Showing papers by "David C. Fritts published in 2014"

Quantifying Kelvin‐Helmholtz instability dynamics observed in noctilucent clouds: 1. Methods and observations

Abstract: Noctilucent clouds (NLCs) have been imaged during two nights in summer 2009 from northern Germany (Kuhlungsborn, 54°N) and middle Norway (Trondheim, 64°N). For the first time a horizontal resolution of 10 to 20 m at the altitude of the clouds (about 83 km) and a temporal resolution of about 1 s was achieved. Additional imaging using a coarser resolution provided monitoring of the larger-scale (~100 km) structures observed in the clouds. Two series of NLC images are described that reveal apparent Kelvin-Helmholtz (KH) billow structures having very different morphologies and apparent transitions to turbulence and mixing. One series exhibits deep KH billows and apparent secondary instabilities in the billow exteriors having streamwise alignment (and spanwise wave number), suggesting a small initial Richardson number (Ri). A second series of images suggests a larger and less unstable Ri, a slower KH billow evolution, shallower billows, and turbulence and mixing confined to the billow cores. We suggest that systematic exploration of these dynamics employing NLC imaging may enable characterization and quantification of KH instability occurrence statistics and of their contributions to turbulence and mixing in the summer mesopause environment with unique sensitivity to their small-scale dynamics.

Quantifying gravity wave momentum fluxes with Mesosphere Temperature Mappers and correlative instrumentation

Abstract: An Advanced Mesosphere Temperature Mapper and other instruments at the Arctic Lidar Observatory for Middle Atmosphere Research in Norway (69.3°N) and at Logan and Bear Lake Observatory in Utah (42°N) are used to demonstrate a new method for quantifying gravity wave (GW) pseudo-momentum fluxes accompanying spatially and temporally localized GW packets. The method improves on previous airglow techniques by employing direct characterization of the GW temperature perturbations averaged over the OH airglow layer and correlative wind and temperature measurements to define the intrinsic GW properties with high confidence. These methods are applied to two events, each of which involves superpositions of GWs having various scales and character. In each case, small-scale GWs were found to achieve transient, but very large, momentum fluxes with magnitudes varying from ~60 to 940 m2 s−2, which are ~1–2 decades larger than mean values. Quantification of the spatial and temporal variations of GW amplitudes and pseudo-momentum fluxes may also enable assessments of the total pseudo-momentum accompanying individual GW packets and of the potential for secondary GW generation that arises from GW localization. We expect that the use of this method will yield key insights into the statistical forcing of the mesosphere and lower thermosphere by GWs, the importance of infrequent large-amplitude events, and their effects on GW spectral evolution with altitude.

Quantifying Kelvin-Helmholtz instability dynamics observed in noctilucent clouds: 2. Modeling and interpretation of observations

Abstract: A companion paper describes high-resolution, ground-based imaging of apparent Kelvin-Helmholtz instabilities (KHI) observed in noctilucent clouds (NLCs) near the polar summer mesopause. Here we employ direct numerical simulations of KHI at Richardson numbers from Ri = 0.05 to 0.20 and relatively high Reynolds numbers to illustrate the dependence of KHI and secondary instabilities on these quantities and interpret and quantify the KHI events described by Baumgarten and Fritts (2014). We conclude that one event triggered by small-scale gravity waves provides clear evidence of strong KHI initiated at Ri ~0.05–0.10. Events arising in a more uniform shear environment exhibit KHI and small-scale dynamics that compare reasonably well with modeled KHI initiated at Ri ~0.20. Our application of numerical modeling in quantifying KHI dynamics observed in NLCs suggests that characteristics of KHI, and perhaps other small-scale dynamics, that are defined well in NLC displays can be used to quantify the dynamics and spatial scales of such events with high confidence. Specifically, our comparisons of KHI observations and modeling appear to indicate a “turbulent” viscosity ~5–40 times the true kinematic viscosity at the NLC altitude. This offers an alternative, or an augmentation, to more traditional radar, lidar, and/or airglow measurements employed for such studies of small-scale dynamics at coarser spatial scales during polar summer.

Investigation of a mesospheric gravity wave ducting event using coordinated sodium lidar and Mesospheric Temperature Mapper measurements at ALOMAR, Norway (69°N)

Abstract: New measurements at the ALOMAR observatory in northern Norway (69°N, 16°E) using the Weber sodium lidar and the Advanced Mesospheric Temperature Mapper (AMTM) allow for a comprehensive investigation of a gravity wave (GW) event on 22 and 23 January 2012 and the complex and varying propagation environment in which the GW was observed. These observational techniques provide insight into the altitude ranges over which a GW may be evanescent or propagating and enable a clear distinction in specific cases. Weber sodium lidar measurements provide estimates of background temperature, wind, and stability profiles at altitudes from ~78 to 105 km. Detailed AMTM temperature maps of GWs in the OH emission layer together with lidar measurements quantify estimates of the observed and intrinsic GW parameters centered near 87 km. Lidar measurements of sodium densities also allow more precise identification of GW phase structures extending over a broad altitude range. We find for this particular event that the extent of evanescent regions versus regions allowing GW propagation can vary largely over a period of hours and significantly change the range of altitudes over which a GW can propagate.

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.

Gravity wave effects on postsunset equatorial F region stability

Abstract: The influence of gravity waves on the stability of the postsunset equatorial F region ionosphere is investigated numerically. For this investigation, we use the output of a direct numerical simulation of waves and turbulence in the mesosphere and lower thermosphere to force a simulation of ionospheric dynamics. Both simulations are cast in three dimensions. The effectiveness of the neutral-plasma coupling involved is generally thought to depend on the dynamo efficiency and spatial resonance of the forcing, which we evaluate. In our simulations, the postsunset equatorial ionosphere could be deformed by neutral waves after 5–15 min most severely when the wavefronts were aligned approximately with the magnetic meridian, despite the fact that the dynamo efficiency is modest even in that case. However, poor spatial resonance limits the subsequent growth of the deformations in our simulations, and the seeding of interchange instabilities does not occur. The coupled simulations predict the formation of intermediate layers in the equatorial valley region (150–250 km apex altitude) under some circumstances that could serve as telltales in nature of the presence of the kind of neutral forcing we simulate.