Richard M. Bevilacqua

The Polar Ozone and Aerosol Measurement (POAM) III instrument and early validation results

Polar Ozone and Aerosol Measurement (POAM) III, a follow-on to the successful POAM II, is a spaceborne experiment designed to measure the vertical profiles of ozone, water vapor, nitrogen dioxide, and aerosol extinction in the polar stratosphere and upper troposphere with a vertical resolution of 1–2 km. Measurements are made by the solar occultation technique. POAM III, now in polar orbit aboard the SPOT 4 satellite, is providing data on north- and south-polar ozone phenomena, including the south-polar ozone hole, and on the spatial and temporal variability of stratospheric aerosols, polar stratospheric clouds, and polar mesospheric clouds. Differences between the POAM III and POAM II instruments are described. First validations of POAM III data products by comparison with Halogen Occultation Experiment and ozonesonde data are presented.

Validation of measurements of water vapor from the Halogen Occultation Experiment (HALOE)

The Halogen Occultation Experiment (HALOE) experiment is a solar occultation limb sounder which operates between 2.45 and 10.0 μm to measure the composition of the mesosphere, stratosphere, and upper troposphere. It flies onboard the Upper Atmosphere Research Satellite (UARS) which was launched in September 1991. Measurements are made of the transmittance of the atmosphere in a number of spectral channels as the Sun rises or sets behind the limb of the atmosphere. One of the channels, at 6.60 μm, is a broadband filter channel tuned to detect absorption in the ν2 band of water vapor. This paper describes efforts to validate the absolute and relative uncertainties (accuracy and precision) of the measurements from this channel. The HALOE data have been compared with independent measurements, using a variety of observational techniques, from balloons, from the ground, and from other space missions, and with the results of a two-dimensional model. The results show that HALOE is providing global measurements throughout the stratosphere and mesosphere with an accuracy within ±10% over most of this height range, and to within ±30% at the boundaries, and to a precision in the lower stratosphere of a few percent. The H2O data are combined with HALOE measurements of CH4 in order to test the data in terms of conservation of total hydrogen, with most encouraging results. The observed systematic behavior and internal consistency of the HALOE data, coupled with these estimates of their accuracy, indicate that the data may be used for quantitative tests of our understanding of the physical and chemical processes which control the concentration of H2O in the middle atmosphere.

Observations of boreal forest fire smoke in the stratosphere by POAM III, SAGE II, and lidar in 1998

A substantial increase in stratospheric aerosol was recorded between May and October 1998 between 55° and 70°N. This phenomenon was recorded in the absence of reported volcanic eruptions with stratospheric impact potential. The POAM III and SAGE II instruments made numerous measurements of layers of enhanced aerosol extinction substantially higher than typical values 3 to 5 km above the tropopause. A comparison of these observations with lidar profiles, TOMS aerosol index data, and forest fire statistics reveals a strong link between stratospheric aerosol and forest fire smoke. Our analysis strongly suggests that smoke from boreal forest fires was lofted across the tropopause in substantial amounts in several episodes occurring in Canada and eastern Russia. Observations reveal a broad zonal increase in stratospheric aerosol that persisted for at least three months.

Pyro‐cumulonimbus injection of smoke to the stratosphere: Observations and impact of a super blowup in northwestern Canada on 3–4 August 1998

[1] We report observations and analysis of a pyro-cumulonimbus event in the midst of a boreal forest fire blowup in Northwest Territories Canada, near Norman Wells, on 3–4 August 1998. We find that this blowup caused a five-fold increase in lower stratospheric aerosol burden, as well as multiple reports of anomalous enhancements of tropospheric gases and aerosols across Europe 1 week later. Our observations come from solar occultation satellites (POAM III and SAGE II), nadir imagers (GOES, AVHRR, SeaWiFS, DMSP), TOMS, lidar, and backscattersonde. First, we provide a detailed analysis of the 3 August eruption of extreme pyro-convection. This includes identifying the specific pyro-cumulonimbus cells that caused the lower stratospheric aerosol injection, and a meteorological analysis. Next, we characterize the altitude, composition, and opacity of the post-convection smoke plume on 4–7 August. Finally, the stratospheric impact of this injection is analyzed. Satellite images reveal two noteworthy pyro-cumulonimbus phenomena: (1) an active-convection cloud top containing enough smoke to visibly alter the reflectivity of the cloud anvil in the Upper Troposphere Lower Stratosphere (UTLS) and (2) a smoke plume, that endured for at least 2 hours, atop an anvil. The smoke pall deposited by the Norman Wells pyro-convection was a very large, optically dense, UTLS-level plume on 4 August that exhibited a mesoscale cyclonic circulation. An analysis of plume color/texture from SeaWiFS data, aerosol index, and brightness temperature establishes the extreme altitude and ‘‘pure’’ smoke composition of this unique plume. We show what we believe to be a first-ever measurement of strongly enhanced ozone in the lower stratosphere mingled with smoke layers. We conclude that two to four extreme pyro-thunderstorms near Norman Wells created a smoke injection of hemispheric scope that substantially increased stratospheric optical depth, transported aerosols 7 km above the tropopause (above 430 K potential temperature), and also perturbed lower stratospheric ozone.

Increases in middle atmospheric water vapor as observed by the Halogen Occultation Experiment and the ground‐based Water Vapor Millimeter‐Wave Spectrometer from 1991 to 1997

Water vapor measurements made by the Halogen Occultation Experiment (HALOE) from 1991 to 1997 are compared with ground-based observations by the Water Vapor Millimeter-wave Spectrometers (WVMS) taken from 1992 to 1997 at Table Mountain, California (34.4°N, 242.3°E), and at Lauder, New Zealand (45.0°S, 169.7°E). The HALOE measurements show that an upward trend in middle atmospheric water vapor is present at all latitudes. The average trend in the HALOE water vapor retrievals at all latitudes in the 40–60 km range is 0.129 ppmv/yr, while the average trend observed by the WVMS instruments in this altitude range is 0.148 ppmv/yr. This trend is occurring below the altitude where changes in Lyman α associated with solar cycle variations should produce a significant increase in water vapor during this period, and is much larger than the ∼0.02 ppmv/yr trend in water vapor associated with increases in methane entering the stratosphere. In addition to the water vapor increase, HALOE measurements show that there is a temporal decrease in methane at altitudes between 40 and 70 km. This indicates an increase in the conversion of the available methane to water vapor, thus contributing to the observed increase in water vapor. The increase in water vapor observed by both instruments is larger than that which would be expected from the sum of all of the above effects. We therefore conclude that there has been a significant increase in the amount of water vapor entering the middle atmosphere. A temperature increase of ∼0.1 K/yr in regions of stratosphere-troposphere exchange could increase the saturation mixing ratio of water vapor by an amount consistent with the observed increase.

Chemical depletion of Arctic ozone in winter 1999/2000

During Arctic winters with a cold, stable stratospheric circulation, reactions on the surface of polar stratospheric clouds (PSCs) lead to elevated abundances of chlorine monoxide (ClO) that, in the presence of sunlight, destroy ozone. Here we show that PSCs were more widespread during the 1999/2000 Arctic winter than for any other Arctic winter in the past two decades. We have used three fundamentally different approaches to derive the degree of chemical ozone loss from ozonesonde, balloon, aircraft, and satellite instruments. We show that the ozone losses derived from these different instruments and approaches agree very well, resulting in a high level of confidence in the results. Chemical processes led to a 70% reduction of ozone for a region ∼1 km thick of the lower stratosphere, the largest degree of local loss ever reported for the Arctic. The Match analysis of ozonesonde data shows that the accumulated chemical loss of ozone inside the Arctic vortex totaled 117 ± 14 Dobson units (DU) by the end of winter. This loss, combined with dynamical redistribution of air parcels, resulted in a 88 ± 13 DU reduction in total column ozone compared to the amount that would have been present in the absence of any chemical loss. The chemical loss of ozone throughout the winter was nearly balanced by dynamical resupply of ozone to the vortex, resulting in a relatively constant value of total ozone of 340 ± 50 DU between early January and late March. This observation of nearly constant total ozone in the Arctic vortex is in contrast to the increase of total column ozone between January and March that is observed during most years.

Photochemistry and transport of carbon monoxide in the middle atmosphere

Abstract Two-dimensional model calculations of the photochemistry and transport of carbon monoxide in the stratosphere, mesosphere, and lower thermosphere are presented. Results are compared to available observations at midlatitudes, where both observation and theory suggest that mesospheric CO abundances are larger on average in winter than in summer. The calculations also indicate that extremely large densities of CO should be found in the polar night mesosphere and upper stratosphere, but at present no high-latitude data are available for direct comparison. However, it is suggested that such a latitudinal distribution implies that the midlatitude region can exhibit unusually large abundances of CO under conditions of large-scale planetary wave activity. Two midlatitude observations during late January 1982 am shown to be consistent with this possibility.

A nonlinear optimization algorithm for WindSat wind vector retrievals

WindSat is a space-based polarimetric microwave radiometer designed to demonstrate the capability to measure the ocean surface wind vector using a radiometer. We describe a nonlinear iterative algorithm for simultaneous retrieval of sea surface temperature, columnar water vapor, columnar cloud liquid water, and the ocean surface wind vector from WindSat measurements. The algorithm uses a physically based forward model function for the WindSat brightness temperatures. Empirical corrections to the physically based model are discussed. We present evaluations of initial retrieval performance using a six-month dataset of WindSat measurements and collocated data from other satellites and a numerical weather model. We focus primarily on the application to wind vector retrievals.

Polar Ozone and Aerosol Measurement (POAM) II stratospheric NO2, 1993–1996

We describe, for the first time, measurements of stratospheric nitrogen dioxide (NO 2 ) by the Polar Ozone and Aerosol Measurement (POAM II) instrument. Measurements span October 1993 through mid-November 1996 and cover latitude ranges from 55° to 72°N and from 63 to 88°S. Comparisons with coincident satellite and space shuttle observations show good agreement and confirm the validity of POAM II measurements for scientific investigations. Overall seasonal variations in both hemispheres are qualitatively consistent with standard photochemistry. In the austral late winter/early spring of 1994, however, anomalously high NO 2 mixing ratios were observed above 22 km. We conclude that these high NO 2 levels resulted from downward transport of NO x -enhanced air from the mesosphere or thermosphere inside the polar vortex. Enhanced NO 2 mixing ratios in 1994 were factors of ∼1.3 and 2.5 larger than at the corresponding times in 1995 and 1996. We conclude that POAM II observations of coincident, localized reductions in ozone of up to 40% were caused by the increased stratospheric NO via the standard catalytic NO cycle.

Ground‐based measurements of water vapor in the middle atmosphere

We present measurements of the middle atmospheric water vapor mixing ratio profile obtained using the ground-based Naval Research Laboratory water vapor millimeter-wave spectrometer (WVMS) instrument at the Jet Propulsion Laboratory Table Mountain Observatory. The measurements cover a period of 262 days from January 23, 1992, to October 13, 1992. During this campaign it was possible to retrieve useful daily mixing ratio profiles for 186 days. We thus have a nearly continuous record of water vapor mixing ratios for altitudes from ≈35 to 75 km. The retrievals are obtained using the optimal estimation method. Details of the error analysis are presented, and a technique is introduced that reduces baseline effects and helps to estimate the baseline error. The high-altitude (≳65 km) data show a sharp rise prior to the expected maximum near the summer solstice and a gradual decline in the following months. The mixing ratios generally peak between 55 and 65 km, at which point the mixing ratios are 6–7 parts per million by volume. The highest peaks occur in January, May, and October.