with collocated satellite derived surface albedo and TOA net incoming solar radiation to determine the absorbed solar radiation (ASR) at the surface and within the atmospheric column.
Objectives:
Homogeneity: GEBA Europe
Inclusion of inhomogeneous SSR time series (2000-2008) into (trend) analysis may lead to false conclusions. We applied 4 absolute single change point homogeneity tests (SNHT [Alexandersson, 1986], Pettitt [1979], Buishand [1981], Von Neumann [1941]) to detect inhomogeneous GEBA records: ●
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1. Homogeneous and spatially representative SSR data set
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Hit rate at 95% and 99% significance level: 3-4 tests → record suspect to be inhomogeneous Problem: some hits do not seem reasonable (Fig. 3, right) Trend of 0.71 Wm-2yr-1 over 183 stations (“ok” by eye) Trend of 0.74 Wm-2yr-1 over 162 stations (“ok” at 99%) Trend of 0.57 Wm-2yr-1 over 136 stations (“ok” at 95%)
183 GEBA stations (ok by eye)
TOA solar fluxes: The annual mean values (average 2001-2005) of TOA incoming and net solar radiation (CERES EBAF) over 50 BSRN sites are 329 Wm-2 and 221 Wm-2, respectively (see Table 2).
162 GEBA stations (ok at 99%)
2. Surface absorbed solar radiation surf. ASR = (1 - surf. albedo)*SSR 3. Atmospheric absorbed solar radiation (TOAnet - surf. ASR) Fig. 1: Global energy balance (Martin Wild, unpublished). Red circles highlight the solar fluxes we consider.
4. Assess global and regional climate model's disposition of solar energy
Surface
Fig. 4: Mean time series (monthly SSR anomalies) of 183 (“ok” by eye) GEBA records (top), and 162 records (“ok” at 99%) (bottom). Trend lines: trend of mean time series (red), average of stations trends (blue).
Spatial variability in SSR over Europe
Data sets
Satellite
Fig. 3: Examples of European GEBA records (2000-2010) that are indicated to be inhomogeneous by 4 absolute single change point homogeneity tests. Monthly anomalies in Wm-2.
Data set
Spatial resolution
Temporal resolution
Period covered
Domain
Quantity
GEBA
In situ, surface sites ~2000
Monthly averages
Many back to 1960's
world
Global radiation
BSRN
In situ, surface sites ~50
Minute values
Back to 1990's
world
Global, diffuse, and direct radiation
CM SAF SIS
0.03° 0.03°
Monthly averages
1983-2005
Meteosat disk
Global radiation
MODIS/Terra+Aqua Gap-Filled, SnowFree
0.0083° (30 arc seconds)
Every 16 days
2001-2010
Global
Land surface albedo (white-sky and black-sky)
CERES EBAF
1°
Monthly averages
2000-2005
Global
TOA solar fluxes
Satellite based estimates: Solar irradiance from CM SAF (2000-2005, 0.03°) is used to analyze the spatial variability in surface solar radiation (SSR) over Europe (Fig. 5, for July). We compute the standard deviation within the 1° surrounding of each grid point in the July climatology (Fig. 6). Surface stations in regions of high variability are expected to be of low spatial representativeness for their larger (1°) surrounding. Surface based estimates: Comparing the annual mean Clearness index (SSR/TOAin) of the European BSRN stations with their surrounding GEBA stations (Fig. 7) supports the finding of Fig. 6: larger variability in the alpine region around Payerne (low spatial representativeness) and higher spatial representativeness of stations like Cabauw.
Disposition of solar energy - BSRN
ASR surface: We combined the monthly BSRN surface radiation records with satellite retrieved surface albedo from MODIS (20012005). Because of missing data in surface albedo only 23 stations can be used (+ spurious spread due to missing months, Fig. 8). The annual mean of surface ASR over the remaining 23 stations is 160 Wm-2. Corresponding values for TOAin and TOAnet are 360 and 234 Wm-2.
Source
TOAin
Present study 50 (TOA)/23 (ASR) BSRN
TOAnet ASR surf. ASR atm.
329
221
160
74
Trenberth et al. [2009]
341
239
161
78
Wild (unpublished)
340
240
160
80
ASR atmosphere: The atmospheric absorption of solar energy is the difference between TOAnet and surface ASR. The annual mean over the 23 BSRN sites is 74 Wm-2. Although we consider the above components over land surface stations only, the retrieved quantities are in reasonable agreement with recent published and unpublished global mean estimates (see Table 2). The CERES EBAF land-only global mean TOAin and TOAnet are 327 and 215 Wm-2, respectively.
Fig. 5: CM SAF Solar irradiance [Wm-2] in July (2000-2005).
Table 2: Comparison of solar disposition estimates based on 50 (TOA) and 23 (ASR) BSRN stations (see Fig. 8) with published global mean estimates by Trenberth et al. [2009] and unpublished results by Wild. Units are Wm-2.
Fig. 8: BSRN stations’ disposition of annual mean solar energy -2 [Wm ] in dependence of latitude: TOAin (black), TOAnet (red), ASR surf. (green), ASR atm. (blue). BSRN sites for which the determination of ASR is possible are labeled. green+blue red because of missing months.
Conclusions and Outlook The homogeneity of worldwide GEBA records has been tested based on four absolute approaches. Still, the handling and interpretation of these tests has to be improved in order to assess the quality of the surface records.
The spatial variability in surface solar radiation is under investigation for Europe with the goal of determining a sub set of surface stations that are spatially representative for their larger (e.g., EBAF grid cell) surrounding. In regions of high variability, e.g., mountain ranges, a denser distribution of surface stations will be needed.
Based on high accuracy BSRN records, high-resolution surface albedo (MODIS), and CERES EBAF TOA fluxes, first estimates of the disposition of solar energy within the climate system have been obtained that are in reasonable agreement with recent global estimates. This analysis will be extended based on denser surface networks, such as GEBA.
Fig. 2: World map with GEBA (blue) and BSRN (red) sites, grid as prescribed by the 1° CERES EBAF data set shown over the polar regions.
Contact:
[email protected]
Fig. 7: Difference in annual Clearness Index of GEBA sites as a function of distance (km) from BSRN site. Blue/red: stations located north/south of BSRN site. Green: above 1000 masl. Larger spread at small distance around Payerne indicates lower spatial representativeness compared to Cabauw and Lindenberg.
The key-reference data set we are establishing will be used to assess global (ECHAM6-HAM) and regional (CCLM) climate models in reproducing the disposition of solar energy.
Fig. 6: CM SAF 1° standard deviation [Wm-2] in July. The red range rings around BSRN sites are of 100 km radius.
Acknowledgment: We thank the Swiss National Science foundation for financial support.
