Convection: the likely source of the medium-scale gravity waves observed in the OH airglow layer near Brasilia, Brazil, during the SpreadFEx campaign

Abstract

Abstract. Six medium-scale gravity waves (GWs) with horizontal wavelengths of λ H =60-160 km were detected on four nights by Taylor et al. (2009) in the OH airglow layer near Brasilia, at 15 • S, 47 • W, during the Spread F Experiment (SpreadFEx) in Brazil in 2005. We reverse and forward ray trace these GWs to the tropopause and into the thermosphere using a ray trace model which includes thermospheric dissipation. We identify the convective plumes, convective clusters, and convective regions which may have generated these GWs. We find that deep convection is the highly likely source of four of these GWs. We pinpoint the specific deep convective plumes which likely excited two of these GWs on the nights of 30 September and 1 October. On these nights, the source location/time uncertainties were small and deep convection was sporadic near the modeled source locations. We locate the regions containing deep convective plumes and clusters which likely excited the other two GWs. The last 2 GWs were probably also excited from deep convection; however, they must have been ducted ∼500-700 km if so. Two of the GWs were likely downwards-propagating initially (after which they reflected upwards from the Earth's surface), while one of the GWs was likely upwards-propagating initially from the convective plume/cluster. We also estimate the amplitudes and vertical scales of these waves at the tropopause, and compare their scales with those from a simple, linear convection model. Finally, we calculate each GW's dissipation altitude, location, and amplitude. We find that the dissiCorrespondence to: S. L. Vadas (vasha@cora.nwra.com) pation altitude depends sensitively on the winds at and above the OH layer. We also find that several of these GWs may have penetrated to high enough altitudes to potentially seed equatorial spread F (ESF) if located somewhat farther from the magnetic equator.

Reconstruction of the gravity wave field from convective plumes via ray tracing

“…For all of the simulations in this paper, in Fritts and Vadas (2008), and in Vadas et al (2009b), the background parameters are constant in time; therefore, ω r is constant in time for each GW. However, for the reverse and forward ray trace simulations performed for this SpreadFEx campaign in Vadas et al (2009a), we assume that the background temperatures and winds vary slowlyenough to approximate that ω r is constant in time. We note that several formulations do take into account changing ω r with time as background conditions change with time (e.g., Jones, 1969).…”

“…Many of these GWs are believed to be Earth-reflected (Samson et al, 1989(Samson et al, , 1990Bristow et al, 1994Bristow et al, , 1996b. Additionally, several of the medium-scale GWs observed during the SpreadFEx experiment were reverse ray traced to convective sources positioned at the reflected source locations (Vadas et al, 2009a). Because the solutions, , are Fourier decompositions in all space, including reflection off the ground is equivalent to inserting a boundary condition at z=0 such that the vertical velocity equals zero there, since air cannot penetrate below the Earth's surface.…”

“…Thus the reflected GWs are actually those upward-propagating GWs from the image body force. These vertical distributions are represented mathematically as We also model a small cluster of convective plumes which appear close together and are nearly simultaneous on satellite images (see Vadas et al, 2009a). If body forces are nearly simultaneous and are separated by less than a few diameters horizontally, the excited GW spectrum is not simply the sum of the excited GW spectrum from each body force individually, because GWs with larger horizontal scales are excited in proportion to the increased size of the cluster (Vadas et al, 2003).…”

“…The first step is to relate GW amplitudes at the largest observed spatial scales to the characteristics of the convective plumes from which they arose. This is accomplished by reverse ray tracing these medium-scale GWs observed in MLT to specific convective cells identified and quantified from satellite imagery (Vadas et al, 2009a). This constrains as fully as possible the convective spatial and temporal scales important during SpreadFEx.…”

“…The second step in linking convection to GWs at the bottomside of the F layer is to calculate the GW amplitudes in the MLT expected to arise for these same convective sources, but for spatial scales not observed easily or directly in the MLT because of steep propagation angles and obscuration by anvil clouds (Vadas et al, 2009a). This is one of the goals of this paper, to quantify the GW source spectra from convection in a form that allows for easy calculation of GW amplitudes at larger spatial scales in the MLT and above.…”