A daytime multispectral technique for detecting supercooled liquid water-topped mixed-phase clouds

1. CIRA A daytime multispectral technique for detecting supercooled liquid water-topped mixed-phase clouds Yoo-Jeong Noh Cooperative Institute for Research in the Atmosphere / Colorado State University with Steven D. Miller (CIRA/Colorado State University) Andrew K. Heidinger (NOAA/NESDIS)

2. Supercooled Liquid Water Generating Cells ~ 1-1.5 km in Length Optically Opaque Mixed-Phase Region (~300-500 m deep) Precipitating Ice Region (~0.2-2.5 km deep) Ice Motivation Mixed-Phase Clouds Significant in-flight icing hazard!

3. Objectives Scientific: Understand spectral reflectance characteristics of supercooled liquid water-topped mixed-phase clouds via radiative model simulations in near IR channels Application: Develop a multispectral satellite detection algorithm for supercooled liquid water-toppedmixed-phase clouds Operational Utility: An objective method for identifying a subset of areas where significant aircraft icing conditions may not be present through a significant depth of cloud, given a widespread field of super-cooled liquid clouds.

4. 1.6 μm 2.2 μm 1.6 μm R(1.6) R(2.2) R(1.6) R(2.2) km 5.5 Liquid (_liquid) 5.0 Liquid (_liquid) 3.0 R(2.2)/R(1.6) for a supercooled liquid top and ice bottom cloud R(2.2)/R(1.6) for a pristine liquid cloud > km 5.5 Liquid (_liquid) 5.0 Ice (_ice) 3.0 Hypothesis Assuming ‘all else being equal’ besides the phase of the cloud particles… 2.2 μm less reflectance more reflectance Differential absorption properties between the liquid and ice in the near infrared

5. R_COMP • We define a liquid-normalized reflectance ratio Observed Simulated for pure-liquid With stronger absorption by ice particles at 1.6 m, we expect the numerator term of R_COMP to exceed the denominator term in the case of liquid-over-ice clouds, such that R_COMP  1.

6. MODIS Level 2 data: Cloud Phase, T_cloud_top, Optical thickness, Effective radius Schematics of our detection algorithm • MODIS IR Cloud Phase improved by A. Heidinger • OT* : a minimum optical thickness to be detected (a function of cloud top effective radius) • R*_COMP : a threshold for the SLW topped pixel Liquid or Mixed phase & T_cloud_top < 273 K & Optical thickness ≥ OT* a-priori database (constructed using SBDART) Using MODIS optical thickness and effective radius, for a all-liquid cloud in the database, compute R_SIM=R_sim(2.1μm)/R_sim(1.6μm) Using MOD021KM data, compute OBS Reflectance Ratio R_OBS=R_obs(2.1μm)/R_obs(1.6μm) R_COMP=R_OBS / R_SIM R_COMP ≥ R*_COMP Flag a likely liquid topped mixed-phase pixel

7. Radiative Transfer Simulation in the Near-Infrared • SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer) model used • Compare with Terra MODIS data (GOES-R ABI in the future) • Sensitivity tests for several variables • A-priori database generation for idealized cloud layers • layer1: 3-5km (ice bottom), layer2: 5-5.5km (liquid top) • Liquid optical thickness = 0~30 for total optical thickness = 0~30 • Liquid sizes = 6, 8, 10, 12, 15, 20 μm when ice = 30 μm • Ice sizes = 30, 50, 70, 100, 120 μm when liquid = 8 μm • Sensor/Solar zenith angle = 0~80° • Sensor azimuth angle = 0~170° • Ocean and vegetation surfaces • Total # of data points =15,909,696

8. A-priori databaseof R_COMPTotal # of data points =15,909,696 with varying sun/sensor angles, liquid/ice particle sizes, and total optical thicknesses over two different types of surfaces Sensor zenith angle Liquid droplet size (Ice particle size) Sensor azimuth angle Liquid-top cloud optical thickness Total optical thickness

9. MASE field exp case 1945 UTC 29 Sept 2006 1915 UTC 05 July 2005 1940 UTC 29 Sept 2006 Comparison of reflectance ratios between MODIS and SBDART For MODIS Liquid cloud pixels with T_cld_top> 283.15, Effective radii < 20, Optical thickness > 1

10. 1.005 ~ 1.010 Determine R*_COMP A particular threshold, R*_COMP (>1) gives indication of a detectable signal for liquid-over ice clouds. R_COMP_SIM = R_SIM (SLW top) / R_SIM (Liquid), where R_SIM = R_sim (2.1μm) / R_sim (1.6μm)

11. Minimum Optical Thickness (OT*) • Use SBDART simulated database focusing on top liquid droplet sizes. • Currently, surface types and ice particle sizes are not considered. The impact of ice sizes can be neglected compared with liquid sizes. • using R_SIM(2.1/1.6) and _liquid=1 intersections, where x=liquid droplet effective radius and y=minimum optical thickness.

12. MODIS Level 2 data: Cloud Phase, T_cloud_top, Optical thickness, Effective radius Schematics of our detection algorithm • MODIS IR Cloud Phase improved by A. Heidinger • OT* : a minimum optical thickness to be detected (a function of cloud top effective radius) • R*_COMP : a threshold for the SLW topped pixel Liquid or Mixed phase & T_cloud_top < 273 K & Optical thickness ≥ OT* a-priori database (constructed using SBDART) Using MODIS optical thickness and effective radius, for a all-liquid cloud in the database, compute R_SIM=R_sim(2.1μm)/R_sim(1.6μm) Using MOD021KM data, compute OBS Reflectance Ratio R_OBS=R_obs(2.1μm)/R_obs(1.6μm) R_COMP=R_OBS / R_SIM R_COMP ≥ R*_COMP Flag a likely liquid topped mixed-phase pixel

13. Apply to Terra MODIS data

14. Terra MODIS L1B (MOD021KM) Data on 31 Oct. 2006 Data scan started at 1625 UTC

15. Terra MODIS L2 (MOD06) products on 31 Oct. 2006

16. 1625 UTC 31 Oct 2006 • Likely • liquid topped • mixed-phase pixels in red R*_comp = 1.005 R*_comp = 1.010 R*_comp = 1.100 Cyan color means pixels having temperatures below 273K and also either water or mixed-phase (6,695 points out of total 20,571 pixels in the domain)

17. Preliminary Validation Exercises

18. C3VP/CLEX-10 Target region CARE Ground Site Sample CloudSat Ground track C3VP/CLEX-10 Field Experiment CLEX (Cloud Layer Experiment) is a series of field experiments funded by the Department of Defense's Center for Geosciences/Atmospheric Research at CIRA/Colorado State University for non-precipitating, mid-level, mixed-phase clouds since 1996. CLEX-10 collaborated with the Canadian CloudSat/CALIPSO Validation Project (C3VP) that took place from 31 October 2006 to 1 March 2007 over Southern Ontario and Quebec.

19. - 12°C 1625 UTC 19 Jan 2007 • Likely • liquid topped • mixed-phase pixels in red R*_comp= 1.005 R*_comp = 1.010 Cyan colormeans pixels having temperatures below 273K and also either water or mixed-phase (15,796 points out of total 18,900 pixels in the domain)

20. - 8°C 1625 UTC 20 Feb 2007 • Likely • liquid topped • mixed-phase pixels in red R*_comp= 1.005 R*_comp = 1.010 Cyan colormeans pixels having temperatures below 273K and also either water or mixed-phase (13,329 points out of total 18,876 pixels in the domain)

21. Conclusions A daytime multispectral algorithm for distinguishing between pristine liquid and liquid-topped ice clouds is in development. The approach takes advantage of differential absorption properties between liquid and ice cloud particles in the near infrared. The technique, applied here to Terra MODIS, is designed with an eye toward the future GOES-R Advanced Baseline Imager. Preliminary case study results show signals near regions of observed liquid-over-ice. The algorithm fails in cases of overriding cirrus.

22. Future Work The algorithm will be tested and validated for more cases with quantitative uncertainty estimates. Additional constraints using various channel combinations to clearly exclude ice phase clouds will be studied. More detailed analysis and simulations using the 2.25 μm will continue in preparation for applications to GOES-R ABI data.

23. CIRA THANK YOU!