Abstract
Cirrus clouds, which cover about 30% of the global land area, contribute to the largest determining factors of radiative forcing in the atmosphere. Of particular importance is scattering in the direct backscattering direction, which is used for retrievals from satellites and ground-based instruments. The colour ratio χ, a ratio of the backscattering coefficients at 1.064 and 0.532 µm, has been used to support phase discrimination in mixed phase clouds [1]. By way of laboratory-produced clouds in the Manchester Ice Cloud Chamber [2], we find direct experimental evidence for the near backscatter enhancement in the scattering phase function for a range of crystal habits representative of naturally occurring cirrus ice crystals. Furthermore, the colour ratio has been investigated for clouds consisting of predominantly ice columns, plates and compacts, respectively. The experimental results are compared with modelling results obtained by our Physical Optics Beam Tracer (PBT) model [3].
Best agreement with measured intensity distributions between 175° and 180° was achieved for rounded, slightly rough hexagonal prisms models. The experimentally obtained mean colour ratios for columns, plates and compacts are 0.485, 0.818 and 0.645, respectively. These findings agree broadly with CALIPSO Cloud Physics Lidar measurements [4] and with simulations by Bi et al. [5], who found colour ratio values less than unity with a peak value near 0.7 for columns and 0.8 for plates for random particle orientation. Colour ratios measured within the study presented here increase with particle size, which is consistent with modelling results in [5] and in this study. Furthermore, our modelling results indicate that surface roughness increases the colour ratio compared to smooth pristine hexagonal prisms, whereas particle rounding and indentations decrease it.
[1] Y.Hu, D. Winker, M. Vaughan, B. Lin, A. Omar et al. J. Atmos. Oceanic Technol., 26, 2293–2309 (2009).
[2] H.R. Smith, P.J. Connolly, A.J. Baran, E. Hesse, A.R.D. Smedley, and A.R. Webb, JQSRT 157, 106–118 (2015).
[3] H. Ballington, E Hesse JQSRT, 323,109054 (2024).
[4] M.A. Vaughan, Z. Liu, M.J. McGill, Y. Hu, M.D. Obland JGR, 115, D14206 (2010).
[5] L. Bi, P. Yang, G.W. Kattawar, B.A. Baum, Y.X. Hu et al. JGR, 114, D00H08 (2009).
Best agreement with measured intensity distributions between 175° and 180° was achieved for rounded, slightly rough hexagonal prisms models. The experimentally obtained mean colour ratios for columns, plates and compacts are 0.485, 0.818 and 0.645, respectively. These findings agree broadly with CALIPSO Cloud Physics Lidar measurements [4] and with simulations by Bi et al. [5], who found colour ratio values less than unity with a peak value near 0.7 for columns and 0.8 for plates for random particle orientation. Colour ratios measured within the study presented here increase with particle size, which is consistent with modelling results in [5] and in this study. Furthermore, our modelling results indicate that surface roughness increases the colour ratio compared to smooth pristine hexagonal prisms, whereas particle rounding and indentations decrease it.
[1] Y.Hu, D. Winker, M. Vaughan, B. Lin, A. Omar et al. J. Atmos. Oceanic Technol., 26, 2293–2309 (2009).
[2] H.R. Smith, P.J. Connolly, A.J. Baran, E. Hesse, A.R.D. Smedley, and A.R. Webb, JQSRT 157, 106–118 (2015).
[3] H. Ballington, E Hesse JQSRT, 323,109054 (2024).
[4] M.A. Vaughan, Z. Liu, M.J. McGill, Y. Hu, M.D. Obland JGR, 115, D14206 (2010).
[5] L. Bi, P. Yang, G.W. Kattawar, B.A. Baum, Y.X. Hu et al. JGR, 114, D00H08 (2009).
| Original language | English |
|---|---|
| Pages | 22 |
| Number of pages | 1 |
| Publication status | Published - 23 Jun 2025 |
| Event | 21st Electromagnetic and Light Scattering Conference - Milazzo, Italy Duration: 23 Jun 2025 → 27 Jun 2025 Conference number: 21 https://portale2.unime.it/els2025/ |
Conference
| Conference | 21st Electromagnetic and Light Scattering Conference |
|---|---|
| Abbreviated title | ELS |
| Country/Territory | Italy |
| City | Milazzo |
| Period | 23/06/25 → 27/06/25 |
| Internet address |