4. Case Study Results: July 9th ARCTAS 2008 Summer Campaign

AbstractCirrus clouds are important modulators of the earth radiation budget and continue to be one of the most uncertain components in weather and climate modeling.

Sunphotometers are widely accepted as one of the most accurate platforms for measuring clear sky aerosol optical depth (AOD). However, such measurements under cirrus are still ambiguous. Derivation of valid AOD under cirrus was previously focused on correction factors, rather than on derivation of cirrus clouds optical thickness (COT).

In the present work we propose a new approach that utilizes the total measured irradiance under a cloudy scene to derive cirrus COT and ice particle effective diameter (Deff) by sunphotometry. For that, we have generated Lookup tables of total transmittance at the sunphotometer FOV due to the direct and scattered irradiance over the spectral range of 400-2200 nm, for a range of COT(0-4), and a range of ice particle effective diameters (10-120mm) by using explicit cirrus optical properties models, for cirrus only and for a two-component model including cirrus and aerosols. The new approach is illustrated here on an airborne case using measured transmittances from the ARCTAS campaign using the 14-channel NASA Ames Airborne Tracking Sunphotometer.

We found that relative uncertainties in COT were much smaller than the ones for Deff. Also, we have shown that for thin cirrus cases (COT<1.0), the aerosol layer between the instrument and the cloud plays an important role, especially in derivation of Deff. The choice of the appropriate cirrus model is still uncertain and may introduce large differences in derived Deff for different models.

1. IntroductionQuantification of cirrus microphysical and optical properties, either by in-situ measurements or remote-sensing platforms, is an ongoing effort that continues to gain much attention.

Sunphotometers are good example of a versatile instrument that is relatively simple to operate and is available both on airborne (e.g. AATS-14 – 14-channel Ames Airborne Tracking Sunphotometer [Russell et al., 2005]) and ground-based platforms (e.g. AERONET [Holben et al., 2001]) and can increase our capability in quantifying cirrus properties such as COT and ice crystal effective diameter.

Although sunphotometery measurements under thin cirrus allow the direct incoming solar radiation to reach the detector (Figure 1), this feature was never exploited as a mean for cirrus COT retrievals. Under cloudy conditions in general, and under cirrus in particular, such instruments admit light from at least a part of the strong forward scattering peak of the cloud particles, thus increasing the incoming radiation that is measured in the detector beyond the direct solar transmittance [Kinne et al.,1997].

In this work, we present a fresh look at the topic of cirrus property retrieval by sunphotometers, and suggest a methodology to exploit the forward scattered irradiance component measured by these instruments. We use the combination of this measure and multi/hyper-spectral capabilities (covering the visible to SWIR range) commonly used by many other cloud retrieval schemes [Nakajima and King 1990] to demonstrate the retrieval feasibility of both cirrus COT and ice particle effective diameter by sunphotometry.

The retrieval is based on the lookup table (LUT) approach (Figure 4), in which we perform a calculation spanning COT values from 0 to 4 in increments of 0.02 from 0 to 1 and 0.1 from 1 to 4. This spans the range of 81 COT values. Ice particle effective diameter values are used at the same nominal values (i.e. 23 values) given by Baum et al. [2011] without interpolation.

Figure 1: Tracking the sun through a thin cirrus layer; Mauna Loa (MLO) observatory, Hawaii, May 2012 .

A13H-0282AGU Fall 2012

2. Forward ModelingThe total non-Rayleigh transmittance (Figure 3b) measured at the sunphotometer FOV (which includes both the direct and forward-scattered component) [Shiobara and Asano 1994]:

[1]

where subscripts correspond to cloud, ozone, NO2, water vapor, O2, O4 (oxygen dimer), CO2, and CH4 optical depths, respectively, m is the cosine of the solar zenith angle (SZA), k is a pre-factor that represents the dependence of the transmittance on the cirrus phase-function and instrument parameters (Eq. 2 and Figure 2),and tc is the spectrally dependent cloud optical thickness (COT).

[2]

Here, w is the wavelength-dependent single scattering albedo for the corresponding ice particle effective diameter (Deff), P is the ice cloud scattering phase-function, q is the scattering angle, and h is the sunphotometer FOV half-angle.

Figure 3: (a) Direct transmittance calculated based on direct attenuation of solar irradiance through a cirrus layer under a range of COT (tcloud) and the appropriate trace gas attenuation terms, (b) Total transmittance (Eq.1) for a 1.85o half-FOV acceptance angle for cloud effective diameters of 10 mm (dotted lines) and 120 mm (thick solid lines). Cirrus optical properties are taken from Baum-Yang SR model. Calculations are made for a solar zenith angle of 30o for a Mid-Latitude summer atmosphere using MODTRAN 5.2.1 in direct transmittance mode.

3. Retrieval Scheme

Figure 4: (a) calculated total transmittance LUT (Eq. 1) for a Mid Latitude atmosphere model at SZA of 30o at wavelengths (a) 2138 versus 670 nm, for the Baum-Yang 2011 Severe roughened (SR) optical properties model. Arrows represent increasing COT values (tcloud increase from 0 to 4), and Deff values (10 to 120 mm). (b) Comparison between two calculated LUTs based on Baum et al. [2011] SR model (green shaded area) and Smooth faceted ice crystal model (magenta shaded area). These models represent a general habit mixture (GHM) that includes all ice crystal particle size distributions and shapes measured by in-situ instruments.

Figure 5: The effect of aerosol layer below clouds on retrieval: a Two component LUTsincluding Baum et al. [2011] cirrus SR model with varying amounts of absorbing aerosol model (OPAC Urban model)

ReferencesBaum B.A., P. Yang, A. J. Heymsfield, C. G. Schmitt, Y. Xie, A. Bansemer, Y. X. Hu ,and Z. Zhang, (2011), Improvements in Shortwave Bulk Scattering and Absorption Modelsfor the Remote Sensing of Ice Clouds, J. Appl. Meteorol. Clim., 50, 1037-1056Holben, B.N., D. Tanré, A. Smirnov, T.F. Eck, I. Slutsker, B Chatenet, F. Lavenue,Y. Kaufman, J.V. Castle, A. Setzer, B. Markham, D. Clark, R. Frouin, N A. Karneli. O'Neill, C. Pietras, R. Pinker, K. Voss, G. Zibordi, (2001), An emerging ground-based aerosol climatology: Aerosol Optical Depth from AERONET, J. Geophys. Res., 106, 12067-12097Jacob D.J., J. H. Crawford, H. Maring, A. D. Clarke, J. E. Dibb, L. K. Emmons, R. A. Ferrare, C. A. Hostetler,P. B. Russell, H. B. Singh, A. M. Thompson, G. E. Shaw, E. McCauley, J. R. Pederson, and J. A. Fisher, (2010), The Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) mission: design, execution, and first results, Atmos. Chem. Phys., 10, 5191–5212Kinne S., T. P. Akerman, M. Shiobara, A. Uchiyama, A. J. Heymsfield, L. Miloshevich, J. Wendell, E. W. Eloranta, C. Purgold, and R. W. Bergstrom (1997), Cirrus Cloud Radiative and Microphysical Properties from Ground Observations and In Situ Measurements during FIRE 1991 and Their Application to Exhibit Problems in Cirrus Solar Radiative Transfer Modeling, J. of Atm. Science,54,2320-2341Nakajima, T. and King, M. D., (1990), Determination of the optical thickness and effective particle radius of clouds from reflected solar radiation measurements. Part I: Theory, J. Atmos. Sci., 47, 1878–1893Russell, P. B., et al., (2005), Aerosol optical depth measurements by airborne Sunphotometer in SOLVE II: Comparisons to SAGE III, POAM III and airborne spectrometer measurements, Atmos. Chem. Phys., 5, 1311 –1339, SRef-ID:1680-7324/acp/2005-5-1311Shiobara M., and Asano S.,(1994), Estimation of Cirrus optical thickness from sun-photometer measurements, J. of Appl. Meteorology,33, 672-681

Table 1: Summary statistics for the ARCTAS case retrievals using the different models

Model used in retrieval Valid retrievals (#)

¥COT £Deff

[m] Retrieved COT/

AATS(COT+AOD) Smooth 88 0.06-0.66 (0.20) 50-115 (80) 1.65-2.50 Smooth w. smoke 25 0.06-0.78 (0.14) 10-65 (15) 0.69-2.10 SR (9w) 160 0.04-3.70 (0.18) 30-120 (100) 1.13-1.97 SR w. smoke (9w) 44 0.10-1.60 (0.57) 10-65 (40) 0.87-1.82 SR (6w) 189 0.02-3.70 (0.18) 10-120 (80) 0.60-1.97 SR w. smoke (6w) 163 0.02-3.60 (0.14) 10-85 (25) 0.40-1.92 SR (2w) 203 0.04-3.70 (0.22) 20-120 (90) 0.99-1.98 SR w. smoke (2w) 190 0.02-3.60 (0.14) 10-115 (25) 0.40-1.92 MLT 156 0.04-3.70 (0.18) 40-120 (120) 1.13-1.96 MLT w. smoke 46 0.08-1.00 (0.55) 10-85 (55) 0.73-1.76 ASC 149 0.04-3.70 (0.16) 10-120 (95) 1.10-1.96 ASC w. smoke 35 0.38-1.00 (0.56) 10-85 (30) 1.25-1.76 ¥ - COT retrieved values are given as range (min-max). Values in parenthesis are the median values. £ - ice particle effective diameter retrieved values are given as range (min-max). Values in parenthesis are the median values.

Figure 6: July 9th 2008 ARCTAS case valid retrieved values of (a) cirrus optical depth for the Baum-Yang 2011 Smooth model (solid blue circles), Baum-Yang 2011 SR model (solid red circles), a two component model including the Baum-Yang 2011 Smooth model and ARCTAS smoke aerosol model with constant AOD value of 0.05 (solid cyan circles), a two component model including the Baum-Yang 2011 SR model and ARCTAS smoke aerosol model with constant AOD value of 0.05 (solid green circles), and AATS total retrieved optical depth (which includes both COT and AOD at l=500 nm) for the corresponding cloud-flagged cases (open black circles), and AQUA-MODIS thin cirrus OT (solid grey circles), (b) same as in (a) but for ice particle effective diameter values

We implement our retrieval scheme on data taken by AATS-14 in July 9th 2008 during the ARCTAS mission [Jacob et al., 2010] on-board of the NASA P-3B aircraft. We compare the various cirrus optical models, two component model (cirrus and smoke aerosol layer) and wavelength-dependent scheme in Figures 6, 7 and Table 1.

Figure 7: Sample spectra of measured and modeled transmittance values of (a) wavelength-dependent retrieval results comparing measured spectrum (light blue circles) with retrieved spectra resulting from 9w retrieval (solid green circles with magenta rectangles), 6w retrieval (solid grey circles with black rectangles), and two wavelengths retrieval (solid red circles with red rectangles showing wavelengths used in the retrieval), (b) valid retrieval case of Deff=120mm with 2011 SR model as best fit, (c) non-valid retrieval case of Deff=120mm, (d) valid retrieval case of SR+smoke, (e) valid retrieval case of SR and SR+smoke models.

Figure 2: Cirrus scattering phase-function dependent pre-factor k (Eq. 2) calculated based on Baum and Yang 2011 (SR) ice crystals model (for half-FOV of 1.85 o and Different Deff).

Summary and ConclusionsIn the present work we propose a method to derive cirrus COT and ice particle effective diameter using the total measured irradiance below cirrus clouds measured by sunphotometers. Under cirrus, the measured irradiance includes both the direct transmitted term and the forward scattered one. This additional information, analyzed over a broad wavelength range (visible to SWIR spectral region), permits the derivation of spectral relationships that allow retrieving COT and effective diameters for a specific set of cirrus optical properties.

We found that the Severe Roughened (SR) cirrus optical model produces more valid retrievals for a larger COT range, with smaller ice particle effective diameter retrieved for the smaller COT and vice versa for the larger COT.

Testing the retrieval scheme with different sets of wavelengths we found that the two-wavelength scheme (visible and SWIR channels), as is the standard for many cloud retrieval schemes to-date is less constrained and can result in spectral mismatches when examined over the whole measurement spectral range.

Comparing the LUTs of the various optical properties cirrus models we find that there is less variation among the mixed habit models (GHM, MLT) than between the mixed habits and the severe roughened aggregate of solid columns (ASC) particle model.

We also concluded that the effect of an aerosol layer between the instrument and the cloud layer proves to be an important variable towards a more accurate retrieval, especially for optically thin cirrus layers (tc<1.0). For this, we show that the clear sky data measured by the sunphotometer can be used for the selection of the appropriate aerosol layer model.

From the analysis made so far we could not decide on the appropriate cirrus model for sunphotometers and conclude that this choice should be based on climatological knowledge from in-situ measurements that can characterize the most probable particle type or mixture for a specific region. This is true for our scheme and for more global schemes of retrievals from space.

Retrieval of Cirrus Properties by Sunphotometry:A New Perspective on an Old Issue

M. Segal-Rosenheimer1, P.B. Russell1, J. M. Livingston2, S. Ramachandran3, J. Redemann1, B.A. Baum4

1NASA Ames Research Center, Moffett Field, CA (michal.segalrozenhaimer@nasa.gov), 2SRI International, Menlo Park, CA,3Physical Research Laboratory, Ahmedabad, India, 4Space Science and Engineering Center, University of Wisconsin-Madison, Madison, WI,USA