Susan J. Hovde

Polar stratospheric cloud processed air and potential voracity in the northern hemisphere lower stratosphere at mid‐latitudes during winter

Small-scale (<1000 km) features in ER-2 measurements of ClO, O3, H2O, N2O, and NOy, outside the lower stratospheric Arctic vortex of 1988–1989 are compared with features on potential vorticity maps from the European Centre for Medium-range Weather Forecasts (ECMWF). The potential vorticity maps are obtained from Tl06 analyses and forecasts. Some of the plots have been truncated to lower resolution (T63 or T42) which smooths out the finer-scale structure. Comparison of these lower resolution plots shows how much detail is lost by excessive smoothing. It is also evident that the forecast plots lose fine-scale structure due to dissipation in the model resulting mainly from horizontal diffusion. We conclude that blobs of air on the maps at latitudes between the vortex edge and 25°N having potential vorticities characteristic of the vortex, did indeed originate from the vortex, but that the real atmosphere is more sharply differentiated (inhomogeneous) than the meteorological analyses, implying that the potential vorticity maps underestimate the amount of peeled-off material. Areal budgets of the ex-vortex air are considered for ER-2 flight days, and are performed for 24-hour forecasts at T63, and analyses at T42, T63, and T106 resolution at θ = 475 K. Finally, it is concluded that the lower stratospheric Arctic vortex of 1988–1989 spread considerable amounts of air to mid-latitudes which had been processed by polar stratospheric clouds, and that this mechanism is a realistic explanation for the wintertime loss of ozone observed over northern mid-latitudes during the last decade.

Exchange between the upper tropical troposphere and the lower stratosphere studied with aircraft observations

Exchange between the upper tropical troposphere and the lower stratosphere is considered by examining WB57F and ER-2 aircraft observations of water, ozone, wind, and temperature in the potential temperature range 360 < θ < 420 K. These processes are examined in part by using the technique of unified scale invariance on the airborne data, as has been done previously for the lower stratospheric polar vortex. Scale invariance is found, on scales from a few hundred meters to the maximum flown, 2700 km (25 great circle degrees). The results apply both to vertical exchange at the tropical tropopause and to isentropic exchange at the subtropical jet stream. All scales participate in the maintenance of the mean state, with substantial contributions from relatively infrequent but intense events in the long tails of the probability distributions. Past data are examined and found to fit this general framework. A unique mapping of tropical tropopause temperature to the total hydrogen content of the middleworld and overworld should not be expected; the head of the tape recorder is at 50-60 hPa rather than 90-100 hPa. The tropical tropopause is observed at potential temperatures θ T greater than the maximum moist static surface values θ W , such that θ T - θ W varies between 10 K in fall and up to 40 K in spring. The meridional gradient of θ T is directed from the subtropical jet stream to the inner tropics, with θ T declining by approximately 10 K from near 30°N to near 10°N in the vicinity of 95°W. The maintenance of these θ T values is discussed. Total water (measured as the sum of vapor and vaporized ice) and ozone, major absorbers of solar radiation and emitters/ absorbers of terrestrial infrared radiation, show scale invariance in the upper tropical troposphere. The implications of this result for the notion of a conservative cascade of energy via fluid dynamics from the largest to the smallest scales are discussed. The scaling exponents H z for total water and ozone in the upper tropical troposphere are not the value, 5/9, expected for a passive scalar, probably indicating the presence of sources and/or sinks operating faster than mixing.

Spread of denitrification from 1987 Antarctic and 1988-1989 Arctic stratospheric vortices

Vertical profiles of N2O and NOy taken by the ER-2 outside the vortex are used to construct average vertical profiles of F(NOy) = NOy/(A-N2O), where A is the tropospheric content of N2O three years prior to the measurements. The southern hemisphere had less nitrous oxide in the range 400 < θ < 470 K, by up to 25% relative to the northern hemisphere. F(NOy) is the ratio of NOy produced to N2O lost in a stratospheric air mass since entry from the troposphere. The profiles of F(NOy) have the following characteristics: (1) Relative to 1991–1992, a year without denitrification inside or outside the vortex, the northern hemisphere in 1988–1989 showed denitrification outside the vortex ranging up to 25% and averaging 17% above θ = 425 K. (2) Relative to the northern hemisphere in 1991–1992, the southern hemisphere in 1987 showed denitrification outside the vortex ranging up to 32% and averaging 20% above θ = 400 K. (3) Below θ = 400 K the southern hemisphere showed enhancements of F(NOy) relative to the northern hemisphere in 1991–1992 ranging up to 200% at θ = 375 K, outside the vortex. Corresponding profiles of residual water, R(H2O) = H2O - 2 [1·6 - CH4], are considered and shown to be consistent with those of F(NOy) in the sense that they show deficits outside the Antarctic vortex, which was both dehydrated and denitrified, but not outside the 1988–1989 Arctic vortex, which was denitrified but not dehydrated. R(H2O) is the water content of stratospheric air with the contribution from methane oxidation subtracted. Comparison of F(NOy) and R(H2O) below 400 K outside the Antarctic vortex leads to the suggestion that dehydration in the Antarctic vortex occurs by the sedimentation of ice crystals large enough to fall out of the stratosphere, whereas denitrification occurs mainly on mixed nitric acid-water crystals which evaporate below the base of the vortex at θ = 400 K but above the tropopause.

Time and temperature dependences of fractional HCl abundances from airborne data in the Southern Hemisphere during 1994

Measurements of HCl and CH4 taken by the aircraft laser infrared absorption spectrometer (ALIAS) on the ER-2 high-altitude research aircraft during the Southern Hemisphere winter of 1994 have been used to examine the abundance of HCl as a fraction of total inorganic chlorine. The fractional abundance of HCl shows a threshold behaviour as a function of temperature history; on a 10 day timescale, the abundance dropped sharply in those air parcels experiencing a temperature < 195 K, but little or no change was seen in parcels which stayed warmer than this temperature. The behaviour mirrors well the temperature behaviour calculated for the transformation of HCl into reactive forms (Cl2, HOCl) from laboratory studies of sulfate aerosols and polar stratospheric clouds. During the course of the winter, the fractional abundance of HCl outside the vortex decreased from its values in late May by about a third, while inside it dropped to near zero by early August. Some recovery was evident in October. Examples of the peel-off of low-HCl air equatorward of the wind maximum were evident in early June. Meteorological trajectories are used to show, in a case study of a flight in early August, that air parcels which experienced temperatures of < 195 K, and as a result had low fractional HCl abundances, did so largely poleward of the maximum in the polar night jet stream. Encountering temperatures of < 195 K during the previous 10 days was a necessary and sufficient condition for the transformation of HCl into reactive forms by heterogeneous reactions. The trajectories further showed that air arriving from sub-tropical latitudes had higher fractional HCl abundances than the air in the middle latitudes, and much higher fractions than the air at high latitudes. The resulting picture is one in which the fractional abundance of HCl in air at mid latitudes was the result of mixing of air from sub-tropical latitudes with air mainly from polward of the jet stream core which has experienced temperatures < 195 K. The sensitivity of the fractional abundance of HCl to the assumption that no HCl enters the stratosphere via the tropical tropopause is examined in the light of an observed profile near the equator with a volume fraction of 0.4 ppb HCl, zero ClO and tropospheric mixing ratios of CFCs at the tropical tropopause.

Intercomparison of HALOE and ER‐2 aircraft H2O and CH4 Observations collected during the Second Airborne Arctic STratospheric Experiment (AASE‐II)

HALOE observations of H2O and CH4 are compared with in situ techniques aboard the ER-2 aircraft during the northern winter of 1991/92, in particular for the dates 911208, 920108, 920217, 920222 and 920320 when the spatial coincidences are close, within ±1° latitude and ±12° longitude. The temporal coincidence is within 4 hours. The version 9 retrievals of HALOE profiles are used, since they contain a self-consistent aerosol correction. The results reveal the limitations of comparing high resolution in situ aircraft data with a remote sounding limb scanner. Of the 5 comparison dates, 3 had HALOE/ER-2 coincidences which occurred near the edge of the Arctic vortex (911208, 920108, 920217); the vertical variability in the HALOE results and the horizontal variability in the ER-2 observations on these days show that the vortex edge is not a region where exact agreement can be expected except by chance. On a 4th comparison date, there was substantial overlap away from the vortex edge, although for some species the aircraft data show considerable variability near the coincidence point. On the 5th comparison date, the ER-2 had no overlap in altitude with the lowest HALOE observations; however, a short linear interpolation over ∼1 km altitude results in smooth composite profiles. Generally speaking the agreement between HALOE and the ER-2 at overlap altitudes is about 12% in the case of water vapor, which shows low horizontal and vertical variability. The difference was shown to contain atmospheric variability in one case by using the HALOE viewpath superimposed on a potential vorticity map to to examine a pair of HALOE observations straddling the coincidence point in longitude. The agreement for methane, with limited data having altitude overlap, is better than 6%.

Molecular velocity distributions and generalized scale invariance in the turbulent atmosphere

Airborne observations of ozone, temperature and the spectral actinic photon flux for ozone in the Arctic lower stratosphere April-September 1997 and January-March 2000 allow a connection to be made between the rate of production of translationally hot atoms and molecules via ozone photodissociation and the intermittency of temperature. Seen in the context of non-equilibrium statistical mechanics literature results from molecular dynamics simulations, the observed correlation between the molecular scale production of translationally hot atoms and molecules and the macroscopic fluid mechanical intermittency of temperature may imply a departure from Maxwell-Boltzmann distributions of molecular velocities, with consequences for chemistry, radiative line shapes and turbulence in the atmosphere, arising from overpopulated high velocity tails of the probability distribution functions (PDFs).

Vertical scaling of temperature, wind and humidity fluctuations: dropsondes from 13 km to the surface of the Pacific Ocean

Observational data were taken in the ‘vertical’ structure at 2 Hz from research dropsondes for temperature, wind speed and relative humidity during the ∼800 s it takes to reach the surface from the ∼13 km altitude of the National Oceanic and Atmospheric Administration (NOAA) Gulfstream 4SP aircraft. The observations were made mainly through the depth of the troposphere above the eastern Pacific Ocean from 15° N to 43° N (dropsondes) and 60° N (aircraft) in 2004. Grand averages of some key figures and of probability distribution functions (PDFs) were formed by compounding the data from the Winter Storms Projects 2004, 2005 and 2006, comprising 246, 324 and 315 (some dropped up to 60° N) useable sondes, respectively. This sizeable data set was used to representatively characterize the statistical fluctuations in the ‘vertical’ structure from 13 km to the surface. The fluctuations are resolved at 5–10 m altitude, so covering up to 3 orders of magnitude of typical tropospheric weighting functions for passive remote sounders. Average ‘vertical’ statistical, multifractal, scaling exponents H, C 1 and α of temperature, wind speed and humidity fluctuations observed at high resolution were computed and are available as potential generators of representative, scale-invariant summaries of the vertical structure of the marine troposphere, for use in design and retrieval of remotely sounded observations.