B. L. Gary

Gravity waves generated by a tropical cyclone during the STEP tropical field program: A case study

Overflights of a tropical cyclone during the Australian winter monsoon field experiment of the Stratosphere-Troposphere Exchange Project (STEP) show the presence of two mesoscale phenomena: a vertically propagating gravity wave with a horizontal wavelength of about 110 km and a feature with a horizontal scale comparable to that of the cyclones entire cloud shield (wavelength of 250 km or greater). The larger feature is fairly steady, though its physical interpretation is ambiguous. The 110-km gravity wave is transient, having maximum amplitude early in the flight and decreasing in amplitude thereafter. Its scale is comparable to that of 100-to 150-km-diameter cells of low satellite brightness temperatures within the overall cyclone cloud shield; these cells have lifetimes of 4.5 to 6 hours. Aircraft flights through the anvil show that these cells correspond to regions of enhanced convection, higher cloud altitude, and upwardly displaced potential temperature surfaces. A three-dimensional transient linear gravity wave simulation shows that the temporal and spatial distribution of meteorological variables associated with the 110-km gravity wave can be simulated by a slowly moving transient forcing at the anvil top having an amplitude of 400–600 m, a lifetime of 4.5–6 hours and a size comparable to the cells of low brightness temperature. The forcing amplitudes indicate that the zonal drag due to breaking mesoscale transient convective gravity waves is definitely important to the westerly phase of the stratopause semiannual oscillation and possibly important to the easterly phase of the quasi-biennial oscillation. There is strong evidence that some of the mesoscale gravity waves break below 20 km as well. The effect of this wave breaking on the diabatic circulation below 20 km may be comparable to that of above-cloud diabatic cooling.

Aircraft observations of thin cirrus clouds near the tropical tropopause

This work describes aircraft-based lidar observations of thin cirrus clouds at the tropical tropopause in the central Pacific obtained during the Tropical Ozone Transport Experiment/Vortex Ozone Transport Experiment (TOTE/VOTE) in December 1995 and February 1996. Thin cirrus clouds were found at the tropopause on each of the four flights which penetrated within 15° of the equator at 200–210 east longitude. South of 15°N, thin cirrus were detected above the aircraft about 65% of the time that data were available. The altitudes of these clouds exceeded 18 km at times. The cirrus observations could be divided into two basic types: thin quasi-laminar wisps and thicker, more textured structures. On the basis of trajectory analyses and temperature histories, these two types were usually formed respectively by (1) in situ cooling on both a synoptic scale and mesoscale and (2) recent (a few days) outflow from convection. There is evidence from one case that the thicker clouds can also be formed by in situ cooling. The actual presence or absence of thin cirrus clouds was also consistent with the temperature and convective histories derived from back trajectory calculations. Notably, at any given time, only a relatively small portion (at most 25%) of the west central tropical Pacific has been influenced by convection within the previous 10 days. The structures of some of the thin cirrus clouds formed in situ strongly resembled long-wavelength (500–1000 km) gravity waves observed nearly simultaneously by the ER-2 on one of the flights. Comparison with in situ water vapor profiles made by the NASA ER-2 aircraft provide some observational support for the hypothesis that thin cirrus clouds play an important role in dehydrating tropospheric air as it enters the stratosphere.

Stratospheric horizontal wavenumber spectra of winds, potential temperature, and atmospheric tracers observed by high‐altitude aircraft

Abstract : Horizontal wavenumber power spectra of vertical and horizontal wind velocities, potential temperatures, and ozone and N(2)O mixing ratios, as measured in the mid-stratosphere during 73 ER-2 flights (altitude approx. 20km) are presented. The velocity and potential temperature spectra in the 100 to 1-km wavelength range deviate significantly from the uniform -5/3 power law expected for the inverse energy-cascade regime of two-dimensional turbulence and also for inertial-range, three-dimensional turbulence. Instead, steeper spectra approximately consistent with a -3 power law are observed at horizontal scales smaller than 3 km for all velocity components as well as potential temperature. Shallower spectra are observed at scales longer than 6 km. For horizontal velocity and potential temperature the spectral indices at longer scales are between -1.5 and -2.0. For vertical velocity the spectrum at longer scales become flat. It is argued that the observed velocity and potential temperature spectra are consistent with gravity waves. At smaller scales, the shapes are also superficially consistent with a Lumley-Shur-Weinstock buoyant subrange of turbulence and/or nonlinear gravity waves. Contemporaneous spectra of ozone and N(sub 2)O mixing ratio in the 100 to 1-km wavelength range do conform to an approximately uniform -5/3 power law. It is argued that this may reflect interactions between gravity wave air-parcel displacements and laminar or filamentary structures in the trace gas mixing ratio field produced by enstropy-cascading two-dimensional turbulence.

Ice nucleation processes in upper tropospheric wave‐clouds observed during SUCCESS

We have compared in situ measurements near the leading-edges of wave-clouds observed during the SUCCESS experiment with numerical simulations. Observations of high supersaturations with respect to ice (>50%) near the leading edge of a very cold wave cloud (T <−60°C) are approximately consistent with recent theoretical and laboratory studies suggesting that large supersaturations are required to homogeneously freeze sulfate aerosols. Also, the peak ice crystal number densities observed in this cloud (about 4 cm−3) are consistent with the number densities calculated in our model. In the warmer wave-cloud (T ≃−37°C) relatively large ice number densities were observed (20–40 cm−3). Our model calculations suggest that these large number densities are probably caused by activation of sulfate aerosols into liquid droplets followed by subsequent homogeneous freezing. If moderate numbers of effective heterogeneous freezing nuclei (0.5–1 cm−3) had been present in either of these clouds, then the number densities of ice crystals and the peak relative humidities should have been lower than the observed values.

Troposphere‐to‐stratosphere transport in the lowermost stratosphere from measurements of H2O, CO2, N2O and O3

The origin of air in the lowermost stratosphere is investigated with measurements from the NASA ER-2 aircraft. Air with high water vapor mixing ratios was observed in the stratosphere at θ∼330–380 K near 40 N in May 1995, indicating the influence of intrusions of tropospheric air. Assuming that observed tracer-tracer relationships reflect mixing lines between tropospheric and stratospheric air masses, we calculate mixing ratios of H2O (12–24 ppmv) and CO2 for the admixed tropospheric air at θ=352–364 K. Temperatures on the 355 K surface at 20–40 N were low enough to dehydrate air to these values. While most ER-2 CO2 data in both hemispheres are consistent with tropical or subtropical air entering the lowermost stratosphere, measurements from May 1995 for θ<362 K suggest that entry of air from the midlatitude upper troposphere can occur in conjunction with mixing processes near the tropopause.

Meridional distributions of NOx, NOy, and other species in the lower stratosphere and upper troposphere during AASE II

The meridional distribution of NOx in the lower stratosphere and upper troposphere is inferred from 10 flights of the NASA DC-8 in the northern winter of 1992, along with like distributions of NOy, NOx/NOy, CO, and C2Cl4. In the lowest few km of the stratosphere there is little vertical gradient in NOx over the range of latitudes measured (40°–90°N). There is a substantial latitudinal gradient, with 50 pptv above the pole and 120 pptv near 40°N. In the uppermost few km of the troposphere, background values range from 30 pptv over the pole to 90 pptv near 40°N. On two occasions higher values, up to 140 pptv in the mean, were seen 2–3 km below the tropopause in association with frontal systems. The meridional distributions of CO and C2Cl4 show the same feature, suggesting that the source of the elevated NOx is near the earths surface.

Observational constraints on the formation of type ia polar stratospheric clouds

On January 11, 1989 during AASE I, Type Ia polar stratospheric clouds (PSCs) were observed over a vast area in the Arctic. Synoptic scale temperature histories for this flight are obtained using the Goddard trajectory model. Mesoscale temperature fluctuations (MTF) are divided into two major categories of background and lee waves. The statistics of background MTFs are derived from measurements by the microwave temperature profiler aboard the ER-2 aircraft. A forecast model is used to determine the occurrence of lee waves. The MTFs are then superimposed upon the Goddard synoptic scale temperatures. Statistical correlations between temperature histories and Type Ia PSC observations indicate that MTFs can not be solely responsible for the formation of Type Ia PSCs in the stratosphere. Instead, on a synoptic scale, the exposure time of air masses to temperatures below the nitric acid trihydrate (NAT) frost point for ∼ 1 day or more should be the main criteria for predicting the occurrence of Type Ia clouds in the stratosphere.

Heterogeneous reaction probabilities, solubilities, and the physical state of cold volcanic aerosols

On 19 January 1992, heterogeneous loss of HNO3, ClNO3, and HCl was observed in part of the Mount Pinatubo volcanic cloud that had cooled as a result of forced ascent. Portions of the volcanic cloud froze near 191 kelvin. The reaction probability of ClNO3 and the solubility of HNO3 were close to laboratory measurements on liquid sulfuric acid. The magnitude of the observed loss of HCl suggests that it underwent a heterogeneous reaction. Such reactions could lead to substantial loss of HCl on background sulfuric acid particles and so be important for polar ozone loss.

A case study of the Mountain Lee Wave Event of January 6, 1992

A mountain wave event, observed at the southern tip of Greenland on January 6, 1992, was corroborated by three experiments: the Meteorological Measurement System (MMS), the Microwave Temperature Profiler (MTP), and the Reactive Nitrogen Instrument (NO/NOy). Gravity wave signatures with classical phase relationship between wind and temperature were observed on both the outbound and inbound legs at different altitudes. The waves showed both vertically propagating and evanescent properties. Characteristics of the dominant wave mode are: wavelength ≈ 35 km; vertical displacement ≈ 0.8 km, and peak-to-peak vertical wind ≈ 6 ms−1. With the prevailing wind at ≈ 37 ms−1, the stratospheric temperature was reduced by 6K to 195.5K within 8 minutes. The implication and potential impact of mountain lee waves on the formation of polar stratospheric clouds (PSCs) are discussed.

Dynamics of Rocky Mountain lee waves observed during SUCCESS

On two days during SUCCESS, the NASA DC-8 flew carefully designed flight tracks to sample wave clouds downstream of the Rocky mountains. Wave morphology was deduced by applying linear perturbation theory to in-situ measurements of the Meteorological Measurement System (MMS) and remotely sensed isentrope profiles of the Micro-wave Temperature Profiler (MTP). Vertical winds from the MMS were consistent with updraft velocities derived from air parcel displacements estimated from the MTP. Derived wave characteristics and cooling rates are useful inputs to cloud microphysics models.