Showing papers in "Geophysical Research Letters in 1998"

The Arctic oscillation signature in the wintertime geopotential height and temperature fields

Abstract: The leading empirical orthogonal function of the wintertime sea-level pressure field is more strongly coupled to surface air temperature fluctuations over the Eurasian continent than the North Atlantic Oscillation (NAO). It resembles the NAO in many respects; but its primary center of action covers more of the Arctic, giving it a more zonally symmetric appearance. Coupled to strong fluctuations at the 50-hPa level on the intraseasonal, interannual, and interdecadal time scales, this "Arctic Oscillation" (AO)can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft. It is proposed that the zonally asymmetric surface air temperature and mid-tropospheric circulation anomalies observed in association with the AO may be secondary baroclinic features induced by the land-sea contrasts. The same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature, sea-level pressure, and 50-hPa height over the past 30 years: parts of Eurasia have warmed by as much as several K, sea-level pressure over parts of the Arctic has fallen by 4 hPa, and the core of the lower stratospheric polar vortex has cooled by several K. These trends can be interpreted as the development of a systematic bias in one of the atmosphere's dominant, naturally occurring modes of variability.

Radar interferogram filtering for geophysical applications

Abstract: The use of SAR interferometry is often impeded by decorrelation from thermal noise, temporal change, and baseline geometry. Power spectra of interferograms are typically the sum of a narrow-band component combined with broad-band noise. We describe a new adaptive filtering algorithm that dramatically lowers phase noise, improving both measurement accuracy and phase unwrapping, while demonstrating graceful degradation in regions of pure noise. The performance of the filter is demonstrated with SAR data from the ERS satellites over the Jakobshavns glacier of Greenland.

New estimates of radiative forcing due to well mixed greenhouse gases

Abstract: We have performed new calculations of the radiative forcing due to changes in the concentrations of the most important well mixed greenhouse gases (WMGG) since pre-industrial time. Three radiative transfer models are used. The radiative forcing due to CO2, including shortwave absorption, is 15% lower than the previous IPCC estimate. The radiative forcing due to all the WMGG is calculated to 2.25 Wm−2, which we estimate to be accurate to within about 5%. The importance of the CFCs is increased by about 20% relative to the total effect of all WMGG compared to previous estimates. We present updates to simple forcing-concentration relationships previously used by IPCC.

SOHO/EIT observations of an Earth-directed coronal mass ejection on May 12, 1997

Abstract: An earth-directed coronal mass ejection (CME) was observed on May 12, 1997 by the SOHO Extreme ultraviolet Imaging Telescope (EIT). The CME, originating north of the central solar meridian, was later observed by the Large Angle Spectrometric Coronagraph (LASCO) as a “halo” CME: a bright expanding ring centered about the occulting disk. Beginning at about 04:35 UT, EIT recorded several CME signatures, including dimming regions close to the eruption, post-eruption arcade formation, and a bright wavefront propagating quasi-radially from the source region. Each of these phenomena appear to be associated with the same eruption, and the onset time of these features corresponds with the estimated onset time observed in LASCO. We discuss the correspondence of these features as observed by EIT with the structure of the CME in the LASCO data.

Potential waves for relativistic electron scattering and stochastic acceleration during magnetic storms

Abstract: The possibility of electron stochastic energization to relativisitic energies (≥ 1 MeV) via resonant wave-particle interactions during a magnetic storm is explored. The minimum electron energy Emin for cyclotron resonant interaction with various electromagnetic waves is calculated for conditions representative of storm-times. Since Emin > 1 MeV for resonance with L-mode ion cyclotron waves, intense stormtime EMIC waves could contribute to relativistic electron loss, but not acceleration. Inside the plasmapause whistler mode waves, and highly oblique magnetosonic waves near the lower hybrid frequency, can resonate with electrons over the important energy range from ∼ 100 keV to ∼ 1 MeV. In low density regions outside the plasmapause, the whistler, RX, LO and Z modes can resonate with electrons over a similar energy range. These waves have the potential to contribute to the stochastic acceleration of electrons up to relativistic energies during magnetic storms.

FAST satellite observations of large‐amplitude solitary structures

Abstract: We report observations of “fast solitary waves” that are ubiquitous in downward current regions of the mid-altitude auroral zone. The single-period structures have large amplitudes (up to 2.5 V/m), travel much faster than the ion acoustic speed, carry substantial potentials (up to ∼100 Volts), and are associated with strong modulations of energetic electron fluxes. The amplitude and speed of the structures distinguishes them from ion-acoustic solitary waves or weak double layers. The electromagnetic signature appears to be that of an positive charge (electron hole) traveling anti-earthward. We present evidence that the structures are in or near regions of magnetic-field-aligned electric fields and propose that these nonlinear structures play a key role in supporting parallel electric fields in the downward current region of the auroral zone.

The Sun's total irradiance: Cycles, trends and related climate change uncertainties since 1976

Abstract: A composite record of the Sun's total irradiance compiled from measurements made by five independent space-based radiometers since 1978 exhibits a prominent 11-year cycle with similar levels during 1986 and 1996, the two most recent minimum epochs of solar activity. This finding contradicts recent assertions of a 0.04% irradiance increase from the 1986 to 1996 solar minima and suggests that solar radiative output trends contributed little of the 0.2°C increase in the global mean surface temperature in the past decade. Nor does our 18-year composite irradiance record support a recent upward irradiance trend inferred from solar cycle length, a parameter used to imply a close linkage in the present century between solar variability and climate change.

Global variations in droughts and wet spells: 1900–1995

Abstract: The Palmer Drought Severity Index (PDSI) was calculated globally using gridded monthly air temperature and precipitation. From 1900 to 1995, there are large multi- year to decadal variations in the percentage areas in severe drought (PDSI +3:0) over many land areas while secular trends are small. Since the late 1970s, however, there have been some in- creases in the combined percentage areas in severe drought and severe moisture surplus, resulting from increases in ei- ther the drought area (e.g., over the Sahel, eastern Asia and southern Africa) or both the drought and wet areas (e.g., over the U.S. and Europe). Although the high per- centages of the dry and wet areas in the recent decades are not unprecedented during this century (except the Sahel), the recent changes are closely relate to the shift in El Ni~ no - Southern Oscillation (ENSO) towards more warm events since the late 1970s and coincide with record high global mean temperatures. Moreover, for any given value of ENSO indices, the PDSI anomalies tend to be larger than would be expected from previous records. These changes are qualita- tively consistent with those expected from increased green- house gases in the atmosphere.

Precipitation sensitivity to global warming: Comparison of observations with HadCM2 simulations

Abstract: Recent century-long experiments performed with global climate models have simulated observed trends in global-mean temperature quite successfully when both greenhouse gas and aerosol forcing has been included. The performance of these same experiments in simulating observed global-scale changes in precipitation has not previously been examined. Here we use a gridded terrestrial precipitation dataset for the period 1900 to 1996 to examine the extent to which observed global and zonal-mean precipitation sensitivities to global warming have been captured by a series of model simulations recently completed by the UK Hadley Centre. There are signs that the model has been able to reproduce at least some of the observed zonal-mean variations in the precipitation sensitivity to warming. Questions remain both about the quality of the observed precipitation data and about the spatial scale at which anthropogenically-forced global climate models can be expected to reproduce observed variations in precipitation.

Nature of surface electrical conductivity in natural sands, sandstones, and clays

Abstract: The electrical conductivity of rocks results from conduction through the bulk solution occupying the pores and from surface conduction occurring at the fluid/grain interface. The nature of the surface electrical conductivity of shaly and clean sands and sandstones is examined. Surface conduction is characterized by the specific surface conductance which is the sum of three contributions: (i) Conduction within the electrical diffuse layer, which makes a negligible contribution to the total specific surface conductance, (ii) Conduction in the Stern layer, which is shown to vary significantly with the salinity of the pore fluid at low salinities (10−6 to 10−3 mol 1−1), but becomes independent of salinity at higher salinities, (iii) A mechanism operating directly on the mineral surface, independent of salinity, and perhaps associated with proton transfer. At salinities higher than 10−3 mol 1−1 and at 25°C, the specific surface conductance of quartz and clays is equal to 8.9×10−9 S and 2.5×10−9 S respectively. Equations describing the influence of surface conductivity and microstructure upon the macroscopic electrical conductivity of sands, sandstones, and shales are also developed.

POLAR observations of coherent electric field structures

Abstract: The POLAR plasma wave instrument often detects coherent electric field structures in the high altitude polar magnetosphere. The structures appear to be positively charged potentials which are found to move both up and down the ambient magnetic field. Typical estimated velocities and parallel scale sizes are the order of 1000 km/s and 100-1000 meters, respectively. We have observed the structures at radial distances of 2.02 to 8.5 Re and L shells of 6 - 12+, although the they are likely to occur over a broader range of space than suggested by this initial study. The structures are responsible for some of the the spectral features of broadband electrostatic noise, and are similar to recent GEOTAIL and FAST observations of solitary waves.

Melting and crystal structure of iron at high pressures and temperatures

Abstract: High-pressure melting, phase transitions and structures of iron have been studied to 84 GPa and 3500 K with an improved laser heated diamond anvil cell technique and in situ high P-T x-ray diffraction. At pressures below 60 GPa, the lower bound on the melting curve is close to those measured by Boehler [1993] and Saxena et al. [1993]; however, at pressures above 60 GPa our data indicate melting at higher temperatures than these studies, but still lower than the melting curve of Williams et al [1990]. The e-γ-1 triple point is 60(±5) GPa and 2800(±200) K, based on our data of the e-γ phase transition and the observation of melting by in situ x-ray diffraction. No solid phases other than e-Fe and γ-Fe were observed in situ at high temperatures (>1000 K) and pressures to 84 GPa. However, the diffraction patterns of temperature quenched products at high pressure can be fit to other structures such as dhcp.

FAST observations in the downward auroral current region: Energetic upgoing electron beams, parallel potential drops, and ion heating

Abstract: Observations of plasma particles and fields by the FAST satellite find evidence of acceleration of intense upgoing electron beams by quasi-static parallel electric fields. The beam characteristics include a broad energy spectrum with peak energies between 100 eV and 5 keV, perpendicular temperatures less than 1 eV, and fluxes greater than 109/cm²sec. Diverging electrostatic shocks associated with the beams have integrated potentials that match the beam energy. These beams are found in regions of downward Birkeland current and account for the total field-aligned current when they are present. The most energetic ion conics in the auroral zone are found coincident with these beams, in agreement with the model for “trapped” conics. The measured particle densities of the electron beams and associated ion conics are approximately equal and typically range from 1 to 10 cm−3, with no evidence for additional cold density. The beams are seen frequently at altitudes between 2000 and 4000 km in the winter auroral zone. Their probability of occurrence has a strong dependence on season and altitude and is similar to that for upgoing ion beams in the adjacent upward current regions. This similarity suggests that the density and scale height of ionospheric ions play an important role in the formation of both types of beams.

Bipolar electrostatic structures in the shock transition region: Evidence of electron phase space holes

Abstract: We present observations of intense, bipolar, electrostatic structures in the transition region of the terrestrial bow shock from the Wind spacecraft. The electric field signatures are on the order of a tenth of a millisecond in duration and greater than 100 mV/m in amplitude. The measured electric field is generally larger on the smaller dipole antenna, indicating a small spatial size. We compare the potential on the two dipole antennas with a model of antenna response to a Gaussian potential profile. This result agrees with a spatial scale determined by convection and gives a characteristic scale size of 2–7 λd. We interpret the observations as small scale convecting unipolar potential structures, consistent with simulations of electron phase space holes and discuss the results in the context of electron thermalization at strong collisionless shocks.

High resolution mapping of TEC perturbations with the GSI GPS Network over Japan

Abstract: Two-dimensional total electron content (TEC) perturbations over Japan are mapped with the Geographical Survey Institute (GSI) GPS network, GEONET (GPS Earth Observation Network). Its spatial resolution is 0.15° latitude × 0.15° longitude and temporal resolution is 30 seconds. Two-dimensional TEC observations with these high resolutions revealed spatial structures and temporal evolutions of traveling ionospheric disturbances (TIDs) in the nighttime mid-latitude ionosphere on July 03, 1997. A preliminary result of the TEC perturbation mapping indicates that it would be a strong tool to investigate the ionospheric structures in detail. Coordinated observations with other ionospheric observation techniques, such as incoherent scatter (IS) radars, airglow imagers, and ionosondes, are needed to clarify the vertical structure of the ionosphere.

GOME observations of tropospheric BrO in northern hemispheric spring and summer 1997

Abstract: Measurements from the Global Ozone Monitoring Experiment GOME have been analysed for tropospheric BrO in the northern hemispheric spring and summer 1997. Tropospheric excess columns have been determined by subtracting measurements from a longitude range which is assumed to represent background conditions. From February until the end of May enhanced tropospheric BrO columns are observed over the Hudson Bay area and parts of the Canadian Arctic. This large and persistent event has not been reported before and can only be explained by a large local source of bromine. In addition, from March to May other smaller and shorter tropospheric BrO events are detectable along the coast lines of the Arctic Sea and over the polar ice. They correspond to the ground-based observations of enhanced tropospheric BrO reported from several stations in the high Arctic.

The 1997 Umbria‐Marche, Italy, Earthquake Sequence: A first look at the main shocks and aftershocks

Abstract: A long sequence of earthquakes, six with magnitudes between 5 and 6, struck Central Italy starting on September 26, 1997, causing severe damages and loss of human lives. The seismogenic structure consists of a NW-SE elongated fault zone extending for about 40 km. The focal mechanisms of the largest shocks reveal normal faulting with NE-SW extension perpendicular to the trend of the Apennines, consistently with the Quaternary tectonic setting of the internal sector of the belt and with previous earthquakes in adjacent regions. Preliminary data on the main shocks and aftershocks show that extension in this region of the Apennines is accomplished by normal faults dipping at low angle (∼40°) to the southwest, and confined in the upper ∼8 km of the crust. These normal faults might have reactivated thrust planes of the Pliocene compressional tectonics. The aftershock distribution and the damage patterns also suggest that the three main shocks ruptured distinct 5 to 15 km-long fault segments, adjacent and slightly offset from one another.

FAST satellite observations of electric field structures in the auroral zone

Abstract: Electric field and energetic particle observations by the Fast Auroral Snapshot (FAST) satellite provide convincing evidence of particle acceleration by quasi-static, magnetic-field-aligned (parallel) electric fields in both the upward and downward current regions of the auroral zone. We demonstrate this by comparing the inferred parallel potentials of electrostatic shocks with particle energies. We also report nonlinear electric field structures which may play a role in supporting parallel electric fields. These structures include large-amplitude ion cyclotron waves in the upward current region, and intense, spiky electric fields in the downward current region. The observed structures had substantial parallel components and correlative electron flux modulations. Observations of parallel electric fields in two distinct plasmas suggest that parallel electric fields may be a fundamental particle acceleration mechanism in astrophysical plasmas.

The Fast Auroral SnapshoT (FAST) Mission

Abstract: The FAST satellite mission investigates plasma processes occurring in the low altitude auroral acceleration region, where magnetic field-aligned currents couple global magnetospheric current systems to the high latitude ionosphere. In the transition region between the hot tenuous magnetospheric plasma and the cold, dense ionosphere, these currents give rise to parallel electric fields, particle beams, plasma heating, and a host of wave-particle interactions. FAST instruments provide observations of plasma particles and fields in this region, with excellent temporal and spatial resolution combined with high quantitative accuracy. The spacecraft data system performs on-board evaluation of the measurements to select data “snapshots” that are stored for later transmission to the ground. New measurements from FAST show that upward and downward current regions in the auroral zone have complementary field and particle features defined by upward and downward directed parallel electric field structures and corresponding electron and ion beams. Direct measurements of wave particle interactions have led to several discoveries, including Debye-scale electric solitary waves associated with the acceleration of upgoing electron beams and ion heating, and the identification of electrons modulated by ion cyclotron waves as the source of flickering aurora. Detailed quantitative measurements of plasma density, plasma waves, and electron distributions associated with auroral kilometric radiation source regions yield a consistent explanation for AKR wave generation.

Magnetostratigraphy and paleoclimatic interpretation of a continuous 7.2 Ma Late Cenozoic Eolian sediments from the Chinese Loess Plateau

Abstract: An almost 300m thick eolian sequence of Late Cenozoic sediments, which includes 162.5m of Quaternary loess-paleosols and 126m of Late Tertiary eolian Red Clay from the central part of the Chinese Loess Plateau, was investigated to determine the magnetostratigraphy. The results show that eolian dust accumulation, and by inference the related East Asia paleomonsoon, had begun by 7.2Ma. As paleomonsoon are largely controlled by the Tibetan Plateau, this implies that the Plateau had reached some critical elevation by 7.2Ma. The section also documents a rapid increase in eolian dust accumulation in the Late Cenozoic at 3.2Ma that is probably due to the influence of global ice volume on the East Asian monsoon.

Ulysses' return to the slow solar wind

Abstract: After ten long years of wandering the uncharted seas, Ulysses returned to his home port of Ithaca. Similarly, after its unprecedented five year odyssey through the previously uncharted regions over the poles of the Sun, the Ulysses spacecraft has returned to the slow, variable solar wind which dominates observations near the ecliptic plane. Solar wind plasma and magnetic field observations from Ulysses are used to examine this return from the fast polar solar wind through the region of solar wind variability and into a region of slow solar wind from the low latitude streamer belt. As it journeyed equatorward, Ulysses encountered a large corotating interaction region and associated rarefaction region on each solar rotation. Due to these repeated interactions, Ulysses also observed numerous shocks, all of which have tilts that are consistent with those expected for shocks generated by corotating interaction regions. Eventually, Ulysses emerged into a region of unusually steady slow solar wind, indicating that the tilt of the streamer belt with respect to the solar heliographic equator was smaller than the width of the band of slow solar wind from the streamer belt.

On the origin of relativistic electrons in the magnetosphere associated with some geomagnetic storms

Abstract: There has been considerable interest of late in the sudden appearance of large fluxes of relativistic electrons (E>1 MeV) at geostationary orbit due to the adverse effect these electrons can have on operational spacecraft. The question arises as to how these electrons are accelerated to relativistic energies in the relatively short time of a few hours. We shall show that large amplitude ULF pulsations are a unique feature of intervals of time in which the fluxes of relativistic electrons rise to high levels at radial distances beyond the normal L-shell range occupied by the outer radiation belt. We use SAMPEX polar orbiter data for the magnetic storm of November 1993 to show that the fluxes of E>400 keV electrons increase simultaneously over a broad range of L-shells and use this fact to suggest that large amplitude ULF pulsations have the potential to supply the energy necessary to create the relativistic electron fluxes.

Distribution of Atlantic and Pacific waters in the upper Arctic Ocean: Implications for circulation

Abstract: The Atlantic and Pacific oceans provide source waters for the Arctic Ocean that can be distinguished by their differing nitrate and phosphate concentration relationships. Using these relationships, we estimate the amount of Atlantic and Pacific waters in the surface layer (top 30 m) of the Arctic Ocean. Atlantic source water is dominant in most of the Eurasian Basin and is present in significant amounts in the Makarov Basin north of the East Siberian Sea. Pacific source water is dominant in most of the Canadian Basin and is present in significant amounts in the Amundsen Basin north of Greenland. We deduce circulation patterns from the distributions of Atlantic and Pacific source waters in the surface layer of the Arctic Ocean and conclude that the flow within the surface layer differs from ice drift along the North American and European boundaries of the Polar Basin.

Chorus source locations from VLF Poynting flux measurements with the Polar spacecraft

Abstract: Previous studies have used indirect evidence to argue that whistler-mode chorus emissions are generated near the magnetic equator. In this paper a spatial survey of wave normals and Poynting vectors computed from three-component electric and magnetic field measurements is used to show that chorus is generated very close to the magnetic equator. One surprising result is that there are almost no chorus emissions propagating toward the magnetic equator, such as might be expected from high-latitude magnetospheric reflections. The absence of a reflected component indicates that the chorus is reabsorbed, probably by Landau damping, before returning to the magnetic equatorial plane.

Heavy precipitation processes in a warmer climate

Abstract: Climate simulations have suggested that a greenhouse-gas induced global warming would also lead to a moistening of the atmosphere and an intensification of the mean hydrological cycle. Here we study possible attendant effects upon the frequency of heavy precipitation events. For this purpose simulations with a regional climate model are conducted, driven by observed and modified lateral boundary conditions and sea-surface temperature distributions. The modifications correspond to a uniform 2K temperature increase and an attendant 15% increase of the specific humidity (unchanged relative humidity). This strategy allows to isolate the effects of an increased atmospheric moisture content from changes in the atmospheric circulation. The numerical experiments, carried out over Europe and for the fall season, indicate a substantial shift towards more frequent events of strong precipitation. The magnitude of the response increases with the intensity of the event and reaches several 10s of percent for events exceeding 30 mm per day. These results appear to apply to all precipitation events dominated by sea-to-land moisture transport.

Experimental phase equilibria constraints on pre‐eruptive storage conditions of the Soufriere Hills magma

Abstract: New experimental results are used to constrain the P. T, X(H 2 O) conditions of the Soufriere Hills magma prior to ascent and eruption. The experiments were performed on a powdered andesite erupted in January, 1996, at an fO 2 corresponding to ∼NNO+1 with P H2 O and temperatures in the range 50 to 200 MPa and 800 to 940°C. Amphibole is stable at P H2 O >115 MPa and temperatures 72 wt% SiO 2 in residual melt) at P H2 O >115 MPa. Analyses of rhyolitic glass inclusions in quartz and plagioclase from recently erupted samples indicate melt water contents of 4.27±0.54 wt% H 2 O and CO 2 contents <60 ppm. The evolved Soufriere Hills magma would therefore be H 2 O-saturated at pressures <130 MPa. These results suggest that the Soufriere Hills magma containing the stable assemblage amphibole, quartz, plagioclase, orthopyroxene, magnetite and ilmenite was stored at P H2 O of 115-130 MPa, equivalent to a minimum depth for a water-saturated magma chamber of 5-6 km depth. Magma temperatures were initially low (820-840°C). Quartz is believed to have been destabilised by a heating event involving injection of new basaltic magma. The stability field of hornblende provides a useful upper limit (∼880°C) for the extent of this reheating.

Spatial structure of sprites

Abstract: A theory of the electrical breakdown (EB) above thunderstorms is developed. The streamer type of the EB is proposed for the explanation of recent observations of fine spatial structures and bursts of blue optical emissions associated with sprites.

Heavy precipitation on the alpine southside: An upper-level precursor

Abstract: Analyses of four separate autumnal events of very heavy precipitation on the southern slopes of the European Alps indicate that each was accompanied by a strikingly similar anomaly at upper-tropospheric levels. It took the form of a narrow (∼500km), deep (∼4km) and elongated (∼2000 km) streamer of intruded stratospheric air [sic. high potential vorticity air] extending north-south from the British Isles to the western Mediterranean. In each case the streamer translated comparatively slowly eastward, and the storm event ensued as its forward flank approached the Alpine ridge. Empirical indicators and theoretical considerations support the thesis that the streamer is an ubiquitous, dynamically significant, and distinctive precursor of storms on the Alpine southside.

Observations of highly nonlinear internal solitons over the continental shelf

Abstract: Surface disturbances caused by strong, near-surface internal waves are widely seen in coastal regions as bands of slick and rough water propagating shoreward. These surface slicks are typically manifestations of optical and radar backscatter properties above internal solitons, and have strong surface current pulses associated with them. During a recent experiment studying coastal internal waves, extremely strong solitary internal wave packets were observed over a three week period on a very shallow and strongly stratified pycnocline off Northern Oregon. During periods of strongest tidal forcing, solitons were consistently observed on the leading edge of a semidiurnal internal tide bore, with pycnocline displacements up to 25m downward from a 7m initial depth in the first few solitons. The extreme nonlinearity of these internal waves is believed to be unique in ocean observations. This note characterizes these highly nonlinear Solitary Internal Waves (SIW) and presents a second order KdV model which reproduces the form of the displacements, and the small change in soliton width with amplitude predicted by this model.

Laminar cirrus observed near the tropical tropopause by LITE

Abstract: The Lidar In-space Technology Experiment (LITE) provided near-global observations of optically thin clouds during a 10-day Space Shuttle mission in September 1994. We report here on layers of cirrus occurring in thin sheets, which we refer to as ‘laminar’ cirrus, observed near the tropical tropopause. The layers were observed to have thicknesses generally between a few hundred meters and one kilometer and to be unusually homogeneous in the horizontal, with extents of up to 2700 km. Layers were observed near and possibly above the mean tropical tropopause, both in clear air and above intense tropical thunderstorms, but only in the tropics (35°N to 20°S). Thin layers near the tropopause were found to be common, but not pervasive, throughout the tropics including regions characterized by large scale subsidence in the middle troposphere.