Gianluca Redaelli

Effects of the PDO Phase on the Tropical Belt Width

Recent studies have shown that the tropical belt (TB) has progressively expanded since at least the late 1970s. This trend has been largely attributed to the radiative forcing due to greenhouse gas (GHG) increase and stratospheric ozone depletion, even if an influence of sea surface temperature (SST) anomalies has been also suggested. The impact of the Pacific decadal oscillation (PDO) on the TB width is investigated in this work. The study is performed by using both Atmospheric Model Intercomparison Project (AMIP) and idealized simulations, produced by the NCAR Community Atmosphere Model, version 3 (CAM3) GCM and reanalysis data [40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40), ERA-Interim, and Modern-Era Retrospective Analysis for Research and Applications (MERRA)]. Reanalyses show that a switch of the PDO from a positive to a negative phase can lead to a significant TB expansion during the equinoxes. This effect, indicating a possible PDO contribution to the widening that characterized the TB width during the last decades, is not correctly reproduced by model simulations. Deficiencies in the sensitivity of model-simulated convective processes to SST anomalies are suggested as a possible cause of the TB widening underestimation.

Simulation of Polar Antarctic trends: Influence of tropical SST

[1] The polar Antarctic atmosphere has been recently characterized by a progressive intensification of the circumpolar westerly winds, both in the stratospheric and tropospheric component. We simulate the response of the polar southern hemisphere atmosphere to a prescribed trend of oceanic equatorial temperature to demonstrate a possible link between the progressive warming of tropical sea surface temperature (SST) and Antarctic climate changes in the last decades. Model simulations produce an atmospheric response suggesting an influence of equatorial SST on polar dynamics, particularly during summer months.

Clouds at the tropical tropopause: A case study during the APE‐THESEO campaign over the western Indian Ocean

In this paper, we report a detailed description of a thin cirrus at the tropopause above a cumulonimbus (Cb) convective cluster observed during the Airborne Platform for Earth Observation–Third European Stratospheric Experiment for Ozone (APE-THESEO) campaign in February–March 1999 in the western Indian Ocean. The thin cirrus (Ci) has an optical depth at 532 nm below 0.1, with extended subvisible stretches, and is located directly below the tropopause, which was supersaturated with respect to ice. A direct comparison between the optical depth retrieved by Meteosat and that obtained by means of the hygrometers installed on the M55-Geophysica aircraft is discussed showing discrepancies ranging from 10 to 20%. Combining satellite and aircraft data, we show that the observed Ci is not due to cirrus outflow from Cb anvils. In the absence of any deeply convective clouds reaching altitudes above 15 km, we propose a possible mechanism of Ci formation based on a net mesoscale transport of water vapor from altitudes above 16 km to the tropopause region around 18 km. This transport could be driven by the critical layer and turbulence induced by gravity waves that could have been generated by lower level Cb cluster activity. The proposed mechanism for high-altitude Ci formation corroborates the new paradigm of a tropical tropopause layer (TTL) or “substratosphere,” several kilometers thick, which is decoupled from the convection-dominated lower troposphere.

Polar stratospheric clouds due to vapor enhancement: HALOE observations of the Antarctic vortex in 1993

Aerosol measurements from the Halogen Occultation Experiment (HALOE) during the Antarctic spring of 1993 are compared with calculations of the volume of different types of polar stratospheric clouds (PSCs) at equilibrium. The observed volumes increased by a factor of ∼30 coincident with water vapor enhancements of ∼3 ppmv, suggesting that the enhancement of water vapor was important in determining PSC growth. The enhanced water vapor was coincident with increased methane mixing ratios, and trajectory analysis suggests that the vapor enhancements were consistent with transport from lower latitudes. The nitric acid distribution was not measured and is treated either as constant or as positively correlated with the observed water. Comparing the observed volumes with model calculations assuming constant nitric acid suggests that the PSCs were composed of liquid ternary H 2 SO 4 -H 2 O-HNO 3 aerosols (LTA) rather than solid nitric acid trihydrate (NAT). However, if the water vapor intrusions were accompanied by enhanced nitric acid, the observations closely match predictions for solid NAT, and those for LTA. These comparisons highlight the importance of the vapor distributions for modeling PSC growth and suggest that vapor resupply is important for late spring PSC growth. This work also highlights some inherent limitations of large field of view limb-viewing instruments for the observation of PSCs.

ULF geomagnetic and polar cap potential signatures in the temperature and zonal wind reanalysis data in Antarctica

In the present study we investigated the possible coupling between geomagnetic activity and the low atmosphere dynamics in the polar cap. We compared daily values of the ERA-Interim temperature and zonal wind over Antarctica, with the daily geomagnetic ULF power, in the Pc5 (1–7 mHz), Pc1, and Pc2 (100 mHz–1 Hz) frequency ranges, at Terra Nova Bay (Antarctica, corrected geomagnetic latitude λ ~ 80°S) and with solar wind data during 2007, in correspondence to the last declining phase of the solar cycle 23. We found a high and statistically significant correspondence of temperature and zonal wind fluctuations in the stratosphere and troposphere with geomagnetic ULF power fluctuations at the ~27 day periodicity, with a substantial reduction at the tropopause height. A similar, clear relationship between the meteorological parameters and the polar cap potential difference was also observed. The results suggest that the changes in the atmospheric conductivity, due to energetic electrons precipitation driven by the ULF waves, as well as the high latitude potential variations, both associated to high geomagnetic activity, can affect the atmospheric dynamics.

Tropical SST Preconditioning of the SH Polar Vortex during Winter 2002

The response of the Southern Hemisphere (SH) polar atmosphere to the tropical sea surface temperature (SST) during the 2002 winter–spring season is investigated by using a general circulation model (GCM). The SH stratospheric winter of 2002 was particularly unusual, characterized by a weaker-than-normal polar vortex during the whole season. It also registered, at the end of September, the first major warming yet observed in the SH. This event is unexpected in the SH, and it is supposed to be induced by a “preconditioning” of the polar vortex starting at the beginning of the winter. Atmospheric GCM experiments with prescribed SST boundary conditions are performed. The sensitivity of the Antarctic dynamics to the tropical SST of 2002 (a year characterized by an El Nino event of moderate intensity) is studied, and the uniqueness of the 2002 tropical oceanic condition is investigated through the comparison of the simulated response of the climatic system to 2002 and 1997 tropical SST (1997 being a year with a strong El Nino event). Model results highlight a primary role played by the tropical SST of 2002 in the development of the peculiar characteristics of the Antarctic dynamics during the winter months that appears to be a necessary condition for the generation of the anomalous destabilization of the polar vortex during the following spring. Results for June 2002 show a strong generation of vertically propagating waves resulting from the tropical SST that, through the perturbation of the westerly jet at middle latitudes, produces a preconditioning of the polar vortex by affecting the wave refraction index. The particular structure of the tropical SST anomalies during the winter of 2002 is thought to have influenced the subsequent preconditioning of the stratospheric vortex.

ULF geomagnetic activity effects on tropospheric temperature, specific humidity, and cloud cover in Antarctica, during 2003–2010

In the present study we investigated the possible relationship between the ULF geomagnetic activity and the variations of several atmospheric parameters. In particular, we compared the ULF activity in the Pc1-2 frequency band (100 mHz - 5 Hz), computed from geomagnetic field measurements at Terra Nova Bay in Antarctica, with the tropospheric temperature T, specific humidity Q and cloud cover (HCC, MCC and LCC) obtained from re-analysis dataset. The statistical analysis was conducted during the years 2003-2010, using correlation and Superposed Epoch Analysis approaches. The results show that the atmospheric parameters significantly change following the increase of geomagnetic activity within 2 days. These changes are evident in particular when the interplanetary magnetic field Bz component is oriented southward (Bz 0). We suggest that both the precipitation of electrons induced by Pc1-2 activity, and the intensification of the polar cap potential difference, modulating the microphysical processes in the clouds, can affect the atmosphere conditions.

Prioritizing Data for Improving the Multidecadal Predictive Capability of Atmospheric Models

AbstractThe prioritization accorded to observation types currently being considered for a space-based climate observing system is extended from a previous study. Hindcast averages and trends from 1970 through 2005 of longitude–latitude maps of 200-hPa geopotential height and of net downward shortwave and longwave radiation at the top of the atmosphere are investigated as relevant tests of climate models for predicting multidecadal surface air temperature change. To discover the strongest tests of climate models, Bayes’s theorem is applied to the output provided by phase 5 of the Coupled Model Intercomparison, and correlations of hindcasts and multidecadal climate prediction are used to rank the observation types and long-term averages versus long-term trends. Spatial patterns in data are shown to contain more information for improving climate prediction than do global averages of data, but no statistically significant test is found by considering select locations on the globe. Eigenmodes of intermodel dif...

Possible nitric acid coating formation over Pinatubo aerosols inferred with a microphysical code: A case study during EASOE

Lidar observations at Sodankyla (Finland) on January 9 and 11 1992, made as part of the European Arctic Stratospheric Ozone Experiment (EASOE), showed very different profiles of backscattering ratio. A prominent layer was seen between potential temperatures of 420 and 445K on January 9, which was nearly absent in the backscattering profile between these isentropic surfaces on the 11th. Examination of the trajectories of the airmasses reaching Sodankyla on these days lead us to suggest that the larger returns on January 9th could arise from the formation of a nitric acid coating on volcanic aerosols. Simulations using a microphysical model produce results consistent with this suggestion. We conclude that nitric acid condensation on January 9 can explain the measurements, but that independent information would be needed to quantify the characteristics of the aerosol substrate.