S. P. Alexander

Radiosonde observations of gravity waves in the lower stratosphere over Davis, Antarctica

Radiosonde observations made from Davis station, Antarctica, (68.6°S, 78.0°E) between 2001 and 2012 are used to compile a climatology of lower stratosphere inertial gravity wave characteristics. Wavelet analysis extracts single wave packets from the wind and temperature perturbations. Wavelet parameters, combined with linear gravity wave theory, allow for the derivation of a wide range of wave characteristics. Observational filtering associated with this analysis preferentially selects inertial gravity waves with vertical wavelengths less than 2–3 km. The vertical propagation statistics show strong temporal and height variations. The waves propagate close to the horizontal and are strongly advected by the background wind in the wintertime. Notably, around half of the waves observed in the stratosphere above Davis between early May and mid-October propagate downward. This feature is distributed over the observed stratospheric height range. Based on the similarity between the upward and downward propagating waves and on the vertical structure of the nonlinear balance residual in the polar winter stratosphere, it is concluded that a source due to imbalanced flow that is distributed across the winter lower stratosphere best explains the observations. Calculations of kinetic and potential energies and momentum fluxes highlight the potential for variations in results due to different analysis techniques.

Temporal and spatial distributions of atmospheric wave energy in the equatorial stratosphere revealed by GPS radio occultation temperature data obtained with the CHAMP satellite during 2001–2006

Using stratospheric temperature profiles derived from GPS radio occultation (RO) measurements made by the German CHAMP satellite from June 2001 to May 2006, we studied the climatological behavior of atmospheric wave activity in the tropics. The wave potential energy, Ep, is calculated from temperature fluctuations with vertical scales shorter than 7 km in a longitude and latitude cell of 20° × 10° at 19–26 km. Ep is then averaged every 3 months (June–July–August (JJA), September–October–November (SON), December–January–February (DJF), March–April–May (MAM)), and the averages are compared with the cloud top temperature from outgoing long-wave radiation (OLR) and the convective rain rate from the TRMM precipitation radar (PR). Ep at 19–26 km in the western Pacific to Indian Ocean is found to show a clear seasonal variation, with a large Ep during DJF and MAM and a considerably enhanced Ep in SON; it becomes minimum during JJA near the equator, when the center of the enhanced Ep region appears over north India and the Indochina peninsula. Localized enhancement of Ep seems to be mainly due to atmospheric gravity waves. In addition, the longitudinally elongated portion of Ep is partially affected by Kelvin wave-like disturbances with short horizontal scales. In DJF and MAM, the convective clouds are located over the western Pacific and around Indonesia, at which time the Kelvin wavelike disturbances are effectively generated. The spatial and seasonal variations of Ep are closely related to the distribution of clouds, implying that convective wave generation is very important in the tropics. However, wave-mean flow interactions due to the wind shear of the QBO become important in the lower stratosphere, which considerably modifies our analysis of the Ep distribution at 19–26 km. Therefore, both wave generation and propagation characteristics must be taken into account in describing the climatological behavior of atmospheric wave activity in the equatorial stratosphere.

Southern Hemisphere Extratropical Gravity Wave Sources and Intermittency Revealed by a Middle-Atmosphere General Circulation Model

AbstractSouthern Hemisphere extratropical gravity wave activity is examined using simulations from a free-running middle-atmosphere general circulation model called Kanto that contains no gravity wave parameterizations. The total absolute gravity wave momentum flux (MF) and its intermittency, diagnosed by the Gini coefficient, are examined during January and July. The MF and intermittency results calculated from the Kanto model agree well with results from satellite limb and superpressure balloon observations. The analysis of the Kanto model simulations indicates the following results. Nonorographic gravity waves are generated in Kanto in the frontal regions of extratropical depressions and around tropopause-level jets. Regions with lower (higher) intermittency in the July midstratosphere become more (less) intermittent by the mesosphere as a result of lower-level wave removal. The gravity wave intermittency is low and nearly homogeneous throughout the SH middle atmosphere during January. This indicates t...

High-Resolution Radio Acoustic Sounding System Observations and Analysis up to 20 km

Abstract The first campaign-based measurements of virtual temperature in the upper-troposphere and lower-stratosphere (UTLS) region were made with the middle- and upper-atmosphere (MU) radar radio acoustic sounding system (RASS) during 4 days in August 1995. This dataset was examined in order to study high-frequency changes in the stability below 20 km, but especially in the UTLS region. Calculations of the WMO tropopause and cold-point tropopause heights showed the latter to be (1.0 ± 0.6) km higher, where 0.6 km is the standard deviation. A diurnal cycle of temperature and wind dominated the spectra, which was identified as the diurnal solar tide. Its phase maximum occurred in the afternoon between 5 and 15 km and showed upward energy propagation above this height. Changes in the UTLS kinetic energy dissipation rate ϵ showed significant high-frequency fluctuations embedded within layers that persisted for at least 1 day. Relative to the WMO tropopause height, the median ϵ increased from (0.5 ± 0.1) × 10...

The Seasonal Cycle of Lower-Tropospheric Gravity Wave Activity at Davis, Antarctica (69°S, 78°E)

AbstractA VHF wind-profiling radar located at Davis in coastal East Antarctica (69°S, 78°E) collected data from September 2009 to August 2011 in the lower troposphere. Gravity wave activity is quantified using the radar’s wind velocity variances. ERA-Interim and Antarctic Mesoscale Prediction System (AMPS) forecast output are used to understand the gravity wave activity in the context of the synoptic-scale meteorology and to identify the likely source of the observed gravity waves. The seasonal cycle of lower-tropospheric gravity wave activity (2.0–3.2-km altitude) obtained from the radar data for waves with ground-based periods of 16 min–12.8 h reveals a maximum in winter and a minimum in summer. The largest gravity wave activity corresponds in time to the presence of a surface depression centered north of Davis that directs strong northeasterly winds along the Antarctic coastline. Case studies indicate that these winds interact with an ice ridgeline located around 60 km northeast and upwind of Davis. Th...

Effects of Atmospheric Stability on Wave and Energy Propagation in the Troposphere

Abstract The very high frequency (VHF) middle and upper atmosphere radar radio acoustic sounding system (MU-RASS) in Shigaraki, Japan, is able to provide tropospheric virtual temperature data with high temporal resolution on the order of a few minutes. The objective of this paper is to test the usefulness of MU-RASS as a tool for examining high-frequency changes in atmospheric stability and its effects on wave and energy propagation. For this study, temperature and wind data below 8-km altitude during a 2-day campaign period in October 2001 were used. A long-lasting inversion layer at 3.5-km altitude dominated the observation period. Large vertical wind perturbations with periods of less than 30 min were observed inside this inversion layer. Wavelet analysis was used to identify the dominant wave period for calculating the wind and temperature variances. The temperature variance characteristics exhibited a combination of the horizontal and vertical wind variance characteristics. In conclusion, the high te...

Characteristics of gravity waves observed with intensive radiosonde campaign during November–December 2005 over western Sumatera

Characteristics of gravity waves are studied using radiosonde campaign conducted during November–December 2005 at Koto Tabang (KT, 0.2°S, 100.32°E). Intensive sounding with hourly launches was also conducted on seven days to study the characteristics of short period (2–6 hours) waves along with EAR data. Gravity waves with period (τ) of 3 hours and vertical wavelength (λz) of 10 km seemed to be generated due to localized convection around KT, which is inferred from X-band Doppler radar. The energy of the gravity wave with period of 2–3 days and vertical wavelength of 3–5 km is largest between 15 and 20 km and 25 and 30 km. We also report the comparison of the wave activity and its interaction with background wind between the three campaigns (CPEA-I, CPEA-II and Nov. 2002). Most of the time waves are propagating towards east and the source of gravity waves is strongly related to the slowly eastward-advecting tropospheric convection, implying that the wave activity was generated at far distant sources located west of KT. A key finding of this study is neither short period nor long-period gravity waves are generated during stationary type of convection. The change in the propagation direction of the short period waves within the event is observed which is not expected.

Diurnal and intraseasonal variation of UTLS vertical wind disturbance in the equatorial region and its relation to tropospheric convective activities

Vertical wind variations in the Upper Troposphere and Lower Stratosphere (UTLS) measured by the Equatorial Atmosphere Radar (EAR) at Kototabang, Sumatra, between 2003 and 2005 but mainly in 2004, have been statistically analyzed to study the characteristics of wind variances associated with convective activity, which is related to gravity wave generation and propagation. The analyses are intended to characterize relatively short period disturbances of less than 12 hours and an energy propagation direction of a relatively high elevation angle, and to relate vertical wind variations to convective activity close to the EAR. Correlation analyses between vertical wind variations and rainfall show that the wind variances have a clear diurnal variation indicating probable effects of tropospheric convection. They also show some intraseasonal variation. However, there are no significant correlations with the Out-going Long-wave Radiation (OLR) anomaly. The correlations between variances at UT and LS suggest that the UTLS coupling of vertical wind variation through upward propagation of gravity wave is similarly evident in the afternoon during both the active and the inactive phase of OLR that is a proxy of large-scale convective activity.