K. Roland Chan

The Meteorological Measurement System on the NASA ER-2 Aircraft

Abstract A Meteorological Measurement System (MMS) was designed and installed on one of the NASA high-altitude ER-2 aircraft (NASA 706). The MMS provides in situ measurements of free-stream pressure (±0.3 mb), temperature (±0.3°C), and wind vector (±1 m s−1). It incorporates a high-resolution inertial navigation system (INS) specially configured for scientific applications, a radome differential pressure system for measurements of the airflow angles, and a compact, computer-controlled data acquisition system to sample, process and store 45 variables on tape and on disc. The MMS hardware and software development is described and resolution and accuracy of the instrumentation discussed. Custom software facilitates preflight system checkout, inflight data acquisition, and fast postflight data download. It accommodates various modes of MMS data: analog and digital, serial and parallel, and synchronous and asynchronous. Flight results are presented to demonstrate the capability of the system.

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.

The role of heterogeneous freezing nucleation in upper tropospheric clouds: Inferences from SUCCESS

A temperature spectrum of heterogeneous freezing nuclei concentrations in continental air in the upper troposphere was determined based on airborne measurements. Numerical model simulations incorporating ice formation by heterogeneous and homogeneous freezing of deliquesced soluble aerosol particles were performed to investigate the effect of the heterogeneous process on the microphysics of upper tropospheric clouds. Heterogeneous freezing nuclei were predicted to cause lower maximum concentrations of ice particles formed in clouds. These nuclei also initiate the first ice formation and act to broaden ice crystal size distributions in upper tropospheric clouds. Observations of ice formation in an orographic wave cloud supported these predictions.

Radon measurements in the lower tropical Stratosphere: Evidence for rapid vertical transport and dehydration of tropospheric air

During the tropical experiment of NASAs Stratosphere-Troposphere Exchange Program (STEP), in situ radon and other trace constituent measurements were made aboard a NASA ER-2 high-altitude research aircraft to investigate the mechanisms of irreversible transfers from the troposphere into the tropical stratosphere. Observations made in and downwind of the cirrus shields of three large tropical cyclones and downwind of the cirrus anvil of a large cumulonimbus cloud cluster showed several clear instances of elevated radon activity occurring simultaneously with low total water mixing ratios. These observations are unambiguous evidence of an effective dehydration process, capable of reducing total water vapor mixing ratios to less than 2.5 ppmv, occurring in conjunction with troposphere-to-stratosphere transport and indicate that rapid localized convection, rather than slow regional mean motions, was responsible for the observed transports and associated with the accompanying dehydration. Radon activities measured in regions of active or recent troposphere-to-stratosphere transport were consistent with the 17 pCi/scm mean value needed to support the observed abundance of stratospheric 210Pb.

Environmental conditions required for contrail formation and persistence

The ambient temperatures and humidities required for contrail for- mation and persistence are determined from in situ measurements during the Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS) exper- iment. Ambient temperatures and water vapor concentrations were measured with the meteorological measurement system, a laser hygrometer, and a cryogenic hygrometer (all onboard the DC-8). The threshold temperatures are compared with theoretical estimates based on simple models of plume evolution. Observed contrail onset temperatures for contrail formation are shown to be 0-2 K below the liquid-saturation threshold temperature, implying that saturation with respect to liquid water must be reached at some point in the plume evolution. Visible contrails observed during SUCCESS persisted longer than a few minutes only when substantial ambient supersaturations with respect to ice existed over large regions. On some occasions, contrails formed at relatively high temperatures (>_ -50oC) due to very high ambient supersaturations with respect to ice (of the order of 150%). These warm contrails usually formed in the presence of diffuse cirrus. Water vapor from sublimated ice crystals that entered the engine was probably necessary for contrail formation in some of these cases. At temperatures above about -50oC, contrails can only form if the ambient air is supersaturated with respect to ice, so these contrails should persist and grow.

Refractive indices of aerosols in the upper troposphere and lower stratosphere

A new instrument for simultaneously measuring aerosol diameter from 0.4–10 µm and the refractive index between 1.30–1.60 has recently been flown on the NASA ER-2 aircraft during a stratospheric measurement campaign. Average stratospheric refractive indices varied from 1.40 to 1.42 over a latitude range from 70°S to 50°N and from 1.34 to 1.46 over a vertical range from 4–20 km. The measured stratospheric refractive indices do not agree well with theoretical predictions and vertical profiles suggest the presence of non-spherical or absorbing particles in the altitude range of 7–9 km.

Heterogeneous reactions on stratospheric background aerosols, volcanic sulfuric acid droplets, and type I polar stratospheric clouds: Effects of temperature fluctuations and differences in particle phase

Northern hemispheric ER-2 (NASA) data from stratospheric aerosol measurements during background conditions, periods disturbed by the influence of Mount Pinatubo, and polar stratospheric cloud (PSC) type I events are used to study the heterogeneous reactions of ClONO2 with H2O and of HOCl and ClONO2 with HCl in comparison to the gas phase reaction rate of OH with HCl. To calculate the reaction rates, the measured data of pressure, temperature, water vapor, and aerosol surface are utilized together with recent laboratory results for the heterogeneous reactive uptake coefficients. Because observations are limited, the mixing ratios of the gas phase species entering these rate calculations (i.e., ClONO2, HOCl, HCl, and N2O5) are taken from a two-dimensional model. It is found that in dense volcanic clouds at temperatures below 200 K the resulting heterogeneous reaction rates of chlorine activation can be of similar magnitude as the gas phase reaction rate. The heterogeneous rates in PSCs can exceed the gas phase rates by more than 2 orders of magnitude. For the ClONO2 and HOCl reactions the measured aerosol surfaces during the PSC events are treated both as liquid (e.g., ternary solution) droplets and as solid NAT to compare the effects of the different phases. The reaction rates on NAT are significantly lower than on liquid droplets. Indeed, this study shows that a transition from liquid ternary solutions to NAT is expected to reduce the rate of chlorine activation based on present chemical understanding and on observed aerosol surface areas. Additionally, the effect of temperature and surface area fluctuations on the heterogeneous reaction rates is discussed.

Wavelet analysis of dynamical processes in cirrus

Vertical air velocity observations obtained from the NASA DC-8 on 21 April 1996 are analyzed using wavelet and spectral analysis methods. Wavelet analysis yielded a very useful characterization of the complex dynamical structures encountered above, below and within the cirrus clouds observed on this flight. Intermittent small groups of apparent gravity waves were found at length scales ranging from 2 km to greater than 40 km. Within the cloud, patches of convective cells were also found. The hierarchy of dynamical structures was well discriminated by the analysis.

Northern hemisphere nitrous oxide morphology during the 1989 AASE and the 1991–1992 AASE II campaigns

Nitrous oxide vertical profiles and latitudinal distributions for the 1989 AASE and 1992 AASE II northern polar winters are developed from the ATLAS N[sub 2]O dataset, using both potential temperature and pressure as vertical coordinates. Morphologies show strong descent occurring poleward of the polar jet. The AASE II morphology shows a mid latitude [open quotes]surface zone,[close quotes] characterized by strong horizontal mixing, and a horizontal gradient south of 30[degrees]N due to the sub-tropical jet. These features are similar to those produced by two-dimensional photochemical models which include coupling between transport, radiation, and chemistry. 18 refs., 4 figs.

Comparisons of the NASA ER-2 Meteorological Measurement System with Radar Tracking and Radiosonde Data

Abstract Measurements of aircraft longitude, latitude, and velocity, and measurements of atmospheric pressure, temperature, and horizontal wind from the meteorological measurement system (MMS) on board the NASA ER-2 aircraft were compared with independent measurements of these quantities from radiosondes and radar tracking of both the ER-2 and radiosonde balloons. In general, the comparisons were good and within the expected measurement accuracy and natural variability of the meteorological parameters. Radar tracking of the ER-2 resolved the velocity and position drift of the inertial navigation system (INS). The rms errors in the horizontal velocity components of the ER-2, due to INS errors, were found to be 0.5 m s−1. The magnitude of the drift in longitude and latitude depends on the sign and magnitude of the corresponding component velocity drift and can be a few hundredths of a degree. The radar altitudes of the ER-2 and radiosondes were used as the basis for comparing measurements of atmospheric pre...