HAO 2012 Profiles In Science: Dr. Nicholas Pedatella

Contact

303-497-1514
nickp@ucar.edu

Dr. Nicholas Pedatella's is an Post Doc I in the High Altitude Observatory of the National Center for Atmospheric Research. His research interest is in the vertical propagation of waves of lower atmospheric origin and their influence on the upper atmosphere. This involves the study of how nonmigrating tides and planetary waves introduce spatial and temporal variations in the ionosphere. I have used a number of different ground and space based observations as well as numerical models of the upper atmosphere to study of this coupling between the lower and upper atmospheres.

Publications

(1) Pedatella, N. M., H.-L. Liu, and M. E. Hagan. 2012: Day-to-day migrating and nonmigrating tidal variability due to the six-day planetary wave, J. Geophys. Res., 117, A06301, doi:10.1029/2012JA017581.

Abstract: To investigate day-to-day variability in the mesosphere and lower thermosphere (MLT), an idealized simulation of a six-day westward propagating zonal wave number-1 planetary wave is performed using the National Center for Atmospheric Research (NCAR) Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM). The six-day planetary wave introduces a six-day periodicity in the zonal mean atmosphere, migrating and nonmigrating tides, as well as in secondary waves that are produced by nonlinear planetary wave-tide interactions. We have further used the linear Global Scale Wave Model (GSWM) to isolate the effect of how the day-to-day changes in zonal mean zonal winds may influence tides in the MLT. The most significant changes are observed in the migrating diurnal tide (DW1), eastward propagating nonmigrating tides with zonal wave numbers-2 and -3 (DE2 and DE3), and a 20 hr eastward propagating wave with zonal wave number-2 (20E2). Because we have included the lower atmospheric source of nonmigrating tides, DE2 and DE3 are present with relatively large amplitudes in the MLT, even in the absence of planetary wave forcing. The 20E2 wave is produced by the nonlinear interaction between the DE3 and the six-day planetary wave, and its large amplitude indicates the importance of including the realistic spectra of nonmigrating tides in numerical simulations of planetary waves. The GSWM simulations reveal that the DW1 is not significantly influenced by the changes in the zonal mean winds. We thus conclude that the DW1 changes are driven by a combination of changes due to nonlinear interaction with the six-day planetary wave as well as changes due to zonal asymmetries that result from the six-day planetary wave. The six-day planetary wave induced changes in zonal mean zonal winds lead to a general reduction in the amplitude of DE2 and DE3, and introduce a slight periodic behavior in these tides. The effect of changing zonal mean zonal winds appears to be the primary driver of the changes in the DE2. However, for DE3, although the changes that can be attributed to zonal mean zonal wind variability are not insignificant, the primary driver of the DE3 perturbations appears to be the nonlinear interaction with the six-day planetary wave. Last, we demonstrate that the day-to-day changes in the DE3 introduce similar day-to-day changes in the daytime wave number-4 longitude structure in the low-latitude ionosphere. These results indicate that short-term variability in the low-latitude ionosphere is likely to be driven by similar short-term variability in nonmigrating tides in the MLT.

Figure 1 caption: (a) Vertical profile of zonal mean zonal wind at the equator simulated by the TIME-GCM without planetary wave forcing at the lower boundary. (b) Temporal and vertical variation of the zonal mean zonal wind at the equator for the TIME-GCM simulation with planetary wave forcing at the lower boundary. (c and d) Same as (a) and (b) except for DW1 in neutral temperature. (e and f) Same as (a) and (b) except for the 20–24E2 nonmigrating tide in zonal neutral wind. (g and h) Same as (a) and (b) except for the DE3 nonmigrating tide in zonal neutral wind. To emphasize the temporal variability, the background value at each height is removed for the results presented in (b). Note that the results on day six are identical to those on day one due to the six day period of the planetary wave.

(2) Pedatella, N. M., H.-L. Liu, and A. D. Richmond. 2012: Atmospheric Semidiurnal Lunar Tide Climatology Simulated by the Whole Atmosphere Community Climate Model, J. Geophys. Res., 117, A06327, doi:10.1029/2012JA017855.

Abstract: The atmospheric semidiurnal lunar tide is added to the Whole Atmosphere Community Climate Model (WACCM) through inclusion of an additional forcing mechanism. The simulated climatology of the semidiurnal lunar tide in surface pressure and zonal and meridional winds in the mesosphere and lower thermosphere (MLT) is presented. Prior observations and modeling results demonstrate characteristic seasonal and latitudinal variability of the semidiurnal lunar tide in surface pressure, and the WACCM reproduces these features. In the MLT, the WACCM simulations reveal a primarily semiannual variation with maxima near December and June solstice. The peak amplitudes in the MLT zonal and meridional winds are ~5–10 ms^−1, and occur at mid to high latitudes in the summer hemisphere. We have further compared the WACCM simulation results in the MLT with those from the Global Scale Wave Model (GSWM). The overall latitude and seasonal variations are consistent between these two models. However, the GSWM peak amplitudes are ~2–3 times larger than those in the WACCM. This is thought to be related to deficiencies in the GSWM and not the WACCM simulations. With the exception of smaller amplitudes during Northern Hemisphere summer months, the WACCM simulations of the semidiurnal lunar tide in the MLT are also shown to be generally consistent with prior observations and modeling results. The reduced amplitudes in the WACCM simulations during Northern Hemisphere summer months are thought to be related to the influence of the cold-pole bias in WACCM on the propagation of the lunar tide during these months.

Figure 2 caption: Seasonal and latitudinal variability of the migrating semidiurnal lunar tide amplitude in zonal neutral wind at (a) 90 km, (b) 95 km, (c) 105 km, and (d) 125 km.

(3) Pedatella, N. M., H.-L. Liu, A. D. Richmond, A. Maute, and T.-W. Fang. 2012: Simulations of solar and lunar tidal variability in the mesosphere and lower thermosphere during sudden stratospheric warmings and their influence on the low-latitude ionosphere, J. Geophys. Res., 117, A08326, doi:10.1029/2012JA017858.

Abstract: Whole Atmosphere Community Climate Model (WACCM) simulations are used to investigate solar and lunar tide changes in the mesosphere and lower thermosphere (MLT) that occur in response to sudden stratosphere warmings (SSWs). The average tidal response is demonstrated based on 23 moderate to strong Northern Hemisphere SSWs. The migrating semidiurnal lunar tide is enhanced globally during SSWs, with the largest enhancements (~60–70%) occurring at mid to high latitudes in the Northern Hemisphere. Enhancements in the migrating solar semidiurnal tide (SW2) also occur up to an altitude of 120 km. Above this altitude, the SW2 decreases in response to SSWs. The SW2 enhancements are 40–50%, making them smaller in a relative sense than the enhancements in the migrating semidiurnal lunar tide. Changes in nonmigrating solar tides are, on average, generally small and the only nonmigrating tides that exhibit changes greater than 20% are the diurnal tide with zonal wave number 0 (D0) and the westward propagating semidiurnal tide with zonal wave number 1 (SW1). D0 is decreased by ~20–30% at low latitudes, while SW1 exhibits a similar magnitude enhancement at mid to high latitudes in both hemispheres. The tidal changes are attributed to a combination of changes in the zonal mean zonal winds, changes in ozone forcing of the SW2, and nonlinear planetary wave-tide interactions. We further investigate the influence of the lunar tide enhancements on generating perturbations in the low latitude ionosphere during SSWs by using the WACCM-X thermosphere to drive an ionosphere-electrodynamics model. For both solar maximum and solar minimum simulations, the changes in the equatorial vertical plasma drift velocity are similar to observations when the lunar tide is included in the simulations. However, when the lunar tide is removed from the simulations, the low latitude ionosphere response to SSWs is unclear and the characteristic behavior of the low latitude ionosphere perturbations that is seen in observations is no longer apparent. Our results thus indicate the importance of variability in the lunar tide during SSWs, especially for the coupling between SSWs and perturbations in the low latitude ionosphere.

Figure 3 caption: (a) Change in the zonal mean vertical drift velocity at the magnetic equator and 300 km. The zonal means are calculated at fixed local times. The changes are with respect to the average local time variation in the vertical drift velocity over the interval shown. (b) 10 hPa daily zonal mean temperature at 90°N (solid) and 60°N (dashed), and zonal wind at 60°N (dotted). Results are for solar minimum simulation with lunar tidal forcing.

(4) Pedatella, N. M., A. D. Maute, and M. E. Hagan. 2012: The comparative importance of DE3, SE2, and SPW4 on the generation of wavenumber-4 longitude structures in the low-latitude ionosphere during September Equinox, Geophys. Res. Lett., 117, A08326, doi:10.1029/2012GL053643 in press.

Abstract: Numerical simulations are performed to investigate the generation of the wave-4 longitude variation in the low-latitude ionosphere due to the diurnal eastward propagating nonmigrating tide with zonal wavenumber 3 (DE3), semidiurnal eastward propagating nonmigrating tide with zonal wavenumber 2 (SE2), and stationary planetary wave 4 (SPW4). From a fixed local time perspective, the DE3, SE2, and SPW4 all appear as wave-4 structures in longitude, and thus each of these waves must be considered as a potential source of the wave-4 variation in the ionosphere. Both the DE3 and SPW4 are found to produce significant wave-4 variations in the equatorial vertical ExB drift velocity, and in the ionospheric peak density (NmF2) at 15°N magnetic latitude. The daytime wave-4 variation in NmF2 is driven by the combination of vertical E x B drift variability and in-situ effects due largely to meridional neutral winds. The simulation results indicate that the SE2 is not a contributor to the wave-4 longitude variation. Our results further demonstrate that the actual wave-4 longitude variation is due to a combination of the DE3 and SPW4. We therefore conclude that, in addition to the DE3, the SPW4 also needs to be considered as an important driver of the wave-4 longitude variation in the low-latitude ionosphere. We additionally present evidence for the generation of the SPW4 due to the nonlinear interaction between the migrating diurnal tide and the DE3, and demonstrate the impact of DE3 variability on the amplitude of the SPW4.

Figure 4 caption: (a) Longitude variation of the simulated vertical E−B drift velocity at the magnetic equator and 300 km altitude. The results shown are for the average between 12 and 15 local time. Individual lines correspond to the TIME-GCM simulation results, climatology from Scherliess and Fejer [1999], and the simulation results for the ionosphere-electrodynamics model. Note that 3 m/s has been subtracted from the TIME-GCM results and the climatological values from Scherliess and Fejer [1999]. (b) Longitude variation of the ionospheric peak density (NmF2) at 15°N magnetic latitude. The results are for the average between 14 and 18 local time. The individual lines correspond to simulation results for different cases.

(5) Pedatella, N. M., and H-L. Liu. 2012: Tidal Variability in the Mesosphere and Lower Thermosphere due to the El-Nino Southern Oscillation, Geophys. Res. Lett., 117, A08326, doi:10.1029/2012GL053383 in press.

Abstract: Whole Atmosphere Community Climate Model (WACCM) simulations are used to investigate the migrating and nonmigrating tidal variability in the mesosphere and lower thermosphere (MLT) due to the El-Nino Southern Oscillation (ENSO). The most notable changes occur in the equatorial region during Northern Hemisphere winter in the diurnal migrating tide (DW1), diurnal eastward propagating nonmigrating tides with zonal wavenumbers 2 and 3 (DE2 and DE3), and the semidiurnal westward propagating nonmigrating tide with zonal wavenumber 4 (SW4). The WACCM simulations indicate that the ENSO represents a source of interannual tidal variability of ~10–30% in the MLT. The tidal changes are attributed to changes in tropical precipitation, altered tidal propagation due to changing zonal mean zonal winds, and changes in planetary wave activity associated with the ENSO. During the El-Nino phase of the ENSO the DE2 and DE3 are decreased, and the DW1 and SW4 are enhanced. The opposite response occurs during the La Nina phase of the ENSO; however, the magnitude of the tidal changes due to El-Nino and La Nina are different. This is especially notable for the DE2 and DE3 which are enhanced by ~2 K during La Nina time periods, and only reduced by ~1 K during El-Nino time periods. The results demonstrate that changing sea surface temperatures associated with the ENSO significantly impact the overall dynamics of the MLT. Our results further suggest that the ENSO is a source of significant interannual variability in the low-latitude ionosphere and thermosphere.

Figure 5 caption: Northern Hemisphere winter (December-February) average Nino 3.4 index and relative tidal temperature amplitude for (a) DW1, (b) DE2, (c) DE3, and (d) SW4. The tidal amplitudes are at an altitude of 100 km at the equator. Individual colors indicate the different ensemble members which are offset in time for clarity. The vertical thin dashed lines denote the boundary between the ensemble members.