David C. Fritts

Bio: David C. Fritts is an academic researcher from Cora. The author has contributed to research in topics: Gravity wave & Thermosphere. The author has an hindex of 66, co-authored 227 publications receiving 14924 citations. Previous affiliations of David C. Fritts include University of Colorado Boulder & National Waste & Recycling Association.

Radar observations of gravity waves over Jicamarca, Peru during the CADRE campaign

Abstract: We present results obtained with the mesosphere-stratosphere-troposphere (MST) radar at Jicamarca, Peru, from three 10-day experiments in January 1993, March 1994, and August 1994. Horizontal and vertical velocities were measured over ranges spanning the lower part of the stratosphere and most of the mesosphere. In the stratosphere, the fluctuating part of the wind field was found to be dominated by inertia-gravity waves. Sinusoids of different period were fit to the velocity time series using a least squares procedure. The dominant periods of the inertia-gravity wave motions were found to be 1.5 days for the January 1993 experiment and 2.1 days for the August 1994 experiment. For the January 1993 experiment, the amplitudes and phases of the inertia-gravity wave oscillations were extracted for the vertical as well as the horizontal components. The vertical amplitude of the 1.5-day period wave was small (<0.1 ms−1), but measurable with the Jicamarca radar. The intrinsic periods of the inertia-gravity waves were inferred from the fits using two different methods. The first method predicted the intrinsic period using the orbital ellipses traced out in time by fits to the horizontal winds. The second method used information taken from the vertical as well as the horizontal fits. The values of intrinsic period made using the second method were found to have much less scatter than the values inferred solely from the orbital ellipses. The momentum flux estimates in both the stratosphere and mesosphere were found to depend significantly on the exact methodology used. A technique was adopted whereby estimates were formed only when radial velocities were measured simultaneously on both beams of a coplanar beam pair. Most of the total wave stress was usually contributed by waves at periods ≥4 hours in the stratosphere and ≥1 hour in the mesosphere. In the stratosphere, localized layers of enhanced momentum flux were sometimes observed. The obvious anticorrelation between the shear of the mean wind and the momentum flux in these layers suggests that they were caused by in situ generation of waves by the Kelvin-Helmholtz instability, rather than gravity waves propagating from lower levels. At short periods, momentum flux spectra in the stratosphere and mesosphere showed numerous positive and negative peaks. A correlation analysis revealed that the high-frequency peaks were associated with discrete wave packets. The short-scale waves associated with these packets were fairly isotropic in their direction of propagation, and due to cancellation they contributed little net momentum.

Computation of clear‐air radar backscatter from numerical simulations of turbulence: 2. Backscatter moments throughout the lifecycle of a Kelvin‐Helmholtz instability

Abstract: [1] Franke et al. (2011) describe a numerical simulation of the instability and turbulent breakdown of Kelvin-Helmholtz (KH) billows at a high Reynolds number, numerical assessment of radar backscatter, and accuracies of inferred Doppler spectral moments for one test volume. Those results suggest a potential for significant measurement biases for radars that obtain backscatter from refractive index fluctuations. We present in this paper the morphology of computed radar moments throughout the KH instability lifecycle for two radar configurations in order to reveal the evolving character of radar backscatter and compare the radar velocity estimates with true velocities throughout the evolution, and to provide guidance, and cautions, for the interpretation of these dynamics in observational data. Results reveal strong variations in backscatter moments and character, and dependence on radar measurement parameters, that should be beneficial in the interpretation of such measurements in the atmosphere. Backscatter power predictions agree reasonably with observations of such events and their temporal evolutions. Our results also reveal a potential for significant measurement or sensitivity biases, some of which were predicted previously. Examples include a lack of significant backscatter power in well-mixed billow cores, suggesting possibly weak turbulence where in fact it may be strongest, maximum backscatter power in the billow exteriors, where refractive index fluctuations are large but turbulence is weak, underestimated vertical velocities within the KH billows at early times, and inferred significant vertical velocities where true vertical velocities are near zero at late stages of restratification, especially in the edge regions of the turbulence layer.

Gravity Wave Dynamics in a Mesospheric Inversion Layer: 1. Reflection, Trapping, and Instability Dynamics.

Abstract: An anelastic numerical model is employed to explore the dynamics of gravity waves (GWs) encountering a mesosphere inversion layer (MIL) having a moderate static stability enhancement and a layer of weaker static stability above. Instabilities occur within the MIL when the GW amplitude approaches that required for GW breaking due to compression of the vertical wavelength accompanying the increasing static stability. Thus, MILs can cause large-amplitude GWs to yield instabilities and turbulence below the altitude where they would otherwise arise. Smaller-amplitude GWs encountering a MIL do not lead to instability and turbulence but do exhibit partial reflection and transmission, and the transmission is a smaller fraction of the incident GW when instabilities and turbulence arise within the MIL. Additionally, greater GW transmission occurs for weaker MILs and for GWs having larger vertical wavelengths relative to the MIL depth and for lower GW intrinsic frequencies. These results imply similar dynamics for inversions due to other sources, including the tropopause inversion layer, the high stability capping the polar summer mesopause, and lower frequency GWs or tides having sufficient amplitudes to yield significant variations in stability at large and small vertical scales. MILs also imply much stronger reflections and less coherent GW propagation in environments having significant fine structure in the stability and velocity fields than in environments that are smoothly varying.

Mesospheric radar echoes at Poker Flat, Alaska: Evidence for seasonally dependent generation mechanisms

Abstract: The long-term character of VHF mesospheric echoes obtained by using the Poker Flat MST radar in Alaska show the following characteristics: (1) The winter time echoes are relatively weak and are only present in the height range 55–80 km, while (2) the summertime echoes are much stronger, with the maximum echo intensity centered at 86 km. The evidence currently available indicates that the winter time echoes arise, from the nonlinear breakup of tropospherically generated gravity waves. The stronger summertime echoes, on the other hand, appear to arise in situ as a result of the combined effects of the steep summertime vertical temperature gradient that exists at these latitudes and the horizontal wind shears.

Modeling the implications of Kelvin-Helmholtz instability dynamics for airglow observations

Abstract: A companion paper describes high-resolution, ground-based imaging of apparent Kelvin-Helmholtz instabilities (KHI) observed in OH airglow at ~87 km over the Andes Lidar Observatory at 30°S. Here we employ direct numerical simulations (DNSs) and large eddy simulations (LESs) of KHI at Richardson numbers from Ri = 0.05 to 0.20 and relatively high Reynolds numbers of Re ~2500 to 10,000 to illustrate the dependence of primary and secondary KHI on these quantities for the purpose of quantifying KHI dynamics observed by ground-based airglow imagers. Our DNS and LES reveal significant variations of both primary and secondary KHI scales and amplitudes with varying Ri and Re. Lower Ri and higher Re yield stronger and deeper initial 2-D KH billows. Low Re for a given Ri either yield larger-scale 3-D secondary instabilities or suppress them altogether. Secondary instability scales decrease as Re increases for a given Ri. Corresponding variations in implied KHI airglow signatures include (1) stronger airglow intensity variations for larger KHI wavelengths and depths (higher Ri), (2) stronger 2-D and 3-D responses for the initial KHI shear layer displaced somewhat from the airglow layer, and (3) stronger 3-D responses for the lower Ri and intermediate Re yielding the larger secondary instability scales.

Linear and nonlinear waves, by G. B. Whitham. Pp.636. £50. 1999. ISBN 0 471 35942 4 (Wiley).

Abstract: to be done in this area. Face recognition is a problem that scarcely clamoured for attention before the computer age but, having surfaced, has involved a wide range of techniques and has attracted the attention of some fine minds (David Mumford was a Fields Medallist in 1974). This singular application of mathematical modelling to a messy applied problem of obvious utility and importance but with no unique solution is a pretty one to share with students: perhaps, returning to the source of our opening quotation, we may invert Duncan's earlier observation, 'There is an art to find the mind's construction in the face!'.

Gravity wave dynamics and effects in the middle atmosphere

Abstract: [1] Atmospheric gravity waves have been a subject of intense research activity in recent years because of their myriad effects and their major contributions to atmospheric circulation, structure, and variability. Apart from occasionally strong lower-atmospheric effects, the major wave influences occur in the middle atmosphere, between ∼ 10 and 110 km altitudes because of decreasing density and increasing wave amplitudes with altitude. Theoretical, numerical, and observational studies have advanced our understanding of gravity waves on many fronts since the review by Fritts [1984a]; the present review will focus on these more recent contributions. Progress includes a better appreciation of gravity wave sources and characteristics, the evolution of the gravity wave spectrum with altitude and with variations of wind and stability, the character and implications of observed climatologies, and the wave interaction and instability processes that constrain wave amplitudes and spectral shape. Recent studies have also expanded dramatically our understanding of gravity wave influences on the large-scale circulation and the thermal and constituent structures of the middle atmosphere. These advances have led to a number of parameterizations of gravity wave effects which are enabling ever more realistic descriptions of gravity wave forcing in large-scale models. There remain, nevertheless, a number of areas in which further progress is needed in refining our understanding of and our ability to describe and predict gravity wave influences in the middle atmosphere. Our view of these unknowns and needs is also offered.

Geophysical fluid dynamics.

Abstract: The potential for sea ice-albedo feedback to give rise to nonlinear climate change in the Arctic Ocean – defined as a nonlinear relationship between polar and global temperature change or, equivalently, a time-varying polar amplification – is explored in IPCC AR4 climate models. Five models supplying SRES A1B ensembles for the 21 st century are examined and very linear relationships are found between polar and global temperatures (indicating linear Arctic Ocean climate change), and between polar temperature and albedo (the potential source of nonlinearity). Two of the climate models have Arctic Ocean simulations that become annually sea ice-free under the stronger CO 2 increase to quadrupling forcing. Both of these runs show increases in polar amplification at polar temperatures above-5 o C and one exhibits heat budget changes that are consistent with the small ice cap instability of simple energy balance models. Both models show linear warming up to a polar temperature of-5 o C, well above the disappearance of their September ice covers at about-9 o C. Below-5 o C, surface albedo decreases smoothly as reductions move, progressively, to earlier parts of the sunlit period. Atmospheric heat transport exerts a strong cooling effect during the transition to annually ice-free conditions. Specialized experiments with atmosphere and coupled models show that the main damping mechanism for sea ice region surface temperature is reduced upward heat flux through the adjacent ice-free oceans resulting in reduced atmospheric heat transport into the region.

Observing Earth's atmosphere with radio occultation measurements using the Global Positioning System

Abstract: The implementation of the Global Positioning System (GPS) network of satellites and the development of small, high-performance instrumentation to receive GPS signals have created an opportunity for active remote sounding of the Earth's atmosphere by radio occultation at comparatively low cost. A prototype demonstration of this capability has now been provided by the GPS/MET investigation. Despite using relatively immature technology, GPS/MET has been extremely successful [Ware et al., 1996; Kursinski et al., 1996], although there is still room for improvement. The aim of this paper is to develop a theoretical estimate of the spatial coverage, resolution, and accuracy that can be expected for atmospheric profiles derived from GPS occultations. We consider observational geometry, attenuation, and diffraction in defining the vertical range of the observations and their resolution. We present the first systematic, extensive error analysis of the spacecraft radio occultation technique using a combination of analytical and simulation methods to establish a baseline accuracy for retrieved profiles of refractivity, geopotential, and temperature. Typically, the vertical resolution of the observations ranges from 0.5 km in the lower troposphere to 1.4 km in the middle atmosphere. Results indicate that useful profiles of refractivity can be derived from ∼60 km altitude to the surface with the exception of regions less than 250 m in vertical extent associated with high vertical humidity gradients. Above the 250 K altitude level in the troposphere, where the effects of water are negligible, sub-Kelvin temperature accuracy is predicted up to ∼40 km depending on the phase of the solar cycle. Geopotential heights of constant pressure levels are expected to be accurate to ∼10 m or better between 10 and 20 km altitudes. Below the 250 K level, the ambiguity between water and dry atmosphere refractivity becomes significant, and temperature accuracy is degraded. Deep in the warm troposphere the contribution of water to refractivity becomes sufficiently large for the accurate retrieval of water vapor given independent temperatures from weather analyses [Kursinski et al., 1995]. The radio occultation technique possesses a unique combination of global coverage, high precision, high vertical resolution, insensitivity to atmospheric particulates, and long-term stability. We show here how these properties are well suited for several applications including numerical weather prediction and long-term monitoring of the Earth's climate.