Lower Stratospheric Heights Research Articles

Possible climatic implications of high-altitude black carbon emissions

Abstract. On account of its strong absorption of solar and terrestrial radiation, black carbon (BC) aerosol is known to impact large-scale systems, such as the Asian monsoon and the Himalayan glaciers, in addition to affecting the thermal structure of the lower atmosphere. While most studies focus on the near-surface abundance and impacts of BC, our study examines the implications of sharp and confined layers of high BC concentration (called elevated BC layers) at altitudes more than 4 km over the Indian region using the online regional chemistry transport model (WRF-Chem) simulations. These elevated BC layers were revealed in the recent in situ measurements using high-altitude balloons carried out on 17 March 2010, 8 January 2011 and 25 April 2011. Our study demonstrates that high-flying aircraft (with emissions from the regionally fine-tuned MACCity inventory) are the most likely cause of these elevated BC layers. Furthermore, we show that such aircraft-emitted BC can be transported to upper tropospheric or lower stratospheric heights ( ∼ 17 km) aided by the strong monsoonal convection occurring over the region, which is known to overshoot the tropical tropopause, leading to the injection of tropospheric air mass (along with its constituent aerosols) into the stratosphere. We show observational evidence for such an intrusion of tropospheric BC into the stratosphere over the Indian region using extinction coefficient and particle depolarisation ratio data from CALIOP Lidar on-board the CALIPSO satellite. We hypothesise that such intrusions of BC into the lower stratosphere and its consequent longer residence time in the stratosphere have significant implications for stratospheric ozone, especially considering the already reported ozone-depleting potential of BC.

Climatology of stratospheric gravity waves and their interaction with zonal mean wind over the tropics using GPS RO and ground-based measurements in the two phases of QBO

The present manuscript deals with the spatial distribution of gravity wave activity over the tropics using ten years (2001–2010) of CHAllenging Mini Payloads (CHAMP) and Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) Global Positioning System (GPS) Radio Occultation (RO) data and ground-based radiosonde measurements over an equatorial station Singapore (1.36°N, 103.98°E) and four tropical stations, Guam (13.48°N, 144.80°E), Palau (7.33°N, 134.48°E) in the northern hemisphere, Darwin (12.41°S, 130.88°E) and Pago-Pago (14.33°S, 170.71°W) in the southern hemisphere from January 2001–December 2010. It also aims to quantify the difference in wave activity in the two phases of QBO, climatologically. Space-time spectra have been constructed over a latitude band of ±10° and decomposing the CHAMP/COSMIC temperature perturbations into symmetric and antisymmetric modes about the equator. Clear signature of equatorial waves with higher wavelength and a constant background of gravity waves (GW) with inertial frequency are prominent in the spectra. Strong GW and mean flow interaction can be seen in the lower stratosphere potential energy density (EP) and momentum flux with enhanced wave activity during the westerly (eastward wind) phase of quasi-biennial oscillation (QBO) (WQBO) over the equatorial and tropical stations like Singapore and Palau/Darwin, respectively. From the latitudinal distribution of energy density, the occurrence of two-peak structure in energy density can be seen in the middle and lower latitudes with an enhancement during the WQBO phase. The EP associated with GWs are calculated at lower stratospheric (19–26 km) heights and are compared with outgoing longwave radiation (OLR) to correlate the wave events with tropical deep convection during the easterly, i.e. westward wind (EQBO) and WQBO phases of QBO. Clear coherence of convection due to Asian summer monsoon with localized enhancement of wave activity over Western Pacific, South America and African region during the WQBO phase is observed at the lower stratospheric heights. Significant enhancement is observed during Northern Hemisphere winter months and minimum during summer. The longitudinally elongated portion of EP over the equator is partially affected by Kelvin wave (KW) like disturbances with short vertical scales and also by inertia GW.

Dynamical influence of gravity waves generated by the Vestfjella Mountains in Antarctica: radar observations, fine-scale modelling and kinetic energy budget analysis

ABstractGravity waves generated by the Vestfjella Mountains (in western Droning Maud Land, Antarctica, southwest of the Finnish/Swedish Aboa/Wasa station) have been observed with the Moveable atmospheric radar for Antarctica (MARA) during the SWEDish Antarctic Research Programme (SWEDARP) in December 2007/January 2008. These radar observations are compared with a 2-month Weather Research Forecast (WRF) model experiment operated at 2 km horizontal resolution. A control simulation without orography is also operated in order to separate unambiguously the contribution of the mountain waves on the simulated atmospheric flow. This contribution is then quantified with a kinetic energy budget analysis computed in the two simulations. The results of this study confirm that mountain waves reaching lower-stratospheric heights break through convective overturning and generate inertia gravity waves with a smaller vertical wavelength, in association with a brief depletion of kinetic energy through frictional dissipation and negative vertical advection. The kinetic energy budget also shows that gravity waves have a strong influence on the other terms of the budget, i.e. horizontal advection and horizontal work of pressure forces, so evaluating the influence of gravity waves on the mean-flow with the vertical advection term alone is not sufficient, at least in this case. We finally obtain that gravity waves generated by the Vestfjella Mountains reaching lower stratospheric heights generally deplete (create) kinetic energy in the lower troposphere (upper troposphere–lower stratosphere), in contradiction with the usual decelerating effect attributed to gravity waves on the zonal circulation in the upper troposphere–lower stratosphere.

Planetary wave and gravity wave influence on the occurrence of polar stratospheric clouds over Davis Station, Antarctica, seen in lidar and radiosonde observations

We present Rayleigh lidar observations of polar stratospheric clouds (PSC) from Davis Station (68.6°S, 78.0°E), Antarctica, from the 2001, 2002, 2003, and 2004 austral winter seasons. The PSC are detected as enhanced backscatter returns at lower stratospheric heights. We use operational radiosonde balloon data to compare the PSC detections with stratospheric temperature and wave properties. As documented by others, we find that the occurrence of PSC is closely tied to average stratospheric temperatures (which, in turn, are closely related to the location of Davis with respect to the Antarctic vortex), with PSC being seen when the temperature falls to near or below the calculated formation temperature of nitric acid trihydrate. There is clear evidence in the sonde data for gravity wave and planetary wave modulation of the stratospheric temperature. We see small‐scale wavelike structure in the PSC profiles, which likely arises from gravity waves, but find little direct correlation between a given (vertical) lidar PSC profile and near‐contemporaneous sonde profile, probably because the sonde flight is not an instantaneous sounding at the exact spatial location of the lidar. The planetary‐wave‐induced temperature variations are of the order of 10 times the size of the gravity wave variations and have a greater and more direct influence in PSC occurrence over Davis.

Comparison of a large‐scale inertia‐gravity wave as seen in the ECMWF analyses and from radiosondes

The location, characteristics and evolution of a large‐scale inertia‐gravity wave that occurred in the lower stratosphere over the North of the British Isles on February 6, 1997, are studied. Numerous high‐resolution radiosondes were available at that time and in that region as part of the FASTEX database. They reveal an intense, large‐scale inertia‐gravity wave (IGW), propagating upwards above the tropopause. Maps of the divergence of the horizontal wind, on isentropic surfaces, were obtained from the ECMWF analyses, and showed a clear pattern of alternating bands of convergence and divergence, at the lower stratospheric heights, in the same geographical region and starting at the same time. The comparison of the characteristics of this IGW in the analyses and in the observations suggests that the ECMWF analyses can be used for qualitative indications regarding the locations most favorable to large‐scale IGW generation and the corresponding orientation of the waves.

VHF radar observation of atmospheric winds, associated shears and <b>C<sup>2</sup></b><b><sub>n</sub></b> at a tropical location: interdependence and seasonal pattern

Abstract. The turbulence refractivity structure constant (C2n ) is an important parameter of the atmosphere. VHF radars have been used extensively for the measurements of C2n. Presently, most of such observations are from mid and high latitudes and only very limited observations are available for equatorial and tropical latitudes. Indian MST radar is an excellent tool for making high-resolution measurements of atmospheric winds, associated shears and turbulence refractivity structure constant (C2n). This radar is located at Gadanki (13.45° N, 79.18° E), a tropical station in India. The objective of this paper is to bring out the height structure of C2n for different seasons using the long series of data (September 1995 – August 1999) from Indian MST radar. An attempt is also made to understand such changes in the height structure of C2n in relation to background atmospheric parameters such as horizontal winds and associated shears. The height structure of C2n, during the summer monsoon and post-monsoon season, shows specific height features that are found to be related to Tropical Easterly Jet (TEJ) winds. It is important to examine the nature of the radar back-scatterers and also to understand the causative mechanism of such scatterers. Aspect sensitivity of the received radar echo is examined for this purpose. It is observed that radar back-scatterers at the upper tropospheric and lower stratospheric heights are more anisotropic, with horizontal correlation length of 10–20 m, as compared to those observed at lower and middle tropospheric heights.Key words. Meteorology and atmospheric dynamics (climatology; tropical meteorology; turbulence)

Vertical Variation of Madden-Jullian Oscillations in the Normal Monsoon Season as Revealed Through MST Radar Wind Data

Mesosphere-Stratosphere-Troposphere (MST) Radar wind data for the period June through September 1996 have been examined to study vertical variation of Madden-Jullian Oscillations in wind and eddy kinetic energy (eke) in the normal monsoon season. The domain of analysis in the vertical is from 6 to 20 km with a height resolution of 150 m. Fast-Fourier-Transformation (FFT) has been applied to zonal (u), meridional(v) components of wind to extract the Madden-Jullian oscillations and eke. There are three dominant modes viz., 50–70, 30–40 and 10–20 day periodicity, which contain considerable fraction of energy and show high degree of vertical variability. The peak amplitude of 50–70 day mode in u, 30–40 mode in v and eke were observed at 16–17 km just below the tropopause level. The peak amplitudes of 30–40 day mode in u and 50–70 day mode in v were found in the height region of 13–16 km. To understand the origin and propagation of these waves, wave energy is calculated. The wave energy is higher at tropospheric heights than at lower stratospheric heights indicating that the origin of these waves is in the troposphere, and a part of the energy leaks into the stratosphere.

Applications of the NERC MST radar facility in mesoscale studies

AbstractThe NERC radar facility located near Aberystwyth (52.4°N,4.1°W) was established for remote sensing of the structure and dynamics of the mesosphere, stratosphere and troposphere; hence the name MST radar. The facility records scatter and reflection from metre‐scale irregularities in refractive index produced by atmospheric waves and turbulence. The measurement of the three‐dimensional wind field at tropospheric and lower stratospheric heights represents the primary capability of the facility and is expected to be particularly useful in short‐range local weather forecasting operations. At the same time, the temporal and height resolution achievable lend themselves to studies of smaller‐scale, time‐varying phenomena, such as waves and turbulence. Particular attention has been paid to the characteristics and vertical propagation of mountain waves and inertia‐gravity waves, and estimates made of the vertical flux of horizontal momentum associated with such waves, known to play a significant role in the large‐scale atmospheric circulation. The turbulence associated with the dissipation of these waves, and also generated in the vicinity of jet streams, has also been examined. Quasi‐specular reflection of radar waves associated with changes in temperature gradients has helped monitor the height and sharpness of the tropopause and also the structure of frontal zones. The technical capabilities of the facility and the continuity of data provided by its all‐weather and extended operation have been shown to be well suited to both mesoscale meteorological and research applications. Copyright © 1999 Royal Meteorological Society

Inertia-gravity waves in the troposphere and lower stratosphere associated with a jet stream exit region

Abstract. Radar measurements at Aberystwyth (52.4° N, 4.1° W) of winds at tropospheric and lower stratospheric heights are shown for 12-13 March 1994 in a region of highly curved flow, downstream of the jet maximum. The perturbations of horizontal velocity have comparable amplitudes in the troposphere and lower stratosphere with downward and upward phase propagation, respectively, in these two height regions. The sense of rotation with increasing height in hodographs of horizontal perturbation velocity derived for hourly intervals show downwards propagation of energy in the troposphere and upward propagation in the lower stratosphere with vertical wavelengths of 1.7 to 2.3 km. The results indicate inertia-gravity waves propagating in a direction similar to that of the jet stream but at smaller velocities. Some of the features observed contrast with those of previous observations of inertia-gravity waves propagating transverse to the jet stream. The interpretation of the hodographs to derive wave parameters has taken account of the vertical shear of the background wind transverse to the direction of wave propagation.Key words. Meteorology and atmospheric dynamics (mesoscale meteorology; middle atmosphere dynamics; waves and tides)

Comparing upper tropospheric and lower stratospheric temperatures: Microwave sounding unit, radiosonde, COSPAR International Reference Atmosphere, and National Center for Environmental Prediction/National Center for Atmospheric Research reanalysis monthly mean climatologies

Climate‐modeling groups are having to assess their models' temperatures at high, thinly observed altitudes in order to investigate near‐tropopause forcings with confidence. To support such analyses, the microwave sounding unit (MSU), GFDL/Oort radiosonde, COSPAR International Reference Atmosphere (CIRA), and new 13‐year National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis Project climatologies are intercompared herein in terms of their monthly mean microwave brightness temperatures Tb. In the lower stratosphere these climatologies agree extremely well. Small differences between NCEP/NCAR and MSU Tb centered at about 80 mbar amount to ≤2 K year round across the tropics and ≤5 K in southern winter polar latitudes. Artificial land‐ocean outlines ≤2 K do appear in maps of the NCEP/NCAR lower stratospheric Tb between 30°S and 30°N. This NCEP/NCAR land‐ocean Tb distinction may be due to sparseness of inputed radiosonde data at lower stratospheric heights not compensated for by satellite retrievals in the NCEP/NCAR assimilation process. In the upper troposphere, good agreement again occurs between the GFDL radiosonde, the CIRA, and the NCEP/NCAR reanalysis Tb. However, the experimental MSU channel 3R climatology has markedly cooler upper tropospheric Tb. Coolness peaks at over 6 K across low latitudes and diminishes to a few kelvins at polar latitudes with little seasonal dependence. The cool bias of the MSU channel 3R is likely due to a combination of channel 3 receiver drift and limb‐darkening problems. Latitudinal correction terms are suggested for this experimental, but valuable, MSU product based on comparisons with calculated NCEP/NCAR reanalysis Tb.

Laser–radar measurements in southern England of aerosols from Mount St Helens

Material from the eruption of Mount St Helens (46°N, 122°W) has been detected at lower stratospheric heights using a laser-radar system at Winkfield (51°N, 1°W). The observation of a distinct layer of ∼1 km thickness near 15.5 km altitude for several days in early June is consistent with the west–east transport of material from above the eruption site expected at this height and also indicates a significant northward spread of the material. In late July less pronounced layers were observed at 17–18 km and 22 km at which level the direction of motion is less certain.