KRISTIAN PAGH NIELSEN, DANISH METEOROLOGICAL INSTITUTE, KPN@DMI.DK. ELSA ANDERSEN, SIMON FURBO & JANNE DRAGSTED, TECHNICAL UNIVERSITY OF

DENMARK (DTU). EMILY GLEESON, MET ÉIRANN

CLOUDS, AEROSOLS AND THE DIRECTIONAL DISTRIBUTION OF SOLAR IRRADIANCES

WHY ARE WE INTERESTED IN DIRECTIONAL IRRADIANCES?

• Solar heating and PV panels• Passive solar heating and illumination of

buildings

WHY ARE WE INTERESTED IN DIRECTIONAL IRRADIANCES?

• Solar heating and PV panels• Passive solar heating and illumination of

buildings• The detailed sunshape (important for

CSP) is not a focus here!

CLOUDS AND RADIATION• Cloud-radiation interactions depend on the following steps:

1. Cloud cover

CLOUDS AND RADIATION• Cloud-radiation interactions depend on the following steps:

1. Cloud cover 2. Cloud physical properties: (Gleeson et al. 2015; ALADIN-HIRLAM Newsletter)

a) Liquid water load [kg/m2] b) Ice water load [kg/m2] c) Effective radii [µm]

CLOUDS AND RADIATION• Cloud-radiation interactions depend on the following steps:

1. Cloud cover 2. Cloud physical properties: (Gleeson et al. 2015; ALADIN-HIRLAM Newsletter)

a) Liquid water load [kg/m2] b) Ice water load [kg/m2] c) Effective radii [µm]

3. Cloud optical properties (Nielsen et al. 2014; Geosci. Model Dev.)

CLOUDS AND RADIATION• Cloud-radiation interactions depend on the following steps:

1. Cloud cover 2. Cloud physical properties: (Gleeson et al. 2015; ALADIN-HIRLAM Newsletter)

a) Liquid water load [kg/m2] b) Ice water load [kg/m2] c) Effective radii [µm]

3. Cloud optical properties (Nielsen et al. 2014; Geosci. Model Dev.) 4. Radiative transfer approximations (Nielsen et al. 2014; Geosci. Model Dev.)

AEROSOLS AND RADIATION• Cloud-radiation interactions depend on the

following steps:

1. Aerosol concentration [g/m3] / load [g/m2] 2. Aerosol optical properties ——————-> 3. Radiative transfer approximations

From Gleeson et al. (2015) Atmos. Chem. Physics COST action ES1004 EuMetChem special issue

AEROSOLS AND RADIATION• Cloud-radiation interactions depend on the

following steps:

1. Aerosol concentration [g/m3] and load [g/m2]

2. Aerosol optical properties ——————-> 3. Radiative transfer approximations

From Gleeson et al. (2015) Atmos. Chem. Physics COST action ES1004 EuMetChem special issue

Simulated errors from assuming typical (IFS_land) optical properties:

AODbroadband vs AOD550nm -30% GHI error

AOD* = (1-a·g2)·AOD +15% GHI error

MEASUREMENTS PERFORMED AT DTUAndersen, Nielsen, Dragsted & Furbo (2015), Energy Procedia, 70: 729-736.

Dragsted & Furbo (2012): DTU Byg, report R-275. Available from http://www.byg.dtu.dk/Publikationer/Byg_rapporter

Skalik et al. (2012): “Evaluation of long term global radiation measurements in Denmark and Sweden”. EuroSun 2012 Proceedings

5/13

MEASUREMENTS PERFORMED AT DTUAndersen, Nielsen, Dragsted & Furbo (2015), Energy Procedia, 70: 729-736.

Dragsted & Furbo (2012): DTU Byg, report R-275. Available from http://www.byg.dtu.dk/Publikationer/Byg_rapporter

Skalik et al. (2012): “Evaluation of long term global radiation measurements in Denmark and Sweden”. EuroSun 2012 Proceedings

1-D THEORETICAL STUDY OF RADIANCES• Calculated with

DISORT/libRadtran (Stamnes et al. 1988, 2000, 2015; Mayer & Kylling 2005)

Accuracy of DISORT spectral radiances demonstrated in (Hestenes et al. 2006, Appl. Opt.)

1-D THEORETICAL STUDY OF RADIANCES• Even in the simple

1-D case, the distribution of directional radiances across the sky is never isotropic!

• Key issue: The effects of inhomogeneous clouds on data at high temporal resolution

HOURLY DTU DATAGood models exist for instance: Skartveit et al. (1998)

For tilted surfaces: Perez (1986, 1987, 1993)

2-MINUTE DTU DATAAt high temporal resolution things are more complicated

10/13

SCATTERED HORIZONTAL IRRADIANCE (SHI) AND CLOUD OPTICAL THICKNESS (TAU)

DIRECTIONAL SCATTERED HORIZONTAL IRRADIANCE AND CLOUD OPTICAL THICKNESS (TAU) / COS(SZA)

DIRECTIONAL SCATTERED HORIZONTAL IRRADIANCE AND CLOUD OPTICAL THICKNESS (TAU) / COS(SZA)

DIRECTIONAL SCATTERED HORIZONTAL IRRADIANCE AND CLOUD OPTICAL THICKNESS (TAU) / COS(SZA)

DIRECTIONAL SCATTERED HORIZONTAL IRRADIANCE AND CLOUD OPTICAL THICKNESS (TAU) / COS(SZA)

DIRECTIONAL SCATTERED HORIZONTAL IRRADIANCE AND CLOUD OPTICAL THICKNESS (TAU) / COS(SZA)

DIRECTIONAL SCATTERED HORIZONTAL IRRADIANCE AND CLOUD OPTICAL THICKNESS (TAU) / COS(SZA)

DIRECTIONAL SCATTERED HORIZONTAL IRRADIANCE AND CLOUD OPTICAL THICKNESS (TAU) / COS(SZA)

DIRECTIONAL SCATTERED HORIZONTAL IRRADIANCE AND CLOUD OPTICAL THICKNESS (TAU) / COS(SZA)

CONCLUSION• Based on detailed irradiance measurement at the DTU

measurement site…

• … a new method is suggested, in which the two physical variables:

• Cloud optical thickness and

• Inhomogeneous cloud reflectance

• are used to form a basis for simulating directional irradiances with high temporal resolution.

13/13

Contact: kpn@dmi.dk

THANK YOU FOR YOUR ATTENTION!