Homework due on Tuesday. Then we will discuss weighting functions for satellite and ground based remote sensing
of the atmosphere at infrared wavelengths.
(read Chapters 6, 8, 9, and 10 in Petty; chapter 4 of Thomas and Stamnes.)
We will set up the next set of problems. Preview available from this training course. Site for blackbody calculations. Atmospheric IR calculator.
Results from problem 3, HW 6. (Note: this solution leaves out the dependence of line strength on temperature, and makes use of the simple exponential (isothermal) atmosphere.) (Here is my solution.)
week 14: 28 Nov 11
Marce discusses her research on aerosol optics on Tuesday. Thursday we will discuss IR weighting functions in preparation for the next homework assignment. Discuss the final exam too.
New anthropogenic radiative forcing history theory.
week 13: 21 Nov 11
Bring questions on the homework to class on Tuesday.
Preview these thoughts on the recent fire. Comments?
We will work on infrared radiative transfer (read Chapters 6, 8, 9, and 10 in Petty; chapter 4 of Thomas and Stamnes.) Statistical Mechanics and Infrared Radiation Transfer are coming together here at the end of the semester. The next homework assignments will involve infrared radiation transfer. Homework 6 has been posted.
week 12: 14 Nov 11
We will work on infrared radiative transfer (read Chapters 6, 8, 9, and 10 in Petty; chapter 4 of Thomas and Stamnes.) Statistical Mechanics and Infrared Radiation Transfer are coming together here at the end of the semester. The next homework assignments will involve infrared radiation transfer. Homework 6 has been posted.
Rayleigh-Jeans approximation: good for microwave remote sensing.
(The chlorine reaction figure was taken from here.)
Review the homework results and hand back the homework. In particular, emphasize the remote sensing opportunity for looking at water cloud reflectivity at non and absorbing wavelengths, to get a measure of the aerosol indirect effect. Discussion: Homework 3 (small ice crystals) and homework 4 (aerosol indirect effect) actually have a lot in common -- both look at the effects of cloud microphysics on cloud albedo.
Homework 5 due on Thursday (will complete the presentation (Presentation for chapter 12 of Petty). Then we will work on infrared radiative transfer (read Chapters 6, 8, 9, and 10 in Petty; chapter 4 of Thomas and Stamnes.) Statistical Mechanics and Infrared Radiation Transfer are coming together here at the end of the semester. The next homework assignments will involve infrared radiation transfer.
Moon corona picture:
Jumping sun dog!!!
week 10: 1 Nov 11
READ CHAPTERS 11 and 12 of PETTY: Also can read chapters 2 through 4 of Thomas and Stamnes.
We will start with the broad perspective of the general equation for radiative transfer, and connect with our study of the 2 stream solution. We will see exactly where absorption, emission, and scattering play roles in radiative transfer in general. (Class lecture notes for chapter 11 of Petty).
Then we will look in detail at molecular scattering, i.e. Rayleigh scattering, and further develop our understanding of this important topic. Then we will look at scattering by particles, especially in the geometrical optics regime, where we will study atmospheric optics associated with the rainbow and the glory. Rainbows are associated with optics of raindrops, and glories are associated with backscattering by cloud droplets. Homework 5 is devoted to this subject. (Presentation for chapter 12 of Petty). Also, it would be good to read section 4.3 of Petty for the rainbow discussion found there.
After this section we will look at absorption of radiation by gases in the IR, and move our attention to the infrared portion of the spectrum.
Awesome website showing development of the rainbow and other atmospheric optics.
week 9: 24 Oct 11
Bring homework questions to class if you have any. New photos in the student section...
Practice with irradiance measurements from around the world. Look especially for the downwelling IR at cold places and warm places.
This section will use the presentation for on Atmospheric Transmission (based loosely on Chapter 7 of Petty's book.)
Continue reading the paper below for the next homework and discussion, relationship of cloud albedo and optical depth.
Also, it's good to look at the original papers by Sean Twomey concerning the effect of pollution on cloud albedo. It is called the Twomey effect!
The Influence of Pollution on the Shortwave Albedo of Clouds. Twomey, S. Journal of Atmospheric Sciences, vol. 34, Issue 7, pp.1149-1154. ABSTRACT: By increasing droplet concentration and thereby the optical thickness of a cloud, pollution acts to increase the reflectance (albedo) of clouds; by increasing the absorption coefficient it acts to decrease the reflectance. Calculations suggest that the former effect (brightening of the clouds in reflection, hence climatically a cooling effect) dominates for thin to moderately thick clouds, whereas for sufficiently thick clouds the latter effect (climatically a warming effect) can become dominant.
Twomey, S. (1991): "Aerosols, Clouds, and Radiation". Atmospheric Environment Vol. 25A, No. 11, pp. 2435-2442. ABSTRACT: Most of the so-called 'CO 2 effect' is, in fact, an 'H20 effect' brought into play by the climate modeler's assumption that planetary average temperature dictates water-vapor concentration (following Clapeyron--Clausius). That assumption ignores the removal process, which cloud physicists know to be influenced by the aerosol, since the latter primarily controls cloud droplet number and size. Droplet number and size are also influential for shortwave (solar) energy. The reflectance of many thin to moderately thick clouds changes when nuclei concentrations change and make shortwave albedo susceptible to aerosol influence.
Figure 7: From the Twomey 1991 publication, cloud susceptibility to modification by aerosol as a function of CCN and cloud reflectivity.
Yangang Liu and Peter H Daum: NATURE | VOL 419 | 10 OCTOBER 2002. Anthropogenic aerosols: Indirect warming effect from dispersion forcing. ABSTRACT: Anthropogenic aerosols enhance cloud reflectivity by increasing the number concentration of cloud droplets, leading to a cooling effect on climate that is referred to as the Twomey effect1,2. Here we show that anthropogenic aerosols exert an additional effect on cloud properties that is derived from changes in the spectral shape of the size distribution of cloud droplets in polluted air and acts to diminish this cooling. This finding could help to improve our understanding of the indirect aerosol effect and its treatment in climate modelling. CHIEF MESSAGE: According to equations (1) and (2), an increase in acts to negate the effect of increased N on effective radius and cloud reflectivity. Because this effect has been largely neglected in estimates of the indirect aerosol effect, cooling by an indirect aerosol effect is likely to have been overestimated.
Yangang Liu, Peter H Daum, Huan Guo, and Yiran Peng (2008). Dispersion bias, dispersion effect, and the aerosol–cloud conundrum. Environ. Res. Lett. 3 (2008) 045021 (8pp). ABSTRACT: This work examines the influences of relative dispersion (the ratio of the standard deviation to the mean radius of the cloud droplet size distribution) on cloud albedo and cloud radiative forcing, derives an analytical formulation that accounts explicitly for the contribution from droplet concentration and relative dispersion, and presents a new approach to parameterize relative dispersion in climate models. It is shown that inadequate representation of relative dispersion in climate models leads to an overestimation of cloud albedo, resulting in a negative bias of global mean shortwave cloud radiative forcing that can be comparable to the warming caused by doubling CO2 in magnitude, and that this dispersion bias is likely near its maximum for ambient clouds. Relative dispersion is empirically expressed as a function of the quotient between cloud liquid water content and droplet concentration (i.e., water per droplet), yielding an analytical formulation for the first aerosol indirect effect. Further analysis of the new expression reveals that the dispersion effect not only offsets the cooling from the Twomey effect, but is also proportional to the Twomey effect in magnitude. These results suggest that unrealistic representation of relative dispersion in cloud parameterization in general, and evaluation of aerosol indirect effects in particular, is at least in part responsible for several outstanding puzzles of the aerosol–cloud conundrum: for example, overestimation of cloud radiative cooling by climate models compared to satellite observations; large uncertainty and discrepancy in estimates of the aerosol indirect effect; and the lack of interhemispheric difference in cloud albedo.
Recent paper (2011) on optical depth relations: Brenguier, J.-L., Burnet, F., and Geoffroy, O.: Cloud optical thickness and liquid water path – does the k coefficient vary with droplet concentration?, Atmos. Chem. Phys., 11, 9771-9786, doi:10.5194/acp-11-9771-2011. ABSTRACT: Cloud radiative transfer calculations in general circulation models involve a link between cloud microphysical and optical properties. Indeed, the liquid water content expresses as a function of the mean volume droplet radius, while the light extinction is a function of their mean surface radius. There is a small difference between these two parameters because of the droplet spectrum width. This issue has been addressed by introducing an empirical multiplying correction factor to the droplet concentration. Analysis of in situ sampled data, however, revealed that the correction factor decreases when the concentration increases, hence partially mitigating the aerosol indirect effect.
Han, Q., W.B. Rossow, J. Chou, and R.M. Welch, 1998: Global survey of the relationships of cloud albedo and liquid water path with droplet size using ISCCP. J. Climate, 11, 1516-1528. (see their website too.) ABSTRACT: "The most common approach used to model the aerosol indirect effect on clouds holds the cloud liquid water path constant. In this case, increasing aerosol increases droplet concentration, decreases cloud droplet size, and increases cloud albedo. The expected decrease in cloud droplet size associated with larger aerosol concentrations has been found to be larger over land than over water and larger in the Northern than in the Southern Hemisphere, but the corresponding cloud albedo increase has not been found. Many previous studies have shown that cloud liquid water path varies with changing cloud droplet size, which may alter the behavior of clouds when aerosols change. This study examines the relationship between geographic and seasonal variations of cloud effective droplet size and cloud albedo, as well as cloud liquid water path, in low-level clouds using International Satellite Cloud Climatology Project data. The results show that cloud albedo increases with decreasing droplet szie for most clouds over continental areas and for all optically thicker clouds, but that cloud albedo decreases with decreasing droplet size for optically thinner clouds over most oceans and the tropical rain forest regions. For almost all clouds, the liquid water path increases with increasing cloud droplet size. "
V. Dallas and J. C. Vassilicos (2011): "Rapid growth of cloud droplets by turbulence" PHYSICAL REVIEW E 84, 046315 (2011). ABSTRACT: Assuming perfect collision efficiency, we demonstrate that turbulence can initiate and sustain the rapid growth of very small water droplets in air even when these droplets are too small to cluster, and even without having to take gravity and small-scale intermittency into account. This is because the range of local Stokes numbers of identical droplets in the turbulent flow field is broad enough even when small-scale intermittency is neglected. This demonstration is given for turbulence which is one order of magnitude less intense than is typical in warm clouds but with a volume fraction which, even though small, is nevertheless large enough for an estimated a priori frequency of collisions to be ten times larger than in warm clouds. However, the time of growth in these conditions turns out to be one order of magnitude smaller than in warm clouds.
week 8: 17 Oct 11
Bring homework questions to class if you have any.
1. Quiz. Under what circumstances does the absorption coefficient scale inversely with wavelength? Significance of this?
This section will use the presentation for on Atmospheric Transmission (based loosely on Chapter 7 of Petty's book.)
Begin reading the paper for the next homework and discussion, relationship of cloud albedo and optical depth.
Han, Q., W.B. Rossow, J. Chou, and R.M. Welch, 1998: Global survey of the relationships of cloud albedo and liquid water path with droplet size using ISCCP. J. Climate, 11, 1516-1528. (see their website too.) ABSTRACT: "The most common approach used to model the aerosol indirect effect on clouds holds the cloud liquid water path constant. In this case, increasing aerosol increases droplet concentration, decreases cloud droplet size, and increases cloud albedo. The expected decrease in cloud droplet size associated with larger aerosol concentrations has been found to be larger over land than over water and larger in the Northern than in the Southern Hemisphere, but the corresponding cloud albedo increase has not been found. Many previous studies have shown that cloud liquid water path varies with changing cloud droplet size, which may alter the behavior of clouds when aerosols change. This study examines the relationship between geographic and seasonal variations of cloud effective droplet size and cloud albedo, as well as cloud liquid water path, in low-level clouds using International Satellite Cloud Climatology Project data. The results show that cloud albedo increases with decreasing droplet szie for most clouds over continental areas and for all optically thicker clouds, but that cloud albedo decreases with decreasing droplet size for optically thinner clouds over most oceans and the tropical rain forest regions. For almost all clouds, the liquid water path increases with increasing cloud droplet size. "
Supplemental reading: Chapter 13 of Petty; Chapters 3 and 7 of Thomas and Stamnes.
week 7: 10 Oct 11
Bring homework questions to class if you have any.
Demonstrations:
1. Multiple scattering by laboratory water droplet cloud at several wavelengths.
2. Diffraction of light by small objects and diffraction gratings.
3. Color of clouds when single scattering dominates (cloud irridescence due to diffraction).
4. Optics of dilute and concentrated milk: Turn milk blue!
"The most common approach used to model the aerosol indirect effect on clouds holds the cloud liquid water path constant. In this case, increasing aerosol increases droplet concentration, decreases cloud droplet size, and increases cloud albedo. The expected decrease in cloud droplet size associated with larger aerosol concentrations has been found to be larger over land than over water and larger in the Northern than in the Southern Hemisphere, but the corresponding cloud albedo increase has not been found. Many previous studies have shown that cloud liquid water path varies with changing cloud droplet size, which may alter the behavior of clouds when aerosols change. This study examines the relationship between geographic and seasonal variations of cloud effective droplet size and cloud albedo, as well as cloud liquid water path, in low-level clouds using International Satellite Cloud Climatology Project data. The results show that cloud albedo increases with decreasing droplet szie for most clouds over continental areas and for all optically thicker clouds, but that cloud albedo decreases with decreasing droplet size for optically thinner clouds over most oceans and the tropical rain forest regions. For almost all clouds, the liquid water path increases with increasing cloud droplet size. "
Supplemental reading: Chapter 13 of Petty; Chapters 3 and 7 of Thomas and Stamnes.
COMING UP: DRAFT!!! of homework 4 on the aerosol indirect effect. EMPHASIZE DRAFT!!! More to be added soon.
This section will use the presentation for Chapter 7 of Petty's book.
week 6: 3 Oct 11
Review: (see below). Practice with irradiance measurements from around the world.
Begin reading An excellent paper on multiple scattering, Craig F. Bohren.
Multiple scattering of light and some of its observable consequences.
American Journal of Physics, 55(6):524--533, June 1987. We will use this
paper for developing a strong intuition for multiple scattering.
Homework. Chapter 2 and homework primer: gas optical depth discussion for problem 3.
(Need to also present cloud optical depth in the geometrical optics limit for problem 2.)
Develop real and imaginary parts of the refractive index.
Define and work with radiance and irradiance.
Chapter 2 presentation.
Sound propagation in the atmosphere and ocean presentation. Ray path theory.
from http://www.kettering.edu/physics/drussell/Demos/refract/refract.html .
week 3: 12 Sep 11
Homework. Chapter 2 and homework primer: gas optical depth discussion for problem 3.
(Need to also present cloud optical depth in the geometrical optics limit for problem 2.) Chapter 2 presentation. Continue with Chapter 2 on basic radiative quantities.
Recommend reading Chapter 2 of Petty also.
Molecular Spectroscopy Database
High Resolution Spectral Modeling using HITRAN
Atlas of UV and Visible Absorption Spectra and Quantum Yield
"HITRAN is an acronym for high-resolution transmission molecular absorption database."
Additonal reading: Arnott's IR radiative transfer notes. An excellent paper on multiple scattering, Craig F. Bohren.
Multiple scattering of light and some of its observable consequences.
American Journal of Physics, 55(6):524--533, June 1987.
