M. Ern

A Comparison between Gravity Wave Momentum Fluxes in Observations and Climate Models

Forthefirsttime,aformalcomparisonismadebetweengravitywavemomentumfluxesinmodelsandthose derivedfromobservations. Althoughgravitywavesoccuroverawiderangeofspatialandtemporalscales,the focusofthispaperisonscalesthatarebeingparameterizedinpresentclimatemodels,sub-1000-kmscales.Only observational methodsthatpermitderivationofgravitywavemomentumfluxesoverlargegeographical areas are discussed, and these are from satellite temperature measurements, constant-density long-duration bal- loons,andhigh-vertical-resolutionradiosondedata.Themodelsdiscussedincludetwohigh-resolutionmodels in which gravity waves are explicitly modeled, Kanto and the Community Atmosphere Model, version 5 (CAM5), and three climate models containing gravity wave parameterizations, MAECHAM5, Hadley Centre Global Environmental Model 3 (HadGEM3), and the Goddard Institute for Space Studies (GISS) model. Measurements generally show similar flux magnitudes as in models, except that the fluxes derived from satellite measurements fall off more rapidly with height. This is likely due to limitations on the observable range of wavelengths, although other factors may contribute. When one accounts for this more rapid fall off, the geographical distribution of the fluxes from observations and models compare reasonably well, except for certain features that depend on the specification of the nonorographic gravity wave source functions in the climate models. For instance, both the observed fluxes and those in the high-resolution models are very small at summer high latitudes, but this is not the case for some of the climate models. This comparison between gravity wave fluxes from climate models, high-resolution models, and fluxes derived from observations in- dicates that such efforts offer a promising path toward improving specifications of gravity wave sources in climate models.

Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) data processing and atmospheric temperature and trace gas retrieval

The Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment aboard the Shuttle Pallet Satellite (SPAS) was successfully flown in early November 1994 (STS 66) and in August 1997 (STS 85). This paper focuses on the first flight of the instrument, which was part of the Atmospheric Laboratory for Application and Science 3 (ATLAS 3) mission of NASA. During a free flying period of 7 days, limb scan measurements of atmospheric infrared emissions were performed in the 4 to 71 μm wavelength region. For improved horizontal resolution, three telescopes (viewing directions) were used that sensed the atmosphere simultaneously. Atmospheric pressures, temperatures, and volume mixing ratios of various trace gases were retrieved from the radiance data by using a fast onion-peeling retrieval technique. This paper gives an overview of the data system including the raw data processing and the temperature and trace gas profile retrieval. Examples of version 1 limb radiance data (level 1 product) and version 1 mixing ratios (level 2 product) of ozone, ClONO 2 , and CFC-11 are given. A number of important atmospheric transport processes can already be identified in the level 1 limb radiance data. Radiance data of the lower stratosphere (18 km) indicate strong upwelling in some equatorial regions, centered around the Amazon, Congo, and Indonesia. Respective data at the date line are consistent with convection patterns associated with El Nino. Very low CFC-11 mixing ratios occur inside the South Polar vortex and cause low radiance values in a spectral region sensitive to CFC-11 emissions. These low values are a result of considerable downward transport of CFC-11 poor air that occurred during the winter months. Limb radiance profiles and retrieved mixing ratio profiles of CFC-11 indicate downward transport over ∼5 km. The accuracy of the retrieved version 1 mixing ratios is rather different for the various trace gases. In the middle atmosphere the estimated systematic error of ozone is ∼14%. Ozone data of correlative satellite measurements are well within this error bar. CRISTA agrees, for example, with Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS) sunset measurements typically within 5%. The random error of version 1 ozone mixing ratios is 4%. Similar values apply to other trace gases. These low random errors allow the identification of small and medium scale horizontal and vertical structures in the measured trace gas distributions. Examples of structures in mixing ratio fields of ozone, ClONO 2 , and CFC-11 are given.

Interaction of gravity waves with the QBO: A satellite perspective

One of the most important dynamical processes in the tropical stratosphere is the quasi-biennial oscillation (QBO) of the zonal wind. Still, the QBO is not well represented in weather and climate models. To improve the representation of the QBO in the models, a better understanding of the driving of the QBO by atmospheric waves is required. In particular, the contribution of gravity waves is highly uncertain because of the small horizontal scales involved, and there is still no direct estimation based on global observations. We derive gravity wave momentum fluxes from temperature observations of the satellite instruments HIRDLS and SABER. Momentum flux spectra observed show that particularly gravity waves with intrinsic phase speeds <30m/s (vertical wavelengths <10km) interact with the QBO. Gravity wave drag is estimated from vertical gradients of observed momentum fluxes and compared to the missing drag in the tropical momentum budget of ERA-Interim. We find reasonably good agreement between their variations with time and in their approximate magnitudes. Absolute values of observed and ERA-Interim missing drag are about equal during QBO eastward wind shear. During westward wind shear, however, observations are about 2 times lower than ERA-Interim missing drag. This could hint at uncertainties in the advection terms in ERA-Interim. The strong intermittency of gravity waves we find in the tropics might play an important role for the formation of the QBO and may have important implications for the parameterization of gravity waves in global models.

Improved Middle Atmosphere Climate and Forecasts in the ECMWF Model through a Nonorographic Gravity Wave Drag Parameterization

Abstract In model cycle 35r3 (Cy35r3) of the ECMWF Integrated Forecast System (IFS), the momentum deposition from small-scale nonorographic gravity waves is parameterized by the Scinocca scheme, which uses hydrostatic nonrotational wave dynamics to describe the vertical evolution of a broad, constant, and isotropic spectrum of gravity waves emanating from the troposphere. The Cy35r3 middle atmosphere climate shows the following: (i) an improved representation of the zonal-mean circulation and temperature structure; (ii) a realistic parameterized gravity wave drag; (iii) a reasonable stationary planetary wave structure and stationary wave driving in July and an underestimate of the generation of stationary wave activity in the troposphere and stationary wave driving in January; (iv) an improved representation of the tropical variability of the stratospheric circulation, although the westerly phase of the semiannual oscillation is missing; and (v) a realistic horizontal distribution of momentum flux in the ...

Quantification of the contribution of equatorial Kelvin waves to the QBO wind reversal in the stratosphere

[1] Both global scale waves (e.g., Kelvin, equatorial Rossby, or Rossby-gravity waves) and mesoscale gravity waves contribute to the wind reversals of the quasi biennial oscillation (QBO). The relative contributions of the different wave types are highly uncertain. In our work we quantify the contribution of equatorial Kelvin waves to the reversal from stratospheric easterlies to westerlies averaged over two QBO cycles in the period 2002-2006. Our analysis is based on longitude-time spectra of temperatures measured by the SABER satellite instrument, as well as temperatures from ECMWF operational analyses. Kelvin waves of zonal wavenumber 1-6 and periods longer than 2.5 days are covered. It is found that the contribution of Kelvin waves is about 30-50% of the observed wind reversal and only 20-35% of the expected total wave forcing. The larger part of the wave forcing therefore has to be contributed by other waves, likely mesoscale gravity waves.

Global distribution of atomic oxygen in the mesopause region as derived from SCIAMACHY O(1S) green line measurements

A new data set of atomic oxygen abundance in the upper mesosphere and lower thermosphere is presented. The data are derived from the nighttime atomic oxygen green line limb emission measurements of the SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) instrument on the European Environmental Satellite. The temporal coverage is October 2002 until April 2012, and the latitudinal extent is 50°S to 80°N at 10 P.M. local time. This data set is compared to other satellite data sets, in particular to recently published data of SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) and the Mass Spectrometer and Incoherent Scatter model. SCIAMACHY atomic oxygen peak abundances are typically 3–6×1011 mol/cm3 at the atomic oxygen maximum region, depending on latitude and season. These values are similar to previous values based on chemiluminescence measurements of the atomic oxygen three-body recombination reaction but at least 30% lower than atomic oxygen abundances obtained from SABER.

Tomographic reconstruction of atmospheric gravity wave parameters from airglow observations

Gravity waves (GWs) play an important role in the dynamics of the mesosphere and lower thermosphere (MLT). Therefore, global observations of GWs in the MLT region are of particular interest. The small scales of GWs, however, pose a major problem for the observation of GWs from space. We propose a new observation strategy for GWs in the mesopause region by combining limb and sub-limb satellite-borne remote sensing measurements for improving the spatial resolution of temperatures that are retrieved from atmospheric soundings. In our study, we simulate satellite observations of the rotational structure of the O2 A-band nightglow. A key element of the new method is the ability of the instrument or the satellite to operate in so-called “target mode”, i.e. to point at a particular point in the atmosphere and collect radiances at different viewing angles. These multi-angle measurements of a selected region allow for tomographic 2-D reconstruction of the atmospheric state, in particular of GW structures. The feasibility of this tomographic retrieval approach is assessed using simulated measurements. It shows that one major advantage of this observation strategy is that GWs can be observed on a much smaller scale than conventional observations. We derive a GW sensitivity function, and it is shown that “target mode” observations are able to capture GWs with horizontal wavelengths as short as∼ 50 km for a large range of vertical wavelengths. This is far better than the horizontal wavelength limit of 100– 200 km obtained from conventional limb sounding.

Observations and Ray Tracing of Gravity Waves: Implications for Global Modeling

Vertical coupling by atmospheric waves is essential for the wind and temperature structure of the middle atmosphere. In particular, momentum carried by atmospheric gravity waves (GWs) governs the global circulation in the mesosphere and is for instance the reason for the cold summer mesopause. However, the small horizontal scales of GWs (tens to thousands of km) are challenging both global modeling and observations from satellite. Further, due to the small scales involved, there is a severe lack of understanding about GWs themselves, as well as dynamical phenomena involving GWs. Until recently, global observations of GWs were sparse and little was known about the global distribution of GWs, as well as their seasonal variation. Therefore, several projects in the priority program Climate And Weather of the Sun-Earth System (CAWSES) of the Deutsche Forschungsgemeinschaft (DFG) have addressed a number of the most pressing problems. Global distributions of GW activity and momentum fluxes have been derived from observations with number of satellite instruments, resulting in the first multi-year global data sets of GW parameters, covering time scales from seasonal variations up to the duration of almost a full 11-year solar cycle. In addition, seasonal and tidal variations of sporadic E layers in the ionosphere were studied in Global Positioning System (GPS) radio occultation data. Satellite observations of GWs and sporadic E layers were complemented by ground-based observations (radar and low-frequency (LF) drift measurements). All these observations, as well as accompanying modeling activities provided important constraints for GW parameterizations. Further activities addressed important aspects of GW propagation usually neglected in global modeling: GW ray tracing studies revealed the importance of non-vertical propagation of GWs and first steps were undertaken to develop an improved GW parameterization based on GW ray tracing techniques.

Seasonal variations of gravity wave variance inferred from CLAES

Gravity wave variances in CLAES temperature data are isolated by a 0-6 zonal wavenumber Kalman filter. Resulting vertical profiles of temperature residuals are analyzed by a combination of Maximum Entropy Method (MEM) and harmonic analysis for gravity waves (GWs). This is the same method previously employed to study GWs in CRISTA data. We obtain nearly 1.5 years of continuous GW data between 34S and 34N and good coverage at higher latitudes depending on UARS yaw maneuvers. Results are compared to CRISTA data and interpreted for different wave sources. A time series of zonal mean GW variance shows the seasonal shift of the tropical maximum of GW variance around the equator. Maximum variances are reached 1-2 months after summer solstice, consistent with the shift of the inner tropical convergence zone. Quiet summer and enhanced winter values at mid and high latitudes are due to a combination of wind filtering and wind modulation. Wind filtering occurs when GWs propagate from tropospheric west winds into the lower stratosphere. There prevailing winds reverse from west wind in winter to east wind in summer, thus causing a critical layer for low phase speed GWs during summer. The term wind modulation is used for the Doppler shift of the GW spectrum by the wind at the observation altitude shifting parts of the GW spectrum in and out the vertical-wavelength visibility limits of the instrument. We find evidence for both processes in the data and indication that GW filtering might be the more important one.

Infrared limb sounding measurements of middle-atmosphere gravity waves by CRISTA

We consider the example of the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment to deduce the sensitivity of an infrared emission limb sounder to gravity waves of different horizontal and vertical wavelength. The sensitivity studies show that gravity waves with horizontal wavelengths of the order of 100-200 km or longer can be detected. The deduced sensitivity factors are validated by comparing CRISTA and data sonde temperature profiles. Analysis of CRISTA temperature data reveals large gravity wave amplitudes in the stratosphere over southernmost South America. The horizontal structure is compared to model calculations. Global distributions are discussed with respect to convective sources, wind modulation, and Coriolis force modulation. It is shown that even the very dense spatial sampling of the CRISTA instrument is insufficient to fully resolve the horizontal structure of the waves which are seen in the vertical. Hence, increased spatial resolution of about 50 X 50 km or better is required to obtain all information the limb sounding technique is capable to provide.