# Presentation: 125592

##  Development of Ice Cloud Microphysical and Optical Models

**Ping Yang****2****, Andrew Heymsfield****3**
**Steve Platnick****4**** and Michael King****4**
-           1 NASA Langley Research Center, Hampton, VA
-           2 Texas A&M University, College Station, TX
-           3 National Center for Atmospheric Research, Boulder, CO
-           4 NASA Goddard Space Flight Center, Greenbelt, MD

** ****Development of Ice Cloud Microphysical and Optical Models**
**for a Variety of Imagers and Interferometers**

***MODIS Science Team Meeting***
***       ******March 22- 24, 2005***

## Bulk Ice Scattering Models

- Incorporate
- 	- latest computational light scattering research
- 	- *in situ* data from multiple field campaigns
- Develop a more comprehensive set of ice scattering models
- Develop similar models for a variety of imagers, interferometers, and other sensors
- Facilitate intercomparison of retrieved ice cloud properties from multiple sensors
- Recent articles awaiting final decision at the *Journal of Applied Meteorology*
*Baum, B. A., A. J. Heymsfield, P. Yang, and S. Thomas: Bulk scattering models for the remote sensing of ice clouds. 1: Microphysical data and models. *
*Baum, B. A., P. Yang, A. J. Heymsfield, S. Platnick, M. D. King, Y.-X. Hu, and S. Thomas: Bulk scattering models for the remote sensing of ice clouds. 2: Narrowband models.**** ***

##           N(D) = NoDe-D

***Gamma size distribution* has the form:***
***          ******N(D) = N******o******D************e******-************D***
***where******    ******D = max diameter***
***	******N******o****** = intercept***
***	************ ******= dispersion***
***	************= slope***
***The intercept, slope, and dispersion values are derived for each PSD by matching three moments (specifically, the 1st, 2nd, and 6th moments)  ***
***Note: when ************ = 0, the PSD reduces to an exponential distribution***
********Heymsfield et al., Observations and parameterizations of particle size distributions in deep tropical cirrus and stratiform precipitating clouds: Results from in situ observations in TRMM field campaigns. J. Atmos. Sci., 59, 3457-3491, 2002.**	*

**Notes:**
This paper by Andy Heymsfield greatly simplified our lives, because instead of trying to analyze all his spectra (a very complex process), he simply provided us with a set of gamma distribution coefficients for all these measurements he’s been taking over the past couple of decades. This really is a fortunate confluence of events. 

## Field Campaign Information

| Field Campaign | Location | Instruments | Number of PSDs |
| --- | --- | --- | --- |
| FIRE-1 (1986) | Madison, WI | 2D-C, 2D-P | 246 |
| FIRE-II (1991) | Coffeyville, KS | Replicator | 22 |
| ARM-IOP (2000) | Lamont, OK | 2D-C, 2D-P, CPI | 390 |
| TRMM KWAJEX (1999) | Kwajalein, Marshall Islands | 2D-C, HVPS, CPI | 418 |
| CRYSTAL-FACE (2002) | Off coast of Nicaragua | 2D-C, VIPS | 41 |

- Probe size ranges are: 2D-C, 40-1000 m; 2D-P, 200-6400 m; HVPS (High Volume Precipitation Spectrometer), 200–6100 m; CPI (Cloud Particle Imager), 20-2000 m; Replicator, 10-800 m; VIPS (Video Ice Particle Sampler): 20-200 m.

**Notes:**
We filtered Andy’s PSD data by cloud temperature; only PSDs with temperatures < -25C were kept to ensure that we were looking at ice clouds.We ended up with 1117 PSDs. Hopefully we will be able to include more PSDs as time goes on.

## Particle Size Distributions

**Particle Size Distributions**

***Midlatitude cirrus characteristics***
*** ******Size sorting more pronounced***
*** ******Small crystals at cloud top***
*** ******More often find pristine particles***
***Tropical cirrus anvil characteristics***
*** ******Form in an environment having much higher vertical velocities***
*** ******Size sorting is not as well pronounced***
*** ******Large crystals often present at cloud top***
*** ******Crystals may approach cm in size.***
*** ******Habits tend to be more complex***
-  Note that CRYSTAL distributions tend to be the narrowest overall

**Notes:**
Here we’re showing 5 of the individual PSDs from our set of 1117 total. Note that the TRMM-KWAJEX PSD seems to look more like an exponential distribution whereas the others tend to look more like the classical gamma distribution. The accuracy of the number of particles at sizes less than 20 microns is suspect since these data were acquired without the use of a small ice particle probe.

## Library of Scattering Properties

**In 0.4-13 ********m Spectral Region**

**Ice particle habits:**
**	**Hexagonal plates
- 	Solid and hollow columns
- 	Aggregates
- 	Droxtals
- 	3D bullet rosettes
**45 size bins ranging from 2 to 9500 ********m**
**Spectral range: **
- 	0.4 to 1.00 m at 0.01-m resolution
- 	1.20 to 1.70 m at 0.01-m resolution
- 	1.80 to 1.95 m at 0.01-m resolution
- 	2.05 to 2.20 m at 0.01-m resolution
- 	3.45 to 4.05 m at 0.05-m resolution
- 	8.10 to 8.95 m at 0.05-m resolution
- 	10 to 13 m at 0.05-m resolution
- Properties for each habit/size bin include volume, projected area,
- maximum dimension, single-scattering albedo, asymmetry factor,
- scattering phase functions, and scattering/extinction cross sections**  **

**Notes:**
Now that we have some detailed particle size distributions to work with, we need to address the idealized particle habits. This slide summarizes what Ping Yang developed for our library.

## Simulated Particle Habits

***Simulated Particle Habits***

***Replicator Particle Habits***

**Notes:**
On the left, one sees visual images of ice crystals as a function of height within the cirrus as captured by the replicator. On the right are presented the “pristine” ice crystal shapes that we are able to simulate for the purposes of light scattering calculations. The new  type of particle we have recently added is the droxtal, which is shaped a bit like a faceted soccer ball. We think that his shape better represents the very small particles than very small columnar crystals or spheres. 

Also, the aggregrate particle is used for the crystals near cloud base, but we are rethinking this issue based on some new work by Andy Heymsfield. More on this later. We are now  beginning to think that the bullet rosette is a more realistic shape for large crystals.

## Ice Particle Habit Distribution

**At this point, we have**
**	****- ice particle scattering library**
**	****- wealth of microphysical data for ice clouds (1117 PSDs)**
**Next issue: develop a ice particle habit distribution that makes sense**
**Note: each idealized ice particle has a prescribed volume, and hence mass**
**Compare *****IWC***** and *****D******m***** computed from integrating over the habit and size distributions to those values estimated from techniques developed by Heymsfield and colleagues from analyses of their *****in situ***** data**

**Notes:**
Now that we have some detailed particle size distributions to work with, we need to address the particle habit distributions. 

##  Ice Water Content and Median Mass Diameter

** ****Ice Water Content and Median Mass Diameter**

**Notes:**
If one makes the assumption that an ice cloud is composed of a single habit and integrates over PSD, this is what ensues. The IWC and Dmm values labeled as “in situ” are those inferred from the data by Andy Heymsfield. The values labeled as “calculated” are derived from the ice particle library based on Ping Yang’s particles and aspect ratios. Andy tells me that his values of IWC are probably good to 20-50% accuracy, maybe better, maybe less, so we don’t want to try to match his results too closely. 

## Guidelines

- 4 size domains defined by particle maximum length
- Droxtals: used only for smallest particles
- Aggregates: only for particles > 1000 ****m
- Plates: used only for particles of intermediate size

**Ice Particle Habit Percentages Based on Comparison of Calculated to In-situ *****D******m***** and *****IWC***

**Proposed ice particle**
**habit mixture**
**Max length < 60 ********m**
- 100% droxtals
**60 ********m < Max length < 1000 ********m**
-  15% bullet rosettes
-  35% hexagonal plates
-  50% solid columns
**1000 ********m  < Max length < 2500 ********m**
- 45% solid columns
- 45% hollow columns
- 10% aggregates
**Max length > 2500 ********m**
- 97% bullet rosettes
-  3% aggregates

**Notes:**
To keep things straightforward, I set up a few guidelines and then fussed around until I arrived at the mixture above. 

## Band-averaged Scattering Properties

- 	a. PSD
- 	b. habit distribution
- 	c. spectral response function

**Band-averaged Scattering Properties**

**Notes:**
Take the next step - to derive a band model for any specific imaging instrument, we discretize the spectral response function for a given band into discrete wavelengths and
Integrate over (a) PSD, (b) habit, and (c) spectral response function weights. 

## MODIS Band-averaged 

**MODIS Band-averaged **
**Single Scattering Albedo**

- For 3 MODIS bands, comparing
- - MODIS V1 models
- - Single habit assumption of solid hexagonal columns
- - Habit mixture
- Habit does not seem to be a factor for smaller *D**eff  *but does for larger *D**eff *
- Several MODIS V1 models are out of range

**Notes:**
For 3 MODIS bands, this shows the single scattering albedo for two different assumptions of habit (mixture and hex columns only), as well as the 12 MODIS V1 models. 

## MODIS Band-averaged Asymmetry Factor

**MODIS Band-averaged Asymmetry Factor**

**Notes:**
For 4 MODIS bands, this shows the asymmetry factor for two different assumptions of habit (mixture and hex columns only), as well as the 12 MODIS V1 models. 

## Small Particle Sensitivity Study

**Small Particle Sensitivity Study**

- Note that CRYSTAL distributions tend to be the narrowest overall
- For all 41 CRYSTAL size distributions, increase the number of particles with *D**max* < 20 m by a factor of 100 or 1000

**Notes:**
On the previous slides, we noted that several of the V1 models have smaller effective diameters than any of the 1117 PSDs. To exolore why, we simply took the CRYSTAL PSDs, since they have the narrowest size distributions, and really ramped up the number of small particles.  

## Slide 15

**Notes:**
On the left, the original figure. On the right, the PSDs in green and red are the results obtained from the modified CRYSTAL PSDs. I can replicate the MODIS V1 model values for each of the bands.
Is this realistic? Probably not - but if we have some measurements of small ice particles, we can narrow this down a bit.

## Slide 16

**Notes:**
Again, by artificially increasing the number of only the smallest particles by orders of magnitude, we can replicate the behavior of the V1 models for asymmetry factor, too.
Note here that the TRMM-KWAJEX data tend to have lower values than those from the other campaigns. 

## Need discrete set of scattering models upon which to base look-up table and associated radiative transfer calculations

- Suggestion that models be evenly spaced in effective diameter

**Development of Band Models**

**Notes:**
Now we’re taking a further step in developing a discrete set of band models for a suite of imagers. That is, we’re taking the results from more than 1000 PSDs and simplifying to a discrete set of about 18 effective diameter models for each imager band.

## Slide 18

**Notes:**
As an example, here’s the asymmetry factor for a single MODIS band, with all 1117 PSDs shown.

## Slide 19

**Notes:**
Here’s a subset of the data from the previous slide - we choose a target effective diameter and average the results from between 10 and 20 individual PSDs that
surround the target diameter. So we can arrive at some mean and standard deviation of each scattering property.

## Ice Scattering Models

**Currently Available**

- Provide microphysical and scattering properties (mean and std. dev.) at *D**eff *from 10 m to 180 m, in increments of 10 m, for
- 	*IWC* 			*D**m*
- 	*Volume**	**	**	**Projected area*
*	**asymmetry factor**	**	**scattering phase function (498 angles)*
*	**single scattering albedo**	**extinction efficiency* */ cross section*
- Models available at ***http://www.ssec.wisc.edu/~baum*** for
- Narrowband imagers:
- 	MODIS		AVHRR		MISR
- 	AATSR		MAS		VIRS
- 	GOES-R Advanced Baseline Imager (ABI)
- 	MSG SEVIRI (Spinning Enhanced Visible InfraRed Imager)
- MWIR/IR/FarIR models: 100 cm-1 to 3250 cm-1 at 1 cm-1 resolution
- Models with full phase matrix (for polarization measurements) available at 7 wavelengths from 0.35 m to 2.1 m

**Notes:**
This is what we provide for each model...