Showing papers by "David C. Fritts published in 2013"

Gravity Wave–Fine Structure Interactions. Part I: Influences of Fine Structure Form and Orientation on Flow Evolution and Instability

Abstract: Four idealized direct numerical simulations are performed to examine the dynamics arising from the superposition of a monochromatic gravity wave (GW) and sinusoidal linear and rotary fine structure in the velocity field. These simulations are motivated by the ubiquity of such multiscale superpositions throughout the atmosphere. Three simulations explore the effects of linear fine structure alignment along, orthogonal to, and at 45° to the plane of GW propagation. These reveal that fine structure alignment with the GW enables strong wave–wave interactions, strong deformations of the initial flow components, and rapid transitions to local instabilities and turbulence. Increasing departures of fine structure alignment from the GW yield increasingly less efficient wave–wave interactions and weaker or absent local instabilities. The simulation having rotary fine structure velocities yields wave–wave interactions that agree closely with the aligned linear fine structure case. Differences in the aligned ...

Fine-Scale Characteristics of Temperature, Wind, and Turbulence in the Lower Atmosphere (0–1,300 m) Over the South Peruvian Coast

Abstract: We report results of preliminary high-resolution in situ atmospheric measurements through the boundary layer and lower atmosphere over the southern coast of Peru. This region of the coast is of particular interest because it lies adjacent to the northern coastal edge of the sub-tropical south-eastern Pacific, a very large area of ocean having a persistent stratus deck located just below the marine boundary layer (MBL) inversion. Typically, the boundary layer in this region during winter is topped by a quasi-permanent, well-defined, and very large temperature gradient. The data presented herein examine fine-scale details of the coastal atmosphere at a point where the edge of this MBL extends over the coastline as a result of persistent onshore flow. Atmospheric data were gathered using a recently-developed in-house constructed, GPS-controlled, micro-autonomous-vehicle aircraft (the DataHawk). Measured quantities include high-resolution profiles of temperature, wind, and turbulence structure from the surface to 1,300 m.

Gravity Wave–Fine Structure Interactions. Part II: Energy Dissipation Evolutions, Statistics, and Implications

Abstract: Part I of this paper employs four direct numerical simulations (DNSs) to examine the dynamics and energetics of idealized gravity wave–fine structure (GW–FS) interactions. That study and this companion paper were motivated by the ubiquity of multiscale GW–FS superpositions throughout the atmosphere. These DNSs exhibit combinations of wave–wave interactions and local instabilities that depart significantly from those accompanying idealized GWs or mean flows alone, surprising dependence of the flow evolution on the details of the FS, and an interesting additional pathway to instability and turbulence due to GW–FS superpositions. This paper examines the mechanical and thermal energy dissipation rates occurring in two of these DNSs. Findings include 1) dissipation that tends to be much more localized and variable than that due to GW instability in the absence of FS, 2) dissipation statistics indicative of multiple turbulence sources, 3) strong influences of FS shears on instability occurrence and turb...

Long-term observations of the quasi two-day wave by Hawaii MF radar

Abstract: [1] A mesospheric horizontal wind data set measured during 1991–2006 by the medium frequency (MF) radar at Kauai, Hawaii (22°N, 160°W) is analyzed to examine the long-term variability of the quasi two-day wave (QTDW). The QTDW over Hawaii is amplified twice a year, with the January and July events most likely being the representation of zonal wave numbers 3 and 4 modes, respectively. The amplitudes of the January monthly mean QTDW in both meridional and zonal winds and the July monthly mean QTDW in meridional component are nearly in phase with the solar cycle but with the solar maxima leading the QTDW maxima by 1 or 2 years. However, the July monthly mean QTDW in zonal wind is more antiphase with the solar cycle. Enhanced QTDW oscillations are evident in both wind components in January 1998, which is likely related to the strong El Nino event during the winter of 1997/1998. The enhanced gravity wave activity and the increased barotropic/baroclinic/inertial instability related to the strengthened stratosphere summer easterly jet might provide additional forcing to amplify the QTDW. Moreover, the enhanced migrating diurnal tide during warm El Nino-Southern Oscillation events could also contribute to the abnormally strong QTDW by increasing the refractive index and thus the growth rate of the QTDW. Additional enhancement of the QTDW with a short period of ~43 h is observed during the major sudden stratospheric warming in January 2006.

Turbulence Dynamics and Mixing Due to Gravity Waves in the Lower and Middle Atmosphere

Abstract: Internal gravity waves contribute to turbulence generation and mixing throughout the atmosphere due to instability accompanying amplitude growth with altitude and/or reduction of intrinsic phase speeds in shear flows. The instability processes accounting for turbulence generation depend on wave and mean flow structures, but in general exhibit an altitude dependence due to increasing wave scales with increasing altitude. Near the tropopause and away from major sources, characteristic vertical scales are a few km, vertical group velocities are small, and the dominant instability appears to be a Kelvin-Helmholtz (KH) shear instability due to inertia-gravity wave motions. At greater altitudes, characteristic vertical scales increase, causing larger vertical group velocities, larger wave energy fluxes, and a preference for faster convective instabilities of the motion field. This paper will review both the reasons for a change in instability character with altitude and the dynamics accompanying the transition to turbulence via. convective wave breaking and shear instability of the motion field. We will also describe the different implications of these wave field instabilities for turbulent mixing and transport of momentum, heat, and constituents.