Assimilation of Infrared Radiance Observations

1. Assimilation of Infrared Radiance Observations Will McCarty Global Modeling and Assimilation OfficeNASA Goddard Space Flight Center 2015 JCSDA Summer Colloquium 28 July 2015

2. What is Infrared Radiation?

3. What is Infrared Radiation?

4. What is Infrared Radiation in Atmospheric Data Assimilation?

5. What is Infrared Radiation in Atmospheric Data Assimilation?

6. What is Infrared Radiation? In General • Anything between the visible and microwave What do we use in data assimilation • Primarily the parts of the IR that are emitted from the earth • As our observations approach the middle-IR, there is a region of overlap thatconsists of a mix of solar reflection and terrestrial emission (arrow)

7. Earth-Emitted Radiation First things first – Units • In the infrared, particularly to the sounding community, units of wavenumber in cm-1 are traditionally used • To the imaging community, units of wavelength in microns (micrometers) are typically used • I have a bad habit of swapping back and forth on the fly • If I start to just throw out numbers, call me out • I will try to keep things somewhat generic by absorber

8. Earth-Emitted Radiation The theoretical emission of the earth is at about 280 K • Warmer than what is expected value from the sun’s directly • greenhouse effect In some regions, the actual emission is less than expected • This is where absorption is occurring • What is seen is the cool top of the greenhouse effect blanket Theoretical Emission as a function of T Measured Emission

9. Earth-Emitted Radiation Major Absorbing Constituents • Carbon Dioxide CO2 Theoretical Emission as a function of T Measured Emission

10. Earth-Emitted Radiation Major Absorbing Constituents • Carbon Dioxide Note: From a weather perspective, we consider CO2 constant and well mixed over short periods. Therefore, temperature is determined from CO2 CO2 Theoretical Emission as a function of T Measured Emission

11. Earth-Emitted Radiation Major Absorbing Constituents • Carbon Dioxide • Water Vapor H2Ov CO2 H2Ov Theoretical Emission as a function of T Measured Emission

12. Earth-Emitted Radiation Major Absorbing Constituents • Carbon Dioxide • Water Vapor • Ozone H2Ov CO2 O3 H2Ov Theoretical Emission as a function of T Measured Emission

13. Earth-Emitted Radiation Major Absorbing Constituents • Carbon Dioxide • Water Vapor • Ozone • Methane H2Ov CH4 CO2 O3 H2Ov Theoretical Emission as a function of T Measured Emission

14. Earth-Emitted Radiation Atmospheric Windows occur in regions of little absorption • Surface-sensitivity H2Ov Window Window CH4 CO2 O3 H2Ov Theoretical Emission as a function of T Measured Emission

15. Back to Simple Radiative Transfer In the previous talk, I referred to radiation as being effected by reflection, absorption, and transmission • Absorption is key in the infrared • The surface reflects, and will be discussed w/ surface emissivity • The atmosphere in the IR generally only scatters with certain-sized particles So how does absorption work? • Quantum physics • Molecules absorb radiation, and re-emit radiation

16. Absorption at the Molecular Level Molecules absorb in electronic, vibrational, and rotational modes

17. Absorption Coefficient The absorption coefficient is a complicated and highly non-linear function of molecule i and line j Line Strengths, Sij, result from many molecular vibrational-rotational transitions of different molecular species and isotopes of those species(blue). Where width of line, ij, is a function of the molecule structure (natural broadening), temperature (doppler broadening) and pressure (collisional broadening)

18. Line Strengths @ 15 μm 600 to 700 cm-1 700 to 800 cm-1 H2O CO2 O3 N2O CO CH4 HNO3 OCS SO2 16.6 to 14.3 μm 14.3 to 12.5 μm

19. Line Strengths @ 10 μm 900 to 1000 cm-1 1000 to 1100 cm-1 H2O CO2 O3 N2O CO CH4 HNO3 OCS SO2 11.1 to 10 μm 10 to 9.1 μm

20. Line Strengths @ 6μm 1250 to 1350 cm-1 1350 to 1450 cm-1 H2O CO2 O3 N2O CO CH4 HNO3 OCS SO2 8.0 to 7.4 μm 7.4 to 6.9 μm

21. Line Strengths @ 4μm 2100 to 2200 cm-1 2300 to 2400 cm-1 H2O CO2 O3 N2O CO CH4 HNO3 OCS SO2 4.8 to 4.5 μm 4.3 to 4.2 μm

22. Atmosphere Transmittance The Optical Depth is the sum of absorption coefficients for all isotopes and species multiplied by the path-length, usually written in terms of pressure levels pi and pj and view angle  The transmittance of a layer is given by the exponential of the optical depth The view angle can be included in the absorption coefficient and transmittance from a level in the atmosphere (at height z) to the top of the atmosphere can be written as

23. Slightly Less Simple Radiative Transfer Let’s make some simple assumptions: • The atmosphere is now discrete isobaric layers • The atmosphere does not reflect • So no scattering • Each layer either transmits or absorbs/emits • The surface does not transmit • Either reflects or emits • No clouds (will quickly address this later)

24. Slightly Less Simple Radiative Transfer

25. Slightly Less Simple Radiative Transfer A Measured Radiance is equal to…

26. Slightly Less Simple Radiative Transfer A Measured Radiance is equal to… The Upward Surface Emission plus…

27. Slightly Less Simple Radiative Transfer A Measured Radiance is equal to… The Upward Surface Emission plus… The Upward Atmospheric Emission plus…

28. Slightly Less Simple Radiative Transfer A Measured Radiance is equal to… The Upward Surface Emission plus… The Upward Atmospheric Emission plus… The Surface Reflection of Downward Atmospheric Emission

29. Slightly Less Simple Radiative Transfer A Measured Radiance is equal to… The Upward Surface Emission plus… The Upward Atmospheric Emission plus… • For the sake of discussion, let’s assume the surface is a blackbody • The surface neither transmits or reflects • Absorptivity = Emissivity = 1.0

30. Slightly Less Simple Radiative Transfer A Measured Radiance is equal to… The Upward Surface Emission plus… The Upward Atmospheric Emission plus… 1 • For the sake of discussion, let’s assume the surface is a blackbody • The surface neither transmits or reflects • Absorptivity = Emissivity = 1.0

31. Slightly Less Simple Radiative Transfer A Measured Radiance is equal to… The Upward Surface Emission plus… The Upward Atmospheric Emission plus… 0 • For the sake of discussion, let’s assume the surface is a blackbody • The surface neither transmits or reflects • Absorptivity = Emissivity = 1.0

32. Slightly Less Simple Radiative Transfer The Upward Surface Emission is the… Blackbody radiation emitted by the surface at a given surface skin temperature

33. Slightly Less Simple Radiative Transfer The Upward Surface Emission is the… Blackbody radiation emitted by the surface at a given surface skin temperature Scaled by the transmissivity from the surface (ps) to the top of the atmosphere (TOA, 0 hPa)

34. Slightly Less Simple Radiative Transfer The Upward Surface Emission is the… Blackbody radiation emitted by the surface at a given surface skin temperature Scaled by the transmissivity from the surface (ps) to the top of the atmosphere (TOA, 0 hPa) Scaled by the surface emissivity (assumed as one in this case)

35. Slightly Less Simple Radiative Transfer The Upward Atmospheric Emission is the… The sum over the vertical of

36. Slightly Less Simple Radiative Transfer The Upward Atmospheric Emission is the… The sum over the vertical of The blackbody radiation emitted by each atmospheric layer at that layer’s temperature

37. Slightly Less Simple Radiative Transfer The Upward Atmospheric Emission is the… The sum over the vertical of The blackbody radiation emitted by each atmospheric layer at that layer’s temperature Scaled by the change in TOA transmittance in that layer

38. Slightly Less Simple Radiative Transfer This term is known as the weighting functionand illustrates the vertical sensitivity of a given channel to the atmosphere

39. The Path so far… The atmosphere is made of varying molecules… These molecules interact with radiation via absorption and emission… This radiation is ultimately emitted to space… Where it is observed by a satellite… End observable – a radiance that is a result of the molecules over the path of the observation Desired observable – the atmospheric distribution of those molecules Thus, an inversion is needed

40. Methods to Assimilated Information from the Infrared Two general methods to IR assimilation • Assimilation of retrieved atmospheric profiles • Direct assimilation of the measured radiances History of both… • Satellite retrievals were the initial approach • Vertical retrievals of temperature and moisture are inverted from the radiances and assimilated in as simple geophysical observations as such • Source of satellite information in NCEP/NCAR Reanalysis (Kalnay et al. 1996) • Advances in the 90s allowed for direct radiance assimilation • With the use of variational methods and fast radiative transfer models (including their tangent linear and adjoint models), the inversion is done in line in the solution • Derber and Wu 1998, McNally et al. 2000

41. Retrievals vs. Radiances A beaten-to-death argument • Radiances are directly measured and uncorrelated • Well, not really…especially in the infrared, but they’re far less correlated in spectral space than retrievals are in the vertical • Retrieval errors are inherently correlated • Retrievals are performed off of some sort of a-priori. • Are you assimilating the prior or the physical information • What is your prior, and would you even want to assimilate it? • Why in the world would you assimilate a climatology-derived prior into a weather model?

42. Retrievals vs. Radiances There’s a way to do the direct comparison correctly, but I’ve never seen it • Physical retrievals have estimates of the averaging kernel, which is analogous to a weighting function or vertical Jacobian • Radiance assimilation should be compared against the proper assimilation of the averaging kernels, not by treating the retrievals as radiosondes • Because they simply are not radiosondes This could be a whole lecture, and radiances basically won this argument

43. IR Radiance Assimilation Traditionally, infrared radiances are only assimilated in those scenes that are clear or for channels insensitive to clouds in cloudy scenes Retrieved Cloud Height

44. IR Radiance Assimilation A key assumption to cloud screening is that an accurate cloud height can be retrieved Cloud Height Retrieval Assumptions: • Single cloud • Flat, infinitesimally thin cloud • Graybody cloud (fractionally cloudy, black cloud)

45. IR Radiance Assimilation A key assumption to cloud screening is that an accurate cloud height can be retrieved CALIPSO Lidar Backscatter Cloud Height Retrieval Assumptions: • Single cloud • Flat, infinitesimally thin cloud • Graybody cloud (fractionally cloudy, black cloud) These assumptions stink G. Marseille, KNMI

46. IR Radiance Assimilation Ultimately, clouds in the infrared are a sharp temperature signal • Generally cold (in an atmosphere of positive lapse rate) Observed minus Forecast signal • Cloudy IR assimilation attempts to include this signal in the solution • What if the observation is clear, but the model erroneously warm at 200 hPa • Cloud retrievals will determine this signal as cold

47. Infrared Instruments In GMAO forward processing, infrared radiances are assimilated from IASI, AIRS, CrIS, GOES Sounder, SEVIRI and HIRS • Heritage “multi”-spectral sounders like HIRS (~ 18 channels) and the GOES Sounder are being phased out • The US HIRS instruments replaced by CrIS from NPP onward (hyperspectral – 1297 ch total, 399 for DA) • The final HIRS launched on MetOp-B. MetOp-C will only fly IASI (hyperspectral – 8461 ch, 616 for DA) • No Sounder in US GEO beginning w/ GOES-R • Hyperspectral sounding potentially in GEO in a number of future longitudes

48. Infrared Instruments Clearly more information from modern hyperspectral sounders (AIRS, IASI, CrIS) vs. older sounders (HIRS, GOES Sounder) Clearly a lot of redundant information as well Taken from a Tony McNally talk

49. Infrared Observation Usage IR observations make up ~65% of the current global observing system But only a small portion of the total number of observations available are utilized: • Spectral Thinning • AIRS: 281 of 2378 channels are available, 124 active (5.2% of total) • IASI: 616 of 8461, 137 active (1.6%) • CrIS: 399 of 1305, 81 active (6.2%) • Spatial Thinning • 1 spectra per instrument for every 145x145 kmthinning mesh (observation footprint size is ~15km) (~1.5% of the previous percentages) • Quality Control • Via traditional means, infrared observations sensitive to clouds are discarded via quality control (~25% of the previous 1.5%)

50. Infrared Observation Impact So based on real numbers: • AIRS: 0.04% of all observations are assimilated • IASI: 0.04% • CrIS: 0.19% So while the instruments provide as much bang as any other satellite instrument type out there, why can we only use such small percentages of the data?