CoRP Symposium, 10-11August 2010, Fort Collins, CO 1 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 Steven D. Miller (CIRA/Colorado State University) Andrew K. Heidinger Andrew K. Heidinger (NOAA/NESDIS) CIRA

CoRP Symposium, 10-11August 2010, Fort Collins, CO 2 Optically Opaque Mixed-Phase Region (~ m deep) Precipitating Ice Region (~ km deep) Generating Cells ~ km in Length Ice Mixed-Phase Clouds Significant in-flight icing hazard! Supercooled Liquid Water Motivation

CoRP Symposium, 10-11August 2010, Fort Collins, CO 3 Objectives u Scientific: Understand spectral reflectance characteristics of supercooled liquid water-topped mixed-phase clouds via radiative model simulations in near IR channels u Application: Develop a multispectral satellite detection algorithm for supercooled liquid water- topped mixed-phase clouds u 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.

CoRP Symposium, 10-11August 2010, Fort Collins, CO 4 Hypothesis km Liquid (  _ liquid ) Ice (  _ ice ) 1.6 μm R(1.6) 2.2 μm R(2.2) 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 > 2.2 μm Differential absorption properties between the liquid and ice in the near infrared km Liquid (  _ liquid ) 1.6 μm R(1.6)R(2.2) phase Assuming ‘all else being equal’ besides the phase of the cloud particles… less reflectancemore reflectance

CoRP Symposium, 10-11August 2010, Fort Collins, CO 5 R_COMP u We define a liquid-normalized reflectance ratio Simulated for pure-liquid Observed  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.

CoRP Symposium, 10-11August 2010, Fort Collins, CO 6 a-priori database (constructed using SBDART) Using MOD021KM data, compute OBS Reflectance Ratio R_OBS=R_obs(2.1μm)/R_obs(1.6μm) 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) R_COMP=R_OBS / R_SIM 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* R_COMP ≥ R*_COMP MODIS Level 2 data: Cloud Phase, T_cloud_top, Optical thickness, Effective radius MODIS Level 2 data: Cloud Phase, T_cloud_top, Optical thickness, Effective radius Flag a likely liquid topped mixed-phase pixel Flag a likely liquid topped mixed-phase pixel Schematics of our detection algorithm

CoRP Symposium, 10-11August 2010, Fort Collins, CO 7 Radiative Transfer Simulation in the Near-Infrared u SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer) model used u Compare with Terra MODIS data (GOES-R ABI in the future) u Sensitivity tests for several variables u A-priori database generation u A-priori database generation for idealized cloud layers t layer1: 3-5km (ice bottom), layer2: 5-5.5km (liquid top) t Liquid optical thickness = 0~30 for total optical thickness = 0~30 t Liquid sizes = 6, 8, 10, 12, 15, 20 μm when ice = 30 μm t Ice sizes = 30, 50, 70, 100, 120 μm when liquid = 8 μm t Sensor/Solar zenith angle = 0~80° t Sensor azimuth angle = 0~170° t Ocean and vegetation surfaces t Total # of data points =15,909,696

CoRP Symposium, 10-11August 2010, Fort Collins, CO 8 A-priori database of R_COMP A-priori database of R_COMP Total # 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 angleSensor azimuth angleLiquid droplet size (Ice particle size) Liquid-top cloud optical thickness Total optical thickness

CoRP Symposium, 10-11August 2010, Fort Collins, CO 9 Comparison of reflectance ratios between MODIS and SBDART For MODIS Liquid cloud pixels with T_ cld_top > , Effective radii < 20, Optical thickness > UTC 29 Sept UTC 29 Sept UTC 05 July 2005 MASE field exp case

CoRP Symposium, 10-11August 2010, Fort Collins, CO 10 Determine R*_COMP ~ R_COMP_SIM = R_SIM (SLW top) / R_SIM (Liquid), where R_SIM = R_sim (2.1μm) / R_sim (1.6μm) A particular threshold, R*_COMP (>1) gives indication of a detectable signal for liquid-over ice clouds.

CoRP Symposium, 10-11August 2010, Fort Collins, CO 11 where x=liquid droplet effective radius and y=minimum optical thickness. Minimum Optical Thickness (OT*) u Use SBDART simulated database focusing on top liquid droplet sizes. t Currently, surface types and ice particle sizes are not considered. The impact of ice sizes can be neglected compared with liquid sizes. t using R_SIM(2.1/1.6) and  _liquid=1 intersections,

CoRP Symposium, 10-11August 2010, Fort Collins, CO 12 a-priori database (constructed using SBDART) Using MOD021KM data, compute OBS Reflectance Ratio R_OBS=R_obs(2.1μm)/R_obs(1.6μm) 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) R_COMP=R_OBS / R_SIM 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* R_COMP ≥ R*_COMP MODIS Level 2 data: Cloud Phase, T_cloud_top, Optical thickness, Effective radius MODIS Level 2 data: Cloud Phase, T_cloud_top, Optical thickness, Effective radius Flag a likely liquid topped mixed-phase pixel Flag a likely liquid topped mixed-phase pixel Schematics of our detection algorithm

CoRP Symposium, 10-11August 2010, Fort Collins, CO 13 Apply to Terra MODIS data

CoRP Symposium, 10-11August 2010, Fort Collins, CO 14 Terra MODIS L1B (MOD021KM) Data on 31 Oct Data scan started at 1625 UTC

CoRP Symposium, 10-11August 2010, Fort Collins, CO 15 Terra MODIS L2 (MOD06) products on 31 Oct. 2006

CoRP Symposium, 10-11August 2010, Fort Collins, CO UTC 31 Oct 2006 Likely liquid topped mixed-phase pixels in red R*_comp = R*_comp = 1.010R*_comp = R*_compDetected supercooled liquid top pixels Cyan color 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)

CoRP Symposium, 10-11August 2010, Fort Collins, CO 17 Preliminary Validation Exercises

CoRP Symposium, 10-11August 2010, Fort Collins, CO 18 C3VP/CLEX-10 Field Experiment u 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 u 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. CARE Ground Site Sample CloudSat Ground track C3VP/CLEX-10 Target region

CoRP Symposium, 10-11August 2010, Fort Collins, CO UTC 19 Jan 2007 Likely liquid topped mixed-phase pixels in red R*_comp = R*_comp = R*_compDetected supercooled liquid top pixels Cyan color Cyan color means pixels having temperatures below 273K and also either water or mixed-phase (15,796 points out of total 18,900 pixels in the domain) - 12°C

CoRP Symposium, 10-11August 2010, Fort Collins, CO UTC 20 Feb 2007 Likely liquid topped mixed-phase pixels in red R*_comp = R*_comp = R*_compDetected supercooled liquid top pixels Cyan color Cyan color means pixels having temperatures below 273K and also either water or mixed-phase (13,329 points out of total 18,876 pixels in the domain) - 8°C

CoRP Symposium, 10-11August 2010, Fort Collins, CO 21 Conclusions u A daytime multispectral algorithm for distinguishing between pristine liquid and liquid-topped ice clouds is in development. u The approach takes advantage of differential absorption properties between liquid and ice cloud particles in the near infrared. u The technique, applied here to Terra MODIS, is designed with an eye toward the future GOES-R Advanced Baseline Imager. u Preliminary case study results show signals near regions of observed liquid-over-ice. The algorithm fails in cases of overriding cirrus.

CoRP Symposium, 10-11August 2010, Fort Collins, CO 22 Future Work u The algorithm will be tested and validated for more cases with quantitative uncertainty estimates. u Additional constraints using various channel combinations to clearly exclude ice phase clouds will be studied. u More detailed analysis and simulations using the 2.25 μm will continue in preparation for applications to GOES-R ABI data.

CoRP Symposium, 10-11August 2010, Fort Collins, CO 23 THANK YOU! CIRA