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Soil moisture content is crucial for the representation and predictability of hydroclimatic extremes of different spatial/temporal scales such as heavy rainfall, droughts and heatwaves. In order to include these effects and the relevant feedback with the atmosphere in a regional climate model, the soil moisture initialization has to be adequate.This study explores the soil moisture precipitation (SM-P) feedback, the soil moisture temperature (SM-T) feedback and the heat fluxes over the entire domain and 3 smaller regions of interest. A hydrostatic version of the Regional Climate Model 4.7 (RegCM4.7) with Arakawa B grid is used to run the simulations. The simulations are performed for the months February to May during the years 2008, 2009 and 2010 with a spatial resolution of 12 km and temporal resolution of 3 hours. The initial and boundary conditions(ICBC) are derived from the ERA5 data. We examine results from simulations initiated using three different soil moisture datasets, namely, the control, dry and wet datasets. The soil moisture data from the ERA5-Land reanalysis is used for the control simulation. A dry/wet simulation is run using dry/wet datasets derived from the ERA5-Land data. This is done by halving/doubling the soil moisture values from ERA5-Land data, giving rise to new soil moisture values with lower/higher soil moisture as compared to the control dataset (ERA5-Land). CMORPH (Climate Prediction Center (CPC) Morphing Technique (MORPH)) and CRU (Climate Research Unit) datasets are used as reference to evaluate the precipitation and temperature values resulting from the control simulation.The results display the mean changes in the dry/wet simulation results with respect to the control simulation. Plots showing the vertical profile changes in relative humidity and air temperature, and changes in lower tropospheric wind and specific humidity, indicates the build-up of the observed precipitation events and temperature patterns induced by the initial soil moisture perturbation. Interestingly the simulation results show negative SM-P feedback. In other words, the average precipitation seemed to increase/decrease for the dry/wet cases with respect to the control simulation. This is contrary to the general expectation that dry/wet soil moisture decreases/increases precipitation. The possible reasons for the negative SM-P feedback and its distribution over the region include the proximity to the ocean, topography, and the pre-monsoon dryline. The SM-T and the heat fluxes on the other hand display expected behaviour with few exceptions in some regions in the dry simulation case.

From 6 to 8 November 2011, a baroclinic wave moved from North Atlantic to Balearic Sea, produced a cut-off, at all altitudes, and developed into a Tropical-Like Cyclone (TLC) characterized by a deep-warm core. This TLC led to a mean sea level pressure minimum of about 987 hPa, 10 m wind speeds higher than 30 m/s around the eye, and very intense rainfall, especially in the Gulf of Lion and the surrounding areas, close to the mountain chains (floods in Genoa and Elba Island).To explore in details the effect of the sea surface temperature on the TLC development, we employed the coupled modeling system COAWST, which consists of the following models: ROMS for the hydrodynamic part, WRF for the meteorological part and SWAN for the surface wave modeling, using a 5 km horizontal grid over all Mediterranean Sea.COAWST was used with different configurations: Stand Alone (SA) approach using only the atmospheric part, atmosphere-ocean coupled mode (AO), and fully coupled version including also surface waves (AOW). Comparing the three runs, the effects of different simulations on the TLC trajectory are significant only in the later stage of the cyclone lifetime. On the other hand, wind intensity is higher in the SA case w.r.t. both coupled runs. When compared to case AO, winds are about 1 m/s larger, even though the spatial distribution is very similar (possibly because of the lower SST produced by case AO). Case AOW produces less intense winds than SA and AO case in the areas where the wave is most developed (differences are about 2-4 m/s), while winds are more intense nearby the cyclone’s eye. Moreover, the inclusion of the wave model (AOW) has implications in the water column, by increasing the mixing in the ocean, changing the depth of the ocean mixed layer along the track of the TLC, increasing the surface drag and the net heat fluxes from ocean to atmosphere, so that eventually SST in AOW run is colder than in AO.It is observed that SST of the SA case is overestimated compared to the coupled cases, and in particular the best performances are observed using the fully coupled case, with the wave motion implementation. The best description of the SST impacts the cyclone intensity and the amount of precipitation at catchment scale. The vertical profiles show that wave induced mixing modifies the mixed layer structure and cools the water column, removing much of the SST (and Ocean Heat Content) anomaly present in the mixed layer.The date chosen for the run initialization appears important: an earlier initial condition allows to properly simulate the evolution of the cyclone from the cyclogenesis between the inclusion and setting-up of air sea interaction effect, through the coupled models.Warming SST in the Mediterranean Sea induced by climate change might increase TLC frequency and/or intensity, potentially becoming more harmful for coastal populations and infrastractures. Fully coupled AOW models might be better suited for studying such aspects. Funding from the STO Office of Chief Scientist (907EUR30) is gratefully acknowledged.

Recent studies exhibit a considerable reduction of summer season precipitation frequency in the new generation of convection-permitting (CP) climate simulations. This seems to over-correct non-CP wet biases with a knock-on effect on summer temperatures via soil moisture-atmosphere feedbacks. However, it is difficult to elucidate which part of the warmth/dryness in CP simulations can be ascribed to land-atmosphere coupling and/or "atmosphere-only" processes. Another layer of uncertainty belongs to the still crude representation of land surface/sub-surface processes that become especially relevant when approaching such high resolution.In this study we explore the modulation of land-atmosphere coupling when moving from a non-CP to a CP-scale climate modeling, considering increasingly sophisticated land surface model configurations. We perform a two-step dynamical downscaling at ~15 km (convection-parameterized) and ~ 3km (convection-permitting) resolutions with the WRF-4.2.1 regional climate model driven by ECMWF-ERA5 reanalysis. The greater alpine region and the extended summer season (May to September) of 2003 are the spatial and temporal domains of interest. A mini multi-physics ensemble is generated with four Noah-MP land-surface model configurations to examine if, and how, including crucial land processes (e.g., vertical soil water transport) modifies hot-temperature forcing mechanisms in the two resolutions. Moreover, each ensemble member is run according to three different initial soil moisture levels, defining reference, anomalously dry- and wet-initialization experiments.Preliminary results show an improved representation of precipitation statistics (seasonal cumulative, frequency, and 99th percentile) from CP simulations, particularly over complex orography. Generally, maximum temperature reproduction benefits from the CP scale. However, localized warm biases persist over flat terrains regardless of the land surface model configuration. Finally, the two resolutions show a substantially different decay of the initial soil moisture state. At CP scale all three runs converge to similar soil moisture at the end of the integration. Conversely, non-CP runs preserve large soil moisture differences until the end of the summer season, signaling longer soil moisture memory and different soil moisture-precipitation feedback.These factors might significantly affect the reproduction and predictability of environmental and societal relevant hydroclimatic extremes on a wide-ranging temporal scale, from seasonal climate predictions to long-term climate projections.

Soil moisture-atmosphere feedback plays an important role in shaping summer temperature variability and eventually in modulating duration, intensity, and predictability of heat waves. Recent studies point out a modulation of summer temperatures introduced by the new generation of km-scale (or convection-permitting, CP) regional climate models (RCMs), compared to convection-parameterized RCMs. Modifications are likely originated from changes in soil moisture-precipitation feedback. This generally turns into an extension of dry spell length (DSL) determining warmer conditions in response to an altered partitioning of surface heat fluxes.In this study, two potentially relevant factors behind modifications in land-atmosphere interactions at the two resolutions are investigated on a seasonal temporal scale. The first, is the underestimation of summer season convective phenomena, as an outcome of a poor sensitivity to triggering factors and driving longer DSL in km-scale simulations. The second is represented by differences in soil moisture memory between RCM and CPRCM.We perform simulations with the ECMWF-ERA5 driven WRF-4.2.1 model consisting of a two-step dynamical downscaling at ~15 km (non-CP scale) and ~ 3km (CP scale) respectively. The greater alpine region and extended summer seasons (May to September) represent spatial and temporal domains.The underestimation of the summer season convection will be explored considering simulations at (i) 15 km with parameterized convection, (ii) 3 km with explicit convection (CPRCM_exp) and (iii) 3 km with parameterized convection (CPRCM_par) according to different numerical schemes. This is to explore whether parameterizing deep convection at CP scale mitigates poor convection-triggering processes sensitivity caused by weak large scale forcing.The soil moisture memory is assessed through autocorrelation analysis applied to three simulations: one standard and two idealized soil-moisture-perturbed-initialization defining anomalously dry- and wet-initialization experiments. Here, ground-water-aware configuration of Noah-MP land surface model (LSM) will be compared to a more simplified LSM configuration.Preliminary results for the 2003 summer season show differences in precipitation statistics between the two different resolutions and between CPRCM_exp/CPRCM_par. We observe an increase in wet-hour frequency, an increase and different spatial pattern of precipitation 99th percentile in CPRCM_par.Concerning soil moisture memory, initial differences are preserved in the two resolutions for the first month run. After that, differences decay in the CPRCM, where all the three simulations converge to similar soil moisture at the end of the run. Differently, RCM preserves a larger difference of soil moisture until the end of the run indicating a longer memory of the initial state, particularly in the wet-initialization experiment.Several reference products will be considered to evaluate resulting modulations, namely if a km-scale deep convection parameterization can be beneficial and if the shorter soil moisture memory resulting in the km-scale simulations represents an improvement.Outcomes might offer insights on the km-scale influence on seasonal time-scale prediction of soil-atmosphere-interaction-driven extreme events.

Heatwaves (HWs) is high-impact phenomena that may have devastating impacts on societies and ecosystems. In the context of numerical modeling, despite driving synoptic scale dynamics are generally well caught, local-scale forcing representation is still challenging, embodying a source of uncertainty and errors. In this work we study two HWs occurring in 2021 summer season (02-05 Aug and 11-14 Aug), on the Italian territory. The first event, is characterized by strong South-Westerly winds and albeit without high geopotential and 850 hPa temperature caused 2 m temperatures above 40 °C in the central Italy east cost (e.g., in the urban area of Pescara), mainly driven by katabatic winds (i.e., Fhoen dynamics). The second HW, has been triggered by a strong advection of hot air from the North African territory and with a major impact on the western coasts of the Italian territory. Here, we leverage a WRF (Weather Research and Forecasting system) numerical model at 1 km resolution over central Italy multi-physics ensemble, including various Planetary Boundary Layer (PBL), land-surface-model (LSM), radiative and surface numerical schemes. The simulations are performed using an approach that takes into account the evolution of sea surface temperature and Marine Mixed Layer Depth (at high resolution, 4.5 km), according to the OML numerical scheme, in order to represent the air-sea interactions more realistically. The preliminary results show a relevant impact exerted by numerical parameterizations, in the prediction of the maximum temperature and humidity, especially in the urban areas. A notable impact is also observed for the simulation of the minimum temperature values. The greatest modulation of HWs reproduction results from the choice of LSMand PBL schemes, and their combinations. These results highlight that the multi-physics approach is also useful to highlight most relevant processes driving local-scale modulation and related uncertainty representing a valuable tool to foster local-scale HWs-related risk mitigation.

Seasonal Climate Predictions (SCPs) represent a challenging intermediate field where aspects typical of the short-term weather forecasts and long-term climate projections interact. Skillful SCPs represent an essential tool to reduce societal vulnerabilities to the inter-annual climate fluctuation through short-term (i.e., next season) climate impact mitigation measures. This is especially true over areas characterized by large climate inter-annual variability as the Mediterranean basin, which is also traditionally characterized by a poor seasonal predictability.The primary research question of present study is to assess the capability of two dynamical downscaling approaches to improve the seasonal inter-annual variability signal coming from the global-scale driving SCP system on the Mediterranean basin.In this work the Weather Research and Forecasting model (WRF3.9.1.1) and the Regional Climatic Model (RegCM4.1) were nested into NCEP’s operational seasonal forecast model Climate Forecast System version 2 (CFSv2) to dynamically downscale seasonal predictions over Mediterranean basin.Using the initial and boundary conditions of an ensemble of the CFSv2 we compare the capability of the two downscaling approaches on improving the large scale CFSv2 prediction of a climatological period of 22-cold seasons (December–February) during 1982–2002.The SCP systems (WRF- and RegCM-based) consist on a double dynamical downscaling where a height-member lagged ensemble of 3-month CFSv2 climate predictions represent the common driving fields. Both the nested models dynamically downscales CFSv2 climate prediction from the original 100 km resolution to 60 km over a domain covering the Mediterranean basin and Central Europe. The first downscaling feeds a second downscaling performed over a domain centered over Central Italy with a resolution of 12 km.Climate variables considered are: 2 m temperature, precipitation, geopotential height at different pressure levels and mean sea level pressure. Results will be discussed by means of mean bias spatial distribution, inter-annual anomaly variability reproduction and probabilistic hit-rate of anomalous seasons, through tercile plots and reliability diagrams of the above mentioned variables.Preliminary results, considering the RegCM, identify temperature variability reproduction benefiting from the downscaling. At the same time, precipitation shows an improved spatial distribution patterns but not improved inter-annual variability representation if compared to the driving CFSv2 reference period climate predictions.

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.

Anticipating seasonal climate anomalies is essential for defining short-term adaptation measures. To be actionable, many stakeholders require seasonal forecasts at the regional scale to be properly coupled to region-specific vulnerabilities. In this study, we present and preliminarily evaluate a regional-scale Seasonal Forecast System (SFS) over Central Italy. This system relies on a double dynamical downscaling performed through the Regional-scale Climate Model (RCM) RegCM4.1. A twelve-member ensemble of the NCEP-CFSv2 provides driving fields for the RegCM. In the first step, the RegCM dynamically downscales NCEP-CFSv2 predictions from a resolution of 100 to 60 km over Europe (RegCM-d1). This first downscaling drives a second downscaling over Central Italy at 12 km (RegCM-d2). To investigate the added value of the downscaled forecasts compared to the driving NCEP-CFSv2, we evaluate the driving CFS, and the two downscaled SFSs over the same (inner) domain. Evaluation involves winter temperatures and precipitations over a climatological period (1982&ndash, 2003). Evaluation for mean bias, statistical distribution, inter-annual anomaly variability, and hit-rate of anomalous seasons are shown and discussed. Results highlight temperature physical values reproduction benefiting from the downscaling. Downscaled inter-annual variability and probabilistic metrics show improvement mainly at forecast lead-time 1. Downscaled precipitation shows an improved spatial distribution with an undegraded but not improved seasonal forecast quality.

The weather forecasts for precipitation have considerably improved in recent years thanks to the increase of computational power. This allows to use both a higher spatial resolutions and the newly developed parameterization schemes for representing sub-grid scale physical processes. However, precipitation estimation is still affected by errors that can impact on the response of hydrological models. To the aim of considering the uncertainties in the precipitation forecast and how they propagate in the hydrological model, an ensemble approach is investigated. A meteo-hydro ensemble system is built to forecast events in a complex orography terrain where catchments of different size are present. In this context, the meteo-hydrological forecast system is implemented and tested for a severe hydrological event occurred over Central Italy on November 15, 2017. During this period, a flash flood hit the Abruzzo region causing precipitation up to 200 mm/24 hours and producing damages with a high impact on social and economic activities.The newly developed meteo-hydro ensemble system is compared with a high resolution deterministic forecast and with the observations over the same area, showing a very good response. In addition, the ensemble allows for an estimation of the predictability of the event a few days in advance and of the uncertainty of this flood. Although the modelling framework is implemented on the basins of Abruzzo region, it is portable and applicable to other areas.

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.

The 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 differences in hindcasts may likely serve as tests of climate models that can improve interdecadal climate prediction, in particular the rate of Arctic tropospheric expansion, which is measurable by Earth radio occultation.

The response of Mediterranean small catchments hydrology to climate change is still relatively unexplored. Regional Climate Models (RCMs) are an established tool for evaluating the expected climate change impact on hydrology. Due to the relatively low resolution and systematic errors, RCM outputs are routinely and statistically post-processed before being used in impact studies. Nevertheless, these techniques can impact the original simulated trends and then impact model results. In this work, we characterize future changes of a small Apennines (Central Italy) catchment hydrology, according to two radiative forcing scenarios (Representative Concentration Pathways, RCPs, 4.5 and 8.5). We also investigate the impact of a widely used bias correction technique, the empirical Quantile Mapping (QM) on the original Climate Change Signal (CCS), and the subsequent alteration of the original Hydrological Change Signal (HCS). Original and bias-corrected simulations of five RCMs from Euro-CORDEX are used to drive the CETEMPS hydrological model CHyM. HCS is assessed by using monthly mean discharge and a hydrological-stress index. HCS shows a large spatial and seasonal variability where the summer results are affected by the largest decrease of mean discharge (down to &minus, 50%). QM produces a small alteration of the original CCS, which generates a generally wetter HCS, especially during the spring season.

During the last decade, Arctic sea ice cover has experienced an accelerated decline that has been suggested to drive the increased occurrence of extremely cold winter events over continental Europe. Observations and modeling studies seem to support the idea that Mediterranean climate is also changing. In this work, the authors estimate potential effects on the Mediterranean Basin, during the winter period, of Arctic sea ice reduction. Two sets of simulations have been performed by prescribing different values of sea ice concentrations (50% and 20%) on the Barents–Kara Seas in the NCAR Community Atmosphere Model, version 3 (CAM3), as representative of idealized present and future sea ice conditions. Global model simulations have then been used to run the Abdus Salam International Centre for Theoretical Physics (ICTP) Regional Climate Model, version 4 (RegCM4), over central Europe and the Mediterranean domain. Simulations provide evidence for a large-scale atmospheric circulation response to sea ice reduction, resembling the negative phase of the Arctic Oscillation (AO) and characterized by a wave activity flux from the North Atlantic toward the Mediterranean Basin, during winter months. An increase in the occurrence and intensity of extreme cold events, over continental Europe, and extreme precipitation events, over the entire Mediterranean Basin, was found. In particular, simulations suggest an increased risk of winter flooding in southern Italy, Greece, and the Iberian Peninsula.

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.

Nitric acid (HNO3) is one of the key products that are operationally retrieved by the European Space Agency (ESA) from the emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) onboard ENVISAT. The product version 4.61/4.62 for the observation period between July 2002 and March 2004 is validated by comparisons with a number of independent observations from ground-based stations, aircraft/balloon campaigns, and satellites. Individual HNO3 profiles of the ESA MIPAS level-2 product show good agreement with those of MIPAS-B and MIPAS-STR (the balloon and aircraft version of MIPAS, respectively), and the balloon-borne infrared spectrometers MkIV and SPIRALE, mostly matching the reference data within the combined instrument error bars. In most cases differences between the correlative measurement pairs are less than 1 ppbv (5–10%) throughout the entire altitude range up to about 38 km (~6 hPa), and below 0.5 ppbv (15–20% or more) above 30 km (~17 hPa). However, differences up to 4 ppbv compared to MkIV have been found at high latitudes in December 2002 in the presence of polar stratospheric clouds. The degree of consistency is further largely affected by the temporal and spatial coincidence, and differences of 2 ppbv may be observed between 22 and 26 km (~50 and 30 hPa) at high latitudes near the vortex boundary, due to large horizontal inhomogeneity of HNO3. Similar features are also observed in the mean differences of the MIPAS ESA HNO3 VMRs with respect to the ground-based FTIR measurements at five stations, aircraft-based SAFIRE-A and ASUR, and the balloon campaign IBEX. The mean relative differences between the MIPAS and FTIR HNO3 partial columns are within ±2%, comparable to the MIPAS systematic error of ~2%. For the vertical profiles, the biases between the MIPAS and FTIR data are generally below 10% in the altitudes of 10 to 30 km. The MIPAS and SAFIRE HNO3 data generally match within their total error bars for the mid and high latitude flights, despite the larger atmospheric inhomogeneities that characterize the measurement scenario at higher latitudes. The MIPAS and ASUR comparison reveals generally good agreements better than 10–13% at 20–34 km. The MIPAS and IBEX measurements agree reasonably well (mean relative differences within ±15%) between 17 and 32 km. Statistical comparisons of the MIPAS profiles correlated with those of Odin/SMR, ILAS-II, and ACE-FTS generally show good consistency. The mean differences averaged over individual latitude bands or all bands are within the combined instrument errors, and generally within 1, 0.5, and 0.3 ppbv between 10 and 40 km (~260 and 4.5 hPa) for Odin/SMR, ILAS-II, and ACE-FTS, respectively. The standard deviations of the differences are between 1 to 2 ppbv. The standard deviations for the satellite comparisons and for almost all other comparisons are generally larger than the estimated measurement uncertainty. This is associated with the temporal and spatial coincidence error and the horizontal smoothing error which are not taken into account in our error budget. Both errors become large when the spatial variability of the target molecule is high.

Polar stratospheric clouds (PSC) were observed with the multi-wavelength lidar of the MOANA project (Modelling and Observations of Aerosols in the Northern Atmosphere) during SESAME (Second European Stratospheric Arctic and Mid-latitude Experiment). The physical state, liquid or solid, of the cloud particles can be inferred from the lidar data. Using isentropic back-trajectories to obtain the thermal history of the sampled air masses, it is possible to reconcile most of the observations with current ideas on PSC formation and evolution. When the cloud particles were identified as liquid, changes in the size distribution of the droplets along the trajectory were calculated using a micro-physical box model. Backscatter ratios calculated from the size distributions are in broad agreement with the lidar data, giving confidence in current understanding of the evolution of ternary solution (H2SO4, HNO3 and H2O) droplets. Results from two soundings are shown which bear on the problem of the formation of solid particles. In the first, solid particles were detected. The air mass had cooled to the frost point 12 hours earlier. In the second no solid particles were detected although the air temperature was below the nitric acid trihydrate existence point, and had decreased by 12K in the previous 14 hours.

A World Climate Research Programme (WCRP) Special Workshop titled the “Climatic effects of ozone depletion in the Southern Hemisphere: Assessing the evidence and identifying gaps in the current knowledge” focused on the current understanding of Southern Hemisphere (SH) ozone depletion, in particular high-latitude ozone depletion, with regards to its impacts on hemispheric climate and its role relative to greenhouse gas (GHG)–induced climate changes. The 2010 United Nations Environment Programme (UNEP)/World Meteorological Organization (WMO) Scientific Assessment of Ozone Depletion, and research published since, provided the starting point for discussion. The workshop was supported by WCRP, the National Science Foundation (NSF; United States), the National Aeronautics and Space Administration (NASA; United States), Agencia Nacional de Promoción Científica y Técnica (Argentina), and the Pontificia Universidad Católica Argentina (Argentina). Fil: Canziani, Pablo Osvaldo. Pontificia Universidad Católica Argentina ; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Oneill, Alan. National Centre For Earth Observations; Reino Unido Fil: Schofield, R.. University of Melbourne; Australia Fil: Raphael, M.. University of California at Los Angeles; Estados Unidos Fil: Marshall,G. J.. British Antarctic Survey; Reino Unido Fil: Redaelli, G.. Universita degli Studi dell'Aquila; Italia

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.

[1] A Singular Value Decomposition analysis is applied to climatological data to determine the modes of variability of monthly mean Sea Surface Temperature (SST) in the tropics coupled with the southern hemisphere stratospheric polar vortex intensity for the 1958 to 2006 period and to identify the tropical region strongly influencing the high latitude stratospheric dynamics. Two subsets of data have been considered by explicitly taking into account the phase of the Pacific Decadal Oscillation (PDO). The study evidences a shift of the tropical region affecting the polar vortex from the Western Pacific to the Indo-Pacific Ocean, driven by the modulation of the SST pattern, due to the phase of the PDO. The analysis shows in both cases (years with positive/negative PDO phases) a high degree of correlation between an SST related index, calculated from NOAA/ERSST and HadISST datasets, and an index of the polar vortex intensity based on ECMWF data.

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 Niño 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 Niño 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.

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), on-board the European ENVIronmental SATellite (ENVISAT) launched on 1 March 2002, is a middle infrared Fourier Transform spectrometer measuring the atmospheric emission spectrum in limb sounding geometry. The instrument is capable to retrieve the vertical distribution of temperature and trace gases, aiming at the study of climate and atmospheric chemistry and dynamics, and at applications to data assimilation and weather forecasting. MIPAS operated in its standard observation mode for approximately two years, from July 2002 to March 2004, with scans performed at nominal spectral resolution of 0.025 cm−1 and covering the altitude range from the mesosphere to the upper troposphere with relatively high vertical resolution (about 3 km in the stratosphere). Only reduced spectral resolution measurements have been performed subsequently. MIPAS data were re-processed by ESA using updated versions of the Instrument Processing Facility (IPF v4.61 and v4.62) and provided a complete set of level-2 operational products (geo-located vertical profiles of temperature and volume mixing ratio of H2O, O3, HNO3, CH4, N2O and NO2) with quasi continuous and global coverage in the period of MIPAS full spectral resolution mission. In this paper, we report a detailed description of the validation of MIPAS-ENVISAT operational ozone data, that was based on the comparison between MIPAS v4.61 (and, to a lesser extent, v4.62) O3 VMR profiles and a comprehensive set of correlative data, including observations from ozone sondes, ground-based lidar, FTIR and microwave radiometers, remote-sensing and in situ instruments on-board stratospheric aircraft and balloons, concurrent satellite sensors and ozone fields assimilated by the European Center for Medium-range Weather Forecasting. A coordinated effort was carried out, using common criteria for the selection of individual validation data sets, and similar methods for the comparisons. This enabled merging the individual results from a variety of independent reference measurements of proven quality (i.e. well characterized error budget) into an overall evaluation of MIPAS O3 data quality, having both statistical strength and the widest spatial and temporal coverage. Collocated measurements from ozone sondes and ground-based lidar and microwave radiometers of the Network for the Detection Atmospheric Composition Change (NDACC) were selected to carry out comparisons with time series of MIPAS O3 partial columns and to identify groups of stations and time periods with a uniform pattern of ozone differences, that were subsequently used for a vertically resolved statistical analysis. The results of the comparison are classified according to synoptic and regional systems and to altitude intervals, showing a generally good agreement within the comparison error bars in the upper and middle stratosphere. Significant differences emerge in the lower stratosphere and are only partly explained by the larger contributions of horizontal and vertical smoothing differences and of collocation errors to the total uncertainty. Further results obtained from a purely statistical analysis of the same data set from NDACC ground-based lidar stations, as well as from additional ozone soundings at middle latitudes and from NDACC ground-based FTIR measurements, confirm the validity of MIPAS O3 profiles down to the lower stratosphere, with evidence of larger discrepancies at the lowest altitudes. The validation against O3 VMR profiles using collocated observations performed by other satellite sensors (SAGE II, POAM III, ODIN-SMR, ACE-FTS, HALOE, GOME) and ECMWF assimilated ozone fields leads to consistent results, that are to a great extent compatible with those obtained from the comparison with ground-based measurements. Excellent agreement in the full vertical range of the comparison is shown with respect to collocated ozone data from stratospheric aircraft and balloon instruments, that was mostly obtained in very good spatial and temporal coincidence with MIPAS scans. This might suggest that the larger differences observed in the upper troposphere and lowermost stratosphere with respect to collocated ground-based and satellite O3 data are only partly due to a degradation of MIPAS data quality. They should be rather largely ascribed to the natural variability of these altitude regions and to other components of the comparison errors. By combining the results of this large number of validation data sets we derived a general assessment of MIPAS v4.61 and v4.62 ozone data quality. A clear indication of the validity of MIPAS O3 vertical profiles is obtained for most of the stratosphere, where the mean relative difference with the individual correlative data sets is always lower than ±10%. Furthermore, these differences always fall within the combined systematic error (from 1 hPa to 50 hPa) and the standard deviation is fully consistent with the random error of the comparison (from 1 hPa to ~30–40 hPa). A degradation in the quality of the agreement is generally observed in the lower stratosphere and upper troposphere, with biases up to 25% at 100 hPa and standard deviation of the global mean differences up to three times larger than the combined random error in the range 50–100 hPa. The larger differences observed at the bottom end of MIPAS retrieved profiles can be associated, as already noticed, to the effects of stronger atmospheric gradients in the UTLS that are perceived differently by the various measurement techniques. However, further components that may degrade the results of the comparison at lower altitudes can be identified as potentially including cloud contamination, which is likely not to have been fully filtered using the current settings of the MIPAS cloud detection algorithm, and in the linear approximation of the forward model that was used for the a priori estimate of systematic error components. The latter, when affecting systematic contributions with a random variability over the spatial and temporal scales of global averages, might result in an underestimation of the random error of the comparison and add up to other error sources, such as the possible underestimates of the p and T error propagation based on the assumption of a 1 K and 2% uncertainties, respectively, on MIPAS temperature and pressure retrievals. At pressure lower than 1 hPa, only a small fraction of the selected validation data set provides correlative ozone data of adequate quality and it is difficult to derive quantitative conclusions about the performance of MIPAS O3 retrieval for the topmost layers.

[1] The atmospheric response to the tendency of oceanic equatorial temperature observed during recent decades has been simulated, leading to the identification of a possible link between tropical Sea Surface Temperature (SST) and Antarctic climate change. The dynamical mechanism, by which changes in tropical SSTs exert an influence on the upper latitude atmospheric dynamics, is investigated. The analysis of the simulations suggests that the perturbations of the atmospheric circulation are induced by changes in the annual cycle of the tropical convective activity, which influences the intensity of the wave forcing. In particular, the introduction of the SST perturbation generates a strong vertical wind shear at middle latitude during the Antarctic winter affecting the propagation of convectively generated waves during the following spring.

[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.

The work presents measurements obtained with the multi-input spectroradiometer GASCOD/A4p, the Fourier transform far infrared interferometer SAFIRE-A and the two-channel chromatograph HAGAR, on board the M-55 Geophysica stratospheric aircraft during the APE-GAIA measurement campaign, held in September-October 1999 over the Antarctic Peninsula. The paper aims to investigate the gas mixing ratio distributions at the flight altitude, during a recovery phase of the reservoir species in the lower stratosphere in the Antarctic spring. Attention is focused on the daytime flight of 23 September, when the aircraft flew inside the vortex at two different altitudes (about 16.5 and 18.5 km). Concentration distributions were examined for NO2, N2O, O3, BrO, ClO, HCl, H2O and HNO3, during flight inside and outside the vortex. Simultaneous concentration measurements of HNO3 and H2O showed a denitrification but not dehydration inside the vortex core. The reformation of chlorine reservoirs is investigated. It was evident that the re-conversion of HCl was favoured in the Cl+CH4 branches, and its high mixing ratio up to 1 ppbv at 425 K surface confirms that low ozone air masses inside the vortex and strong sunlight greatly favour a more rapid formation of passive chlorine reservoirs.

This paper presents two case studies of transport of dry air in the free troposphere measured by a ground based Raman LIDAR in the Northern-Italy, during the Mesoscale Alpine Programme (MAP). Two observations characterized by the presence of anomalously dry layers, below 6 km height, were analyzed using Lagrangian techniques. These events are related to upper-tropospheric, high Potential Vorticity (PV) streamers crossing the Alpine region. These are interpreted as small-scale features of stratospheric intrusions associated with the PV ridge during its phase of dissipation. One of the measurements also shows the presence of two distinct dehydrated structures associated with the same event. The water vapor concentration also suggests dilution processes of dry stratospheric air in the troposphere. Lagrangian simulations allowed to successfully reproduce the observed water vapor distribution and the air parcel histories confirmed the stratospheric origin of the dry layers.

The APE-THESEO campaign was held from 15 February to 15 March 1999 from the Seychelles in the western Indian Ocean. APE-THESEO stands for ‘Airborne Platform for Earth observation — (contribution to) the Third European Stratospheric Experiment on Ozone’. The campaign aimed to study processes controlling the low water content of the stratosphere, including the mechanisms of cloud formation in the tropical tropopause region, and transport processes, studied using measurements of long-lived trace gases and ozone. Two aircraft were used: the high-altitude research aircraft, M-55 Geophysica, which can fly up to 21 km, and the Falcon of the Deutsches Zentrum fur Luft- und Raumfahrt, a tropospheric aircraft. Seven flights were performed, including the first simultaneous in situ and remote sensing of sub-visible cirrus clouds, the first interception of sub-visible cirrus using in-flight guidance from a path-finding aircraft, and guided descent of a high-altitude research aircraft into the anvil cloud at the top of a tropical cyclone. In this paper we describe the payload used, the objectives for each flight, and the meteorological conditions encountered. First results show that a new type of cloud, which we have dubbed ultra-thin tropical tropopause cloud (UTTC), has been observed frequently, and observed to cover areas of 105 km2. The frequent coincidence of the tropopause and hygropause implies that the western Indian Ocean played an important role in the dehydration of the lower tropical stratosphere during the season of the mission. UTTCs, sub-visible cirrus, and visible cirrus, have all been implicated in the observed dehydration. Tracer measurements indicate little mixing of stratospheric air into the upper tropical troposphere.

[1] 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.

Atmospheric vertical profiles of temperature, pressure and concentrations of O3, H2O, CH4, HNO3, N2O and NO2, in the altitude range from 12 to 68 km are routinely retrieved from the measurements of the MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) instrument operating on board the ENVISAT satellite. Some internal consistency checks are performed on these measurements in order to verify the quality of the measurement system that includes the satellite measurements and the data processing. Good internal agreement is in general obtained, but in some cases the comparison with the a-priori understandings identifies the presence of some biases that need to be further investigated.

During May 1999, data collected with two of University of L'Aquila lidar systems, Italy show an increase in the stratospheric aerosol content at altitudes between 15 and 18 km. Our initial hypothesis is that these aerosols originated from the Shishaldin (Alaska) volcanic eruption of 19 April 1999. SAGE II observations, taken between Julian day 100 and 180, at latitudes above 40°N, show several sightings that are good candidates for volcanic aerosols. A trajectory analysis shows that a few SAGE II measurements are in space and time compatible with the Shishaldin eruption, and the aerosol enhancements observed by L'Aquila lidars have been also sampled by SAGE II, later and in different locations.

A technique based on conservative properties of certain meteorological fields is used to compare ozone measurements from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) with soundings from a lidar system operated at midlatitudes by the University of L'Aquila, Italy. A few typical cases are analyzed in connection with the position of the vortex relative to the observing station, and it is shown that in general lidar observations taken within the vortex compare well with the UARS data, regardless of whether they are coincident with a satellite overpass. It is shown that such analysis may be useful for comparing measurements of the same quantity taken at different sites using different measurement techniques.

Numerical experiments and statistical analyses are conducted to determine the skill of different Lagrangian techniques for the construction of tracer distributions. High-resolution potential vorticity (PV) maps are calculated from simulations of the 1996/97 arctic winter stratospheric dynamics using two different numerical schemes—reverse domain filling trajectories (RDF) and contour advection with surgery (CAS)—and data from three meteorological agencies (NCEP, the Met Office, and ECMWF). The PV values are then converted into ozone (O3) concentrations and statistically compared to in situ O3 data measured by the electro chemical ozone cell (ECOC) instrument during the Airborne Polar Experiment (APE) using cross correlation, rms differences, and the Kolmogorov–Smirnov (KS) test. Results indicate that while Lagrangian techniques are successful in increasing the presence of lower-scale tracer structures with respect to the plain meteorological analyses, they significantly improve the statistical ag...

In-situ measurements of ozone and water vapour, in the Antarctic lower stratosphere, were made as part of the APE-GAIA mission in September and October 1999. The measurements show a distinct difference above and below the 415K isentrope. Above 415K, the chemically perturbed region of low ozone and water vapour is clearly evident. Below 415K, but still above the tropopause, no sharp meridional gradients in ozone and water vapour were observed. The observations are consistent with analyses of potential vorticity from the European Centre for Medium Range Weather Forecasting, which show smaller radial gradients at 380K than at 450K potential temperature. Ozone loss in the chemically perturbed region above 415K averages 5ppbv per day for mid-September to mid-October. Apparent ozone loss rates in the sub-vortex region are greater, at 7ppbv per day. The data support, therefore, the existence of a sub-vortex region in which meridional transport is more efficient than in the vortex above. The low ozone mixing ratios in the sub-vortex region may be due to in-situ chemical destruction of ozone or transport of ozone-poor air out of the bottom of the vortex. The aircraft data we use cannot distinguish between these two processes. Key words. Meteorology and atmospheric dynamics polar meteorology) – Atmospheric composition and structure (middle atmosphere–composition and chemistry)

[1] In this work, observations of polar stratospheric clouds (PSC) carried out during one flight of the stratospheric research aircraft Geophysica deployed in the Airborne Polar Experiment–Geophysica Aircraft in Antarctica (APE-GAIA) campaign in September–October 1999 are presented. An analysis of data from the two lidars and a backscatter sonde on board the Geophysica aircraft is presented, coupled with temperature measurements. The observations are analyzed with the aid of a mesoscale model to provide air mass thermal histories and a microphysical box model to simulate PSC formation. The results obtained from this comparison are discussed in view of the theories for PSC formation processes.

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.