3.5 Parametrization of Circumsolar Radiation
3.5.2 Treatment of Multiple Scattering Layers – The Adding Method
So far the developed parameterization allows to calculate circumsolar radiation caused by a single layer of scattering particles only. However at times more than one layer has to be
3.5 Parametrization of Circumsolar Radiation 59
SEVIRI Measurement R(0.6 µm), R(1.6 µm)
APICS
cloud property retrieval
-> effective radius
-> optical thickness
ECMWF IFS aerosol concentration
Circumsolar radiation due to cirrus clouds or aerosol LUT-assissted parameterization
based on concept of apparent optical thickness A priori Information
Ground Albedo
Cirrus Cloud Mask
60 3. Tools and Methods
dealt with; be it in the case of a cirrus above an aerosol layer or when an external mixture11 of aerosols is to be considered, which can be regarded as multiple individual layers as well. The latter is the case if IFS data is to be evaluated, because the model output is an external mixture of aerosol components. The optical thickness of several scattering layers is additive, and since the parameterization is based on the apparent optical thickness, it was obvious to test whether it is additive as well.
For this two different test scenarios were considered. The first one places cirrus clouds over aerosol layers, the second one deals with external aerosol mixtures. In both cases reference simulations were performed with MYSTIC, treating the multiple layers explicitly. The sum of the parameterized values of apparent optical thickness for the individual layers computed according to Eq. 3.15 were then compared to these simulations. The summation ofτapp values of individual layers or components is called “adding method” in the following. The tests shown in the following were performed for a field of view of 3.0◦. The standard deviation in τapp,bb derived from the MYSTIC reference simulations due to Monte Carlo noise is always smaller than 0.8%.
The cirrus clouds in the first test scenario were always composed of HEY solid-columns. The ice optical thickness at 550 nm was varied between the three values of 0.1, 0.5 and 1.2 and the effective radius was set to 5µm, 10µm, 20µm, 30µm, 40µm, 50µm, 70µm or 90µm. For the aerosol layer optical properties for the three different size bins of the IFS dust aerosol were variantly used at aerosol optical thickness values at 550 nm of 0.1, 0.3 and 1.2. In total 3·8·3·3 = 216 scenes were created. Figure 3.15 compares the resulting broad band apparent optical thickness values as well as the resulting CSR values. The error in the apparent optical thickness due to simply adding values of two layers instead of explicitly simulating it stays below 4%. All cases with errors >1.1 % employ aerosol with the highest AOT considered of 1.2. The error in the CSR is in general higher than in the apparent optical thickness since for its calculation two τapp-values need to be determined (comp. Eq. 3.21) which are both prone to errors. The adding of apparent optical thickness values seems by trend to introduce a negative bias in the CSR since for over 90% of the considered test cases the parameterized CSR is smaller than the MYSTIC reference value.
For the second test scenario several external aerosol mixtures were considered. Again the broad band apparent optical thickness and the CSR were calculated – once explicitly with MYSTIC and once by adding the parametrized apparent optical thickness values for
11External mixture means that particles of different type are present, whereas internal mixture indicates that individual particles are composed of several components (e.g. particles with a coating).
3.5 Parametrization of Circumsolar Radiation 61
the individual aerosol species. Overall 230 scenes with a total aerosol optical thickness at 550 nm of either 0.05, 0.10, 0.15, 0.3, 0.4, 0.5, 0.7, 0.9, 1.1, 1.5, 2.0, 3.0 or 5.0 were generated by randomly mixing eight aerosol components from OPAC, namely insoluble
(inso), water-soluble(waso),soot, mineral nucleation mode (minm),mineral accumulation mode (miam), mineral coarse mode (micm), sea salt accumulation mode (ssam) and sea salt coarse mode (sscm). That is, after randomly picking one total aerosol optical thickness level τ, each aerosol component i was assigned a random number ti ∈ [0,1], from which
the partial aerosol optical thickness values τi were computed as
τi =τ ti P aer.components ti . (3.27)
Figure 3.16 shows that the relative error in the apparent optical thickness is mostly below 2%. Only for aerosol optical thickness values larger than 1 the error can reach values in the order of 10%. Like in the first test case the errors in CSR can be larger than in apparent optical thickness. At times they reach values in the order of 15%. Below a total aerosol optical thickness of≈0.5 the “adding method” underestimates the CSR values from MYSTIC in tendency while for larger values mostly an overestimation can be observed.
Considering the results of the two test cases I assume that the error in CSR due to ap- plication of the “adding method” is on average below 5%. For individual setups the tests showed errors in the range of up to 15%. These errors seem to be acceptable if one consid- ers the greatly enhanced flexibility obtained by the adding method. Note that if linearity can be assumed, i.e. ∆kaτs 1 (comp. Eq. 3.23), also the CSR is additive and CSR
62 3. Tools and Methods
Figure 3.15: Values of apparent optical thickness (left) and CSR (right) for 216 setups of a two layer scene with a cirrus over an aerosol layer. The cirrus is composed of HEY “solid- columns” of varyingreff and an ice optical thickness at 550 nm of 0.1, 0.5 or 1.2. The aerosol layer is composed either of the small, medium or large dust component of the ECMWF IFS aerosol (Sect. 3.3.2) with an aerosol optical thickness of 0.1, 0.3 or 1.2. The diagrams are primarily sorted by increasing combined optical thickness (aerosol + cirrus) at 550 nm (τ550nm) and secondly by apparent optical thickness. Upper left: Broad band apparent optical thickness explicitly simulated with MYSTIC (blue), apparent optical thickness (bb) obtained by adding apparent optical thickness values of the individual layers computed using the k-LUTs (Sect. 3.5, green), combined optical thickness τ550nm (red). Lower left: Relative deviation of the parametrized apparent optical thickness to the MYSTIC reference. Upper right: Resulting CSR values. Lower right: Relative deviation of the parametrized CSR to the MYSTIC reference.
3.5 Parametrization of Circumsolar Radiation 63
Figure 3.16: Values of apparent optical thickness (left) and CSR (right) for 230 setups with a random external mixture of eight OPAC aerosol components (inso, waso, soot, minm, miam, micm, ssam, sscm). The diagrams are primarily sorted by increasing combined optical thickness which was set to either 0.05, 0.10, 0.15, 0.3, 0.4, 0.5, 0.7, 0.9, 1.1, 1.5, 2.0, 3.0 or 5.0 and secondly by apparent optical thickness. See Fig. 3.15 for description of panels.
64 3. Tools and Methods