R. G. Grainger

Radiative forcing from the 1991 Mount Pinatubo volcanic eruption

Volcanic sulfate aerosols in the stratosphere produce significant long-term solar and infrared radiative perturbations in the Earths atmosphere and at the surface, which cause a response of the climate system. Here we study the fundamental process of the development of this volcanic radiative forcing, focusing on the eruption of Mount Pinatubo in the Philippines on June 15, 1991. We develop a spectral-, space-, and time-dependent set of aerosol parameters for 2 years after the Pinatubo eruption using a combination of SAGE II aerosol extinctions and UARS-retrieved effective radii, supported by SAM II, AVHRR, lidar and balloon observations. Using these data, we calculate the aerosol radiative forcing with the ECHAM4 general circulation model (GCM) for cases with climatological and observed sea surface temperature (SST), as well as with and without climate response. We find that the aerosol radiative forcing is not sensitive to the climate variations caused by SST or the atmospheric response to the aerosols, except in regions with varying dense cloudiness. The solar forcing in the near infrared contributes substantially to the total stratospheric heating. A complete formulation of radiative forcing should include not only changes of net fluxes at the tropopause but also the vertical distribution of atmospheric heating rates and the change of downward thermal and net solar radiative fluxes at the surface. These forcing and aerosol data are available for GCM experiments with any spatial and spectral resolution.

Residual circulation in the stratosphere and lower mesosphere as diagnosed from microwave limb sounder data

Results for the residual circulation in the stratosphere and lower mesosphere between September 1991 and August 1994 are reported. This circulation is diagnosed by applying an accurate radiative transfer code to temperature, ozone, and water vapor measurements acquired by the Microwave Limb Sounder (MLS) onboard the Upper Atmosphere Research Satellite (UARS), augmented by climatological distributions of methane, nitrous oxide, nitrogen dioxide, surface albedo, and cloud cover. The sensitivity of the computed heating rates to the presence of Mt. Pinatubo aerosols is explored by utilizing aerosol properties derived from the measurements obtained by the Improved Stratospheric and Mesospheric Sounder instrument, also onboard UARS. The computed vertical velocities exhibit a Semiannual oscillation (SAO) around the tropical stratopause, with the region of downward velocities reaching maximum spatial extent in February and August. This behavior reflects the semiannual oscillation in temperature and ozone and mimics that seen in past studies of the October 1978–May 1979 period based on data from the Limb Infrared Monitor of the Stratosphere onboard the Nimbus 7 satellite. The SAO vertical velocities are stronger during the northern winter phase, as expected if planetary waves from the winter hemisphere are involved in driving the SAO. A possible quasi-biennial oscillation (QBO) signal extending from the middle into the upper stratosphere is also hinted at, with the equatorial vertical velocities in the region 10–1 hPa significantly smaller (or even negative) in 1993/94 than in 1992/93. Despite the short data record, the authors believe that this pattern reflects a QBO signal rather than a coincidental interannual variability, since the time–height section of vertical velocity at the equator resembles that of the zonal wind. Wintertime high-latitude descent rates are usually greater in the Northern Hemisphere, but they also exhibit significant variability there. In the three northern winters analyzed in this study, strong downward velocities are diagnosed in the lower stratosphere during stratospheric warmings and are associated with enhanced wave forcing (computed as the momentum residual) in the mid- and upper stratosphere. The implications of the computed circulation for the distribution of tracers are illustrated by the example of the “double-peaked” structure in the water vapor distribution measured by MLS.

Measurements of the evolution of the Mt. Pinatubo aerosol cloud by ISAMS

Measurements by the Improved Stratospheric and Mesospheric Sounder (ISAMS) on the Upper Atmosphere Research Satellite (UARS) are being used to study the spatial and temporal evolution of the volcanic stratospheric aerosol from Mt. Pinatubo. The maximum opacity of the aerosol cloud moved from a position south of the Equator at an altitude of about 26 km in early October 1991, became located over the Equator by mid-January 1992, and descended in altitude to about 21 km by July 1992. Dispersal of the cloud was more rapid in the Southern Hemisphere and penetration to the southern polar region occurred earlier than transport to the corresponding northern polar area. The area weighted global mean stratospheric optical thickness between 15 km and 35 km at 12.1 μm remained at about 5.5×10−3 from November 1991 through to April 1992. The estimated aerosol mass loading is 19–26 megatonnes for this period and by the end of July 1992 it had declined to 15–21 megatonnes.

Use of volcanic aerosols to study the tropical stratospheric reservoir

Aerosol data obtained by the advanced very high resolution radiometer on NOAA 11, the improved stratospheric and mesospheric sounder on the upper atmospheric research satellite, one airborne lidar system, and several ground-based lidar systems up to 2-1/2 years after the eruption of Mount Pinatubo are used to study stratospheric dynamics. In particular, this study focuses on the tropical stratospheric reservoir and transport from it to northern midlatitudes following the eruption of Mount Pinatubo. This includes: The build-up and removal rates for sulfate aerosol, the position and motion of the center of the reservoir, and the position and width of its boundaries at altitudes of the volcanic aerosols. Ozone data from the total ozone mapping spectrometer were also used to study the position and width of the reservoir boundaries. In addition, ground-based lidar stratospheric aerosol data are used to study aerosol transport from the reservoir to the northern hemisphere as it relates to winds in the tropical stratosphere. Finally, historical in situ and satellite data were used to examine how the time and location of volcanic injections into the stratosphere affect the aerosol decay rates and seasonal variations of aerosol optical depth in the midlatitude stratosphere.

Stratospheric aerosol effective radius, surface area and volume estimated from infrared measurements

A technique is presented for estimating the effective radius, surface area density, and volume density of stratospheric aerosols from infrared emission measurements. These parameters are required to assess the perturbation of the climate and chemical balance of the stratosphere following the largest volcanic eruption so far this century: that of Mount Pinatubo in the Philippines. The method uses a relationship between the surface area density and the volume density derived from balloon-borne measurements of the Mount Pinatubo aerosol cloud made at Laramie, Wyoming. It is shown that the aerosol emission value is well approximated by a linear function of effective radius and aerosol volume density. The technique relies on knowing the refractive index of the aerosol cloud, which is assumed to be composed of liquid spheres of sulphuric acid and water. It is shown that the uncertainties in the current knowledge of the refractive index of sulphuric acid solutions limit the accuracy of the inversion technique. As a case study, the aerosol effective radius, surface area density, and volume density are determined from emission measurements at 12.1 μm of the Mount Pinatubo aerosol cloud made by the improved stratospheric and mesospheric sounder (ISAMS) carried on the Upper Atmospheric Research Satellite (UARS). From these measurements it is shown that five months after the eruption the core of the Mount Pinatubo cloud had a size distribution with an effective radius of 0.5 μm, a surface area density of 35 μm2 cm−3, and a volume density of 6 μm3 cm−3.

Infrared absorption by volcanic stratospheric aerosols observed by ISAMS

The Improved Stratospheric and Mesospheric Sounder (ISAMS) aboard the Upper Atmosphere Research Satellite (UARS) senses in 14 wideband channels in the infrared. The absorption by the Mt. Pinatubo aerosol cloud for nine of the channels was averaged over heights from 20 km to 30 km for a 60° latitude band centred on the Equator. The absorption spectrum for sulphuric acid-water aerosols was calculated for wavelengths from 4 μm to 17 μm and investigated as a function of the particle size distribution and the particle composition. The infrared spectrum is shown to be more sensitive to changes in particle composition than to drop size; the ISAMS results are consistent with drops composed of a 59% to 77% solution of sulphuric acid in water.

Global evolution of the Mt. Pinatubo volcanic aerosols observed by the infrared limb-sounding instruments CLAES and ISAMS on the Upper Atmosphere Research Satellite

The cryogenic limb array etalon spectrometer (CLAES) and the improved stratospheric and mesospheric sounder (ISAMS) instruments on board the Upper Atmosphere Research Satellite (UARS) have been used to produce global information on the Mt. Pinatubo volcanic aerosol for the period from October 1991 to April 1993. The satellite infrared extinction measurements near 12 μm are converted into the aerosol-related parameters necessary for modelling the effects of the volcanic aerosol on the aeronomy of the stratosphere and are presented as zonal mean distributions for 80°S to 80°N averaged over ∼35-day periods. The aerosol composition is derived from the CLAES and ISAMS temperature measurements and the water vapour abundances are obtained from the microwave limb sounder (MLS). The aerosol volume density is obtained from the extinction measurements from which the surface area density and the effective particle radius are estimated. The maximum aerosol surface area density has a value of about 50 μm 2 cm -3 at a height of 24 km at the equator in October 1991, before decaying exponentially with a time constant of 443 ± 10 days. The surface area density remained well above preeruption values in April 1993. The effective particle radius in the tropics decays monotonically from 0.65 μm in October 1991 to 0.4 μm in April 1993. The global aerosol sulphate mass loading is 19.5 Mt in October 1991 and decays exponentially with a time constant of 342 ± 8 days to a value of 4.3 Mt by April 1993. Four months after the eruption the calculated optical thickness at 1.02 μm was ∼0.25 in the tropics. Rate constants are derived for the heterogeneous reactions of N 2 O 5 and ClONO 2 on the sulphate aerosols. The application of the aerosol parameters to the investigation of tracer transport, heterogeneous chemistry, and radiative transfer is discussed.

Calculation of Mie derivatives

Analytical expressions are found for the derivatives of commonly used Mie scattering parameters, in particular the absorption and the scattering efficiencies, and for the angular intensity functions. These expressions are based on the analytical derivatives of the Mie scattering amplitudes a(n) and b(n) with respect to the particle size parameter and complex refractive index. In addition, analytical derivatives are found for the volume absorption and scattering coefficients, as well as for the intensity functions of a population of particles with log normal size distribution. These derivatives are given with respect to the total number density, to the median radius and spread of the distribution, and to the refractive index. Comparison between analytically and numerically computed derivatives showed the analytical version to be 2.5 to 6.5 times as fast for the single-particle and particle-distribution cases, respectively.

Cloud retrievals from satellite data using optimal estimation: evaluation and application to ATSR

Clouds play an important role in balancing the Earth’s radiation budget. Hence, it is vital that cloud climatologies are produced that quantify cloud macro and micro physical parameters and the associated uncertainty. In this paper, we present an algorithm ORAC (Oxford-RAL retrieval of Aerosol and Cloud) which is based on fitting a physically consistent cloud model to satellite observations simultaneously from the visible to the mid-infrared, thereby ensuring that the resulting cloud properties provide both a good representation of the short-wave and long-wave radiative effects of the observed cloud. The advantages of the optimal estimation method are that it enables rigorous error propagation and the inclusion of all measurements and any a priori information and associated errors in a rigorous mathematical framework. The algorithm provides a measure of the consistency between retrieval representation of cloud and satellite radiances. The cloud parameters retrieved are the cloud top pressure, cloud optical depth, cloud effective radius, cloud fraction and cloud phase. The algorithm can be applied to most visible/infrared satellite instruments. In this paper, we demonstrate the applicability to the Along-Track Scanning Radiometers ATSR-2 and AATSR. Examples of applying the algorithm to ATSR-2 flight data are presented and the sensitivity of the retrievals assessed, in particular the algorithm is evaluated for a number of simulated single-layer and multi-layer conditions. The algorithm was found to perform well for single-layer cloud except when the cloud was very thin; i.e., less than 1 optical depths. For the multi-layer cloud, the algorithm was robust except when the upper ice cloud layer is less than five optical depths. In these cases the retrieved cloud top pressure and cloud effective radius become a weighted average of the 2 layers. The sum of optical depth of multi-layer cloud is retrieved well until the cloud becomes thick, greater than 50 optical depths, where the cloud begins to saturate. The cost proved a good indicator of multi-layer scenarios. Both the retrieval cost and the error need to be considered together in order to evaluate the quality of the retrieval. This algorithm in the configuration described here has been applied to both ATSR-2 and AATSR visible and infrared measurements in the context of the GRAPE (Global Retrieval and cloud Product Evaluation) project to produce a 14 yr consistent record for climate research.

Validation studies using multiwavelength Cryogenic Limb Array Etalon Spectrometer (CLAES) observations of stratospheric aerosol

Validation studies of multiwavelength Cryogenic Limb Array Etalon Spectrometer (CLAES) observations of stratospheric aerosol are discussed. An error analysis of the CLAES aerosol extinction data is presented. Aerosol extinction precision values are estimated at latitudes and times at which consecutive Upper Atmosphere Research Satellite (UARS) orbits overlap. Comparisons of CLAES aerosol data with theoretical Mie calculations, based upon in situ particle size measurements at Laramie, Wyoming, are presented. CLAES aerosol data are also compared to scaled aerosol extinction measured by the Stratospheric Aerosol and Gas Experiment (SAGE II) and Atmospheric Trace Molecule Spectroscopy (ATMOS) experiments. Observed and calculated extinction spectra, from CLAES, Improved Stratospheric and Mesospheric Sounder (ISAMS), and Halogen Occultation Experiment (HALOE) data, are compared. CLAES extinction data have precisions between 10 and 25%, instrumental biases near 30%, and accuracies between 33 and 43%.