Search Results
The revised method for retrieving daytime distributions of atomic oxygen and odd-hydrogens in the mesopause region from satellite observations
Atomic oxygen (O) and atomic hydrogen (H) in the mesopause region are critical species, governing chemistry, airglow, and energy budget. However, they cannot be directly measured by satellite remote sensing techniques and so inference techniques, by airglow observations, are used. In this work, we retrieved daytime O and H distributions at ~ 77 km–100 km from the data of observations by the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument at the TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) satellite in 2003–2015. The retrieval approach considered the reaction H + O3 → O2 + OH in the ozone balance equation. Moreover, we revised all quenching and spontaneous emission coefficients according to latest published data. We then calculated daytime distributions of OH and HO2 at these altitudes with the use of their conditions of photochemical equilibrium.
Hydrological extremes in the Aksu-Tarim River Basin: Mid-latitude dynamics
Analyses of precipitation (1961–2010) from 39 meteorological stations in the Tarim River Basin revealed a trend from dryer towards wetter conditions induced by an increase of the number of wet extremes. A first (1961–1986) and second (1987–2010) period are the basis for a dynamical analysis of changing drought and wetness extremes which are closely related to cyclonic activity over the European continent and circulation anomalies in the Northern Hemisphere mid-latitudes. Wave train, cyclone tracks, water flux and potential vorticity (PV) front analysis of the wet and dry months show the following result: (1) The extreme wet and dry cases in winter and summer are characterized by distinguished wave train patterns upstream of the Tarim River Basin. All wave trains originate in the Atlantic–European sector pointing towards wave train dynamics as one possible mechanism underlying the connection patterns observed. (2) The selected extreme cases show that exceptional precipitation events can be connected to characteristic cyclone tracks and a PV front in the upper troposphere even if cyclone tracks never cross the Tarim Basin. Extremely wet winters are characterized by cyclone tracks close to the western and northern boundary of the Tarim Basin whereas, during extremely dry winters, such cyclone tracks are absent. Wet summers are characterized by long-lived cyclonic anomalies at the north western corner of the Tarim River Basin [see also (3)]. During dry summers such anomalies are absent. (3) On a more local level the hydrological extreme events are linked to special dynamical structures of the upper tropospheric PV front. In winter strong (extreme) precipitation is connected to a strong non-linear wave development or a wave-breaking event over the Tarim River Basin. Together with non-linear wave development moisture and precipitation areas are advected towards the Tarim River Basin. In dry winters the upper tropospheric PV front is much more zonally oriented and wave-breaking is less frequent. Strong precipitation events are connected to strong breaking events and to the formation of long-lived nearly stationary cyclones over or north of the Tarim River Basin during extremely wet summer months.
Solar and lunar daily geomagnetic variations and their equivalent current systems observed by Swarm
This paper describes solar and lunar daily variations of the geomagnetic field over low- and mid-latitude regions, using vector magnetometer data from Swarm satellites at altitudes of ∼500 km during the solar minimum years of 2017–2020. The average solar variation of the geomagnetic field is within the range of ±14 nT, while the lunar variation is within ±2 nT. The latter is comparable to the ocean tidal field. A spherical harmonic analysis is performed on the solar and lunar variations to evaluate their internal and external equivalent current systems. The results show that both the solar and lunar variations are mainly of internal origin, which can be attributed to combined effects of ionospheric dynamo currents and induced underground currents. Global patterns of the internal solar and lunar current systems are consistent with the corresponding external current systems previously reported based on ground observations. The Swarm external currents are mainly in the meridional direction, and are likely associated with interhemispheric field-aligned currents. Both the internal and external current systems depend on the season and longitude.
Note on consistency between Kalogerakis–Sharma Mechanism (KSM) and two-step mechanism of atmospheric band emission (762 nm)
For more than 30 years, a two-step mechanism was used to explain observed Atmospheric band emission (762 nm) in mesopause region. A new mechanism, which leads to the formation of electronically excited molecular oxygen that gives this emission, was proposed recently. We show, based on an analytical solution, that the fit-functions for Atmospheric band volume emission in the case of the two-step mechanism and the new Kalogerakis–Sharma Mechanism (KSM) have analogous expression. This derivation solves the problem of consistency between the well-known two-step mechanism and the newly proposed KSM.
Hydrological extremes in the Aksu‑tarim river Basin: Climatology and regime shift
Precipitation data between 1961 and 2010 from 39 meteorological stations in the Tarim River Basin are analyzed to classify and investigate hydrological drought and wetness conditions by using the standardized precipitation index (SPI). The leading time and spatial variability of hydrological drought has been investigated by applying a principal component analysis and Varimax rotation to the SPI on a time scale of 24 months. The results suggest that the western basin is characterized by a clear tendency towards wetter conditions after the middle of the 1980s, which results from an increase in the number of wet extremes and can be considered as a regime shift. Subdividing the period of analysis into two parts (1961–1986 and 1987–2010) this change can be clearly seen in a shift of the probability distribution function of precipitation events. Composite analyses of monthly mean geopotential height fields and wind fields of the ERA-40 data set show that enhanced wetness in the Tarim River Basin after the middle of 1980s is closely related to cyclonic anomalies on the European continent and circulation anomalies over mid-latitude of the Northern Hemisphere. Further correlation analysis between the principal components of SPI and large circulation indices shows that hydrological extremes in the Tarim River Basin correlate with indices related to the polar vortex and subtropical high.
On the role of anisotropic MF/HF scattering in mesospheric wind estimation
The Saura radar is designed and used to measure winds and electron densities at polar latitudes (69∘N) within the D region, namely between 50 and 100 km altitude. A relatively narrow radar beam can be generated and steered into distinct pointing directions as a rather large antenna array is used. From the observed radial velocities of the individual pointing directions, the horizontal and vertical wind fields can be obtained using the Doppler beam swinging (DBS) method. With recent upgrades to the radar, the interferometric capabilities are largely improved allowing simultaneous application of different wind estimation techniques now, and also echo localization. In recent studies, Saura DBS winds assuming isotropic scattering were found to be underestimated in comparison with highly reliable winds observed with the MAARSY MST radar in the presence of polar mesospheric summer echoes (PMSE). This underestimation has been investigated by analyzing the scattering positions as well as applying the imaging Doppler interferometry technique. Besides this, Saura winds derived with the classical DBS method seem to be error prone at altitudes above 90 km and even below this altitude for periods of enhanced ionization, e.g., particle precipitations. Various methods taking into account the scattering positions have been used to correct the wind underestimation. These winds are compared to MST radar winds during PMSE, and an optimal combination of these methods for the Saura radar is presented. This combined wind data appears to be reliable; it shows reasonable amplitudes as well as tidal structures for the entire altitude region.