Hiroshi Kanzawa

Abrupt termination of the 1997–98 El Niño in response to a Madden–Julian oscillation

The role of the Madden–Julian oscillation—a global atmospheric wave in the tropics that is associated with convective activity and propagates eastwards with a period of about 30–60 days (refs 1, 2)—in triggering El Niño events has been discussed before. But its possible connection with a termination of El Niño has yet to be investigated, despite the difficulty in explaining the timing of El Niño terminations by the basic wind-induced oceanic-wave processes. For the extreme 1997–98 event, the mechanism of both onset and termination have been investigated, but the reason for the abruptness of the termination has yet to be resolved. Here we present global data of precipitation, sea surface temperatures and wind speeds that show a precipitation system associated with an exceptionally strong Madden–Julian oscillation travelling around the Equator in May 1998. The propagation of this atmospheric system was associated with an abrupt intensification of the easterly trade winds over the eastern equatorial Pacific Ocean. Combined with the already shallow equatorial thermocline in the central and eastern Pacific Ocean at that time, these strong winds provided the triggering mechanism for the observed accelerated ending of the 1997–98 El Niño event.

Improved Limb Atmospheric Spectrometer (ILAS) for stratospheric ozone layer measurements by solar occultation technique

The Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) obtained 8 months of data for trace gas species from November 1996 to June 1997 for studies of the stratospheric ozone layer over the high-latitude regions in both the Southern and Northern Hemispheres. The atmospheric parameters observed by ILAS were vertical profiles of ozone, nitric acid, nitrogen dioxide, nitrous oxide, methane, water vapor, aerosol extinction coefficient (at 780 nm), temperature, and pressure. Version 3.10 of the operational data processing algorithm was used to retrieve the corresponding geophysical profiles, which have been compared whenever possible with independent correlative measurements. Selected examples of ILAS products for ozone, nitric acid, and aerosol extinction coefficient (at 780 nm) are provided. The corresponding data is now publicly available for further analyses by the atmospheric community.

Ozone depletion in and below the Arctic vortex for 1997

The winter 1996/97 was quite unusual with late vortex formation and polar stratospheric cloud (PSC) development and subsequent record low temperatures in March. Ozone depletion in the Arctic vortex is determined using ozonesondes. The diabatic cooling is calculated with PV-theta mapped ozone mixing ratios and the large ozone depletions, especially at the center of the vortex where most PSC existence was predicted, enhances the diabatic cooling by up to 80%. The average vortex chemical ozone depletion from January 6 to April 6 is 33, 46, 46, 43, 35, 33, 32 and 21 % in air masses ending at 375, 400, 425, 450, 475, 500, 525, and 550 K (about 14–22 km). This depletion is corrected for transport of ozone across the vortex edge calculated with reverse domain-filling trajectories. 375 K is in fact below the vortex, but the calculation method is applicable at this level with small changes. The column integrated chemical ozone depletion amounts to about 92 DU (21%), which is comparable to the depletions observed during the previous four winters.

Diurnal and nocturnal distribution of stratospheric NO2 from solar and stellar occultation measurements in the Arctic vortex: Comparison with models and ILAS satellite measurements

NO2 mixing ratio profiles were measured at sunset between 14 and 30 km using the Limb Profile Monitor of the Atmosphere (LPMA) experiment and during the night between 13 and 31 km using the Absorption par Minoritaires Ozone et NOx (AMON) experiment inside the Arctic vortex, both on February 26, 1997. Coinciding profiles measured by the Improved Limb Atmospheric Spectrometer (ILAS) instrument on board ADEOS have been used to check the consistency between the satellite and balloon profiles for NO2, O3, and HNO3. A box model has been used for the photochemical correction of the LPMA NO2 profiles at sunset. The resulting NO2 balloon-borne profiles of LPMA and AMON are compared to each other after accounting for the day/night photochemical variation using the box model initialized with measurements. The comparisons thus performed show an average difference less than 9% between the two measurements (considered to sample similar air masses) when the box model is initialized with little chlorine activation (i.e., when the major burden of chlorine is stored in ClONO2) for a 1 day integration. The comparison with the Reprobus 3-D chemistry transport model (CTM) seasonal simulations clearly confirms an underestimation of NO2 by the model below 25 km, in the altitude range where aerosols lead to a complete removal of NOx in the model. Recent updates of rate coefficients for conversion of HNO3 into NO2 only slightly improve the NO2 model results in vortex conditions. These results suggest that a source of NO2 is still lacking in the CTM.

ILAS observations of chemical ozone loss in the Arctic vortex during early spring 1997

Chemical ozone loss rates were estimated for the Arctic stratospheric vortex by using ozone profile data (Version 3.10) obtained with the Improved Limb Atmospheric Spectrometer (ILAS) for the spring of 1997. The analysis method is similar to the Match technique, in which an air parcel that the ILAS sounded twice at different locations and at different times was searched from the ILAS data set, and an ozone change rate was calculated from the two profiles. A statistical analysis indicates that the maximum ozone loss rate was found on the 450 K potential temperature surface in February, amounting to 84 ppbv/day. The integrated ozone loss for two months from February to March 1997 showed its maximum of 1.5±0.1 ppmv at the surface that followed the diabatic descent of the air parcels and reached the 425 K level on March 31. This is about 50% of the initial (February 1) ozone concentration. The present study demonstrated that data from a solar occultation sensor with a moderate altitude resolution can be used for the Match analysis.

Validation of ILAS Version 3.10 ozone with ozonesonde measurements

Ozone (O3) measurements made with the Improved Limb Atmospheric Spectrometer (ILAS) onboard the Advanced Earth Observing Satellite (ADEOS) were validated with correlative ozonesonde measurements conducted at five stations, Andoya, Kiruna and Yakutsk in the Northern Hemisphere, and Neumayer and Syowa in the Southern Hemisphere. The ILAS Version 3.10 O3 vertical profiles were compared with 79 correlative ozonesonde measurements that were made within 500 km and 3 hours in distance and time differences, respectively. The comparisons indicate that ILAS O3 typically has an accuracy within 20% between 12 and 35 km. The precision of the ILAS O3 is estimated to be ±10–25% between 12 and 20 km, ±5–7% between 20 and 30 km, and ±5% between 30 and 40 km.

Improved limb atmospheric spectrometer (ILAS) project: ILAS instrument, performance, and validation plan

Ozone layer observation will be conducted with the solar occultation sensor ILAS (improved limb atmospheric spectrometer) on board the ADEOS (Advanced Earth Observing Satellite; to be launched in August 1996) to provide vertical profiles of ozone, methane, water vapor, nitrogen dioxide, nitric acid, and nitrous oxide from absorption measurements in the infrared region, and temperature and pressure profiles from measurements of absorption due to oxygen molecules in the visible region. Optical properties of stratospheric aerosol and polar stratospheric clouds (PSCs) are also derived from visible and infrared extinction measurements. Using the ILAS flight model, optical performance data was obtained from the experiments with a gas cell and a black body light source. Field experiments have been planned for the post-launch validation, which includes field campaigns using large balloons at Kiruna (Sweden) and ground-based remote sensors at Kiruna, Alaska, Syowa Station and other locations. This paper briefly describes the ILAS instrument, its performance evaluation, laboratory experiments to determine the instrument function, data processing algorithms and validation plans.

ILAS-II instrument and data processing system for stratospheric ozone layer monitoring

The Improved Limb Atmospheric Spectrometer-II (ILAS-II) is a satellite-borne solar occultation sensor developed by the Environment Agency of Japan for measuring ozone, other gas species, and aerosols/PSCs that are related to the ozone chemistry in the stratosphere. The ILAS-II instrument will be installed on board the ADEOS-II satellite that will be put into a sun-synchronous polar orbit by the National Space Development Agency of Japan (NASDA) in November 2001. The ILAS-II measurement is a continuation of that of ILAS on board ADEOS, which obtained data from November 1996 to June 1997. The main components of ILAS-II are four spectrometers and a sun-edge sensor. The spectrometers include an infrared spectrometer to cover about 6 to 12 micrometer in wavelength, a mid-infrared spectrometer 3 to 5.7 micrometer, a narrow band spectrometer around 12.8 micrometer, and a visible spectrometer 753 to 784 nm. The first two spectrometers are used for measuring gas and aerosol/PSC profiles, while the third is for ClONO2 measurements. The visible spectrometer is used for pressure/temperature measurements as well as aerosol/PSC extinction coefficients. The ILAS_II instrument has already completed its development and environment tests, and now is undergoing satellite system environment tests at NASDA. This paper outlines the characteristics and performance results from laboratory tests along with the present status of development of its data processing algorithm and operational software.

Ozone and temperature profiles measured above Kiruna inside, at the edge of, and outside the Arctic Polar Vortex in February and March 1997

Ozone depletion above Kiruna (67.9°N, 21.1°E), Sweden, was investigated using daily ozone and temperature measurements by ozonesondes between 1 February and 25 March 1997. Using UKMO Ertels potential vorticity (EPV) and wind fields, three dynamically distinct regions were defined on a grid of isentropic surfaces viz.: the polar vortex boundary region characterized by steep EPV gradients, the area poleward of the boundary region (inside the polar vortex), and the area equatorward of the boundary region (outside the polar vortex). Due to dynamically induced displacements of the vortex, measurements were made in all three regions. By calculating the isentropic EPV at each measurement point and comparing it with the values defining the equatorward and poleward edges of the vortex boundary region, all ozone and temperature measurements could be binned according to their position relative to the vortex edge. Since the data outside the polar vortex were highly variable, mean ozone profiles and their standard deviations were calculated and compared only for the two other regions. To investigate whether differences between these mean profiles were indicative of ozone loss, the temporal evolution of ozone mixing ratios measured along several isentropic surfaces was examined, taking into account the diabatic descent of airmasses. Finally, ozone loss rates were calculated for six potential temperature surfaces and loss rates of up to 0.63ppm/month were found inside the Arctic vortex at surfaces descending from approximately 475 K (1 February) to 460 K (25 March).

ILAS for stratospheric ozone layer monitoring: outline of data processing (Version 3.00 and 3.10) and validation experiments

The Improved Limb Atmospheric Spectrometer (ILAS) developed by the Environment Agency of Japan was successfully flown in space on board the Sun synchronous polar-orbiting satellite ADEOS (Advanced Earth Observing Satellite) on August 17, 1996, ADEOS, with ILAS and other sensors on board, performed successfully until June 30, 1997, after which an unexpected sudden failure of the ADEOS satellite solar battery system prevented further measurements. ILAS made over 6700 measurements of the stratospheric ozone layer with the solar occultation technique, producing vertical profiles of ozone, other ozone-chemistry related gases, aerosol extinction coefficients, temperature, and pressure in the high-latitude stratosphere. The ILAS data processing software so far used was Version 3.00, and algorithm tests are currently being made for the improved Version 3.10 software which includes correction of nongas contribution due to aerosols and polar stratospheric clouds (PSCs). The present paper describes mainly the status of data processing and validation and the results of validation and comparison with other independent data sets.