Abstract. The paper presents the first 2 years of continuous surface ozone (O 3 ) observations and systematic assessment of the influence of stratospheric intrusions (SI) at the Nepal Climate Observatory at Pyramid (NCO-P; 27 • 57 N, 86 • 48 E), located in the southern Himalayas at 5079 m a.s.l.. Continuous O 3 monitoring has been carried out at this GAW-WMO station in the framework of the Ev-K2-CNR SHARE and UNEP ABC projects since March 2006. Over the period March 2006-February 2008, an average O 3 value of 49±12 ppbv (±1δ) was recorded, with a large annual cycle characterized by a maximum during the pre-monsoon (61±9 ppbv) and a minimum during the monsoon (39±10 ppbv). In general, the average O 3 diurnal cycles had different shapes in the different seasons, suggesting an important interaction between the synoptic-scale circulation and the local mountain wind regime.Short-term O 3 behaviour in the middle/lower troposphere (e.g. at the altitude level of NCO-P) can be significantly affected by deep SI which, representing one of the most important natural input for tropospheric O 3 , can also influence the regional atmosphere radiative forcing. To identify days possibly influenced by SI at the NCO-P, a specially designed statistical methodology was applied to the time series of observed and modelled stratospheric tracers. On this basis, during the 2-year investigation, 14.1% of analysed days were found to be affected by SI. The SI frequency showed a clear seasonal cycle, with minimum during the summer monsoon Correspondence to: P. Cristofanelli (p.cristofanelli@isac.cnr.it) (1.2%) and higher values during the rest of the year (21.5%). As suggested by back-trajectory analysis, the position of the subtropical jet stream could play an important role in determining the occurrence of deep SI transport on the southern Himalayas.We estimated the fraction of O 3 due to SI at the NCO-P. This analysis led to the conclusion that during SI O 3 significantly increased by 27.1% (+13 ppbv) with respect to periods not affected by such events. Moreover, the integral contribution of SI (O 3S ) to O 3 at the NCO-P was also calculated, showing that up to 13.7% of O 3 recorded at the measurement site could be possibly attributed to SI. On a seasonal basis, the lowest SI contributions were found during the summer monsoon (less than 0.1%), while the highest were found during the winter period (up to 24.2%). Even considering the rather large uncertainty associated with these estimates, the obtained results indicated that, during non-monsoon periods, high O 3 levels could affect NCO-P during SI, thus influencing the variability of tropospheric O 3 over the southern Himalayas.
Knowledge of the precipitation contribution to the Antarctic surface mass balance is essential for defining the ice-sheet contribution to sea-level rise. Observations of precipitation are sparse over Antarctica, due to harsh environmental conditions. Precipitation during the summer months (November–December–January) on four expeditions, 2015–16, 2016–17, 2017–18 and 2018–19, in the Terra Nova Bay area, were monitored using a vertically pointing radar, disdrometer, snow gauge, radiosounding and an automatic weather station installed at the Italian Mario Zucchelli Station. The relationship between radar reflectivity and precipitation rate at the site can be estimated using these instruments jointly. The error in calculated precipitation is up to 40%, mostly dependent on reflectivity variability and disdrometer inability to define the real particle fall velocity. Mean derived summer precipitation is ~55 mm water equivalent but with a large variability. During collocated measurements in 2018–19, corrected snow gauge amounts agree with those derived from the relationship, within the estimated errors. European Centre for the Medium-Range Weather Forecasts (ECMWF) and the Antarctic Mesoscale Prediction System (AMPS) analysis and operational outputs are able to forecast the precipitation timing but do not adequately reproduce quantities during the most intense events, with overestimation for ECMWF and underestimation for AMPS.
Low-cost sensors based on the optical particle counter (OPC) are increasingly being used to collect particulate matter (PM) data at high space and time resolution. In spite of their huge explorative potential, practical guidelines and recommendations for their use are still limited. In this work, we outline a few best practices for the optimal use of PM low-cost sensors based on the results of an intensive field campaign performed in Bologna (44°30′ N, 11°21′ E; Italy) under different weather conditions. Briefly, the performances of a series of sensors were evaluated against a calibrated mainstream OPC with a heated inlet, using a robust approach based on a suite of statistical indexes capable of evaluating both correlations and biases in respect to the reference sensor. Our results show that the sensor performance is sensibly affected by both time resolution and weather with biases maximized at high time resolution and high relative humidity. Optimization of PM data obtained is therefore achievable by lowering time resolution and applying suitable correction factors for hygroscopic growth based on the inherent particle size distribution.
This work analyzes and classifies stratospheric airmass transport events (ST) detected at the Nepal Climate Observatory-Pyramid (NCO-P; 278579N, 868489E, 5079 m MSL) Global Atmospheric Watch-World Meteorological Organization station from March 2006 to February 2008. For this purpose, in situ ozone (O 3 ), meteorological parameters (atmospheric pressure and relative humidity), and black carbon (BC) are analyzed. The paper describes the synoptic-scale meteorological scenarios that are able to favor the development of ST over the southern Himalaya, by analyzing the meteorological fields provided by the ECMWF model (geopotential height, wind speed, and potential vorticity), satellite Ozone Monitoring Instrument data (total column ozone), and three-dimensional back trajectories calculated with the Lagrangian Analysis Tool (LAGRANTO) model. The study, which represents the first ''continuous'' classification of ST in the southern Himalaya, permitted classification of 94% of ST days within four synoptic-scale scenarios: stratospheric potential vorticity structures (PVS), subtropical jet stream (SJS), quasi-stationary ridges (QSR), and monsoon depressions (MD). SJS and PVS were the most frequent scenarios (48% and 30% of occurrences, respectively), QSR occurred for 12% of the ST days, and MD were detected only during the monsoon season (3%). SJS and PVS scenarios presented a peak frequency during the nonmonsoon seasons, when the jet stream and westerly disturbances influence atmospheric circulation over the southern Himalaya. During the identified ST, significant variations of O 3 (124%) and BC (256%) were recorded relative to the averaged 2-yr mean values. On average, PVS and SJS were the most effective synoptic-scale scenarios in modifying the O 3 and BC levels at NCO-P from postmonsoon to premonsoon seasons, and ST is one of the leading processes in defining the ''background'' BC variability at NCO-P.Corresponding author address: P. Cristofanelli,
Abstract. This paper concerns an in-depth analysis of an exceptional incursion of mineral dust over southern Europe in late March 2020 (27–30 March 2020). This event was associated with an anomalous circulation pattern leading to several days of PM10 (particulate matter with an aerodynamic diameter less than 10 µm) exceedances in connection with a dust source located in central Asia; this is a rare source of dust for Europe, which is more frequently affected by dust outbreaks from the Sahara Desert. The synoptic meteorological configuration was analyzed in detail, and the aerosol evolution during the transit of the dust plume over northern Italy was assessed at high time resolution by means of optical particle counting at three stations, namely Bologna, Trieste, and Mt. Cimone, allowing for the revelation of the transport timing among the three locations. Back-trajectory analyses supported by Copernicus Atmosphere Monitoring Service (CAMS) maps allowed for the location of the mineral dust source area in the Aralkum region. Therefore, the event was analyzed by observing the particle number size distribution with the support of chemical composition analysis. It is shown that the PM10 exceedance recorded is associated with a large fraction of coarse particles, which is in agreement with mineral dust properties. Both the in situ number size distribution and the vertical distribution of the dust plume were cross-checked using lidar ceilometer and aerosol optical depth (AOD) data from two nearby stations and showed that the dust plume (in contrast to those originating from the Sahara Desert) traveled close to the ground (up to a height of about 2 km). The limited mixing layer height caused by high concentrations of absorbing and scattering aerosols caused the mixing of mineral dust with other locally produced ambient aerosols, thereby potentially increasing its morbidity effects.
Snow plays a crucial role in the hydrological cycle and energy budget of the Earth, and remote sensing instruments with the necessary spatial coverage, resolution, and temporal sampling are essential for snowfall monitoring. Among such instruments, ground-radars have scanning capability and a resolution that make it possible to obtain a 3D structure of precipitating systems or vertical profiles when used in profiling mode. Radars from space have a lower spatial resolution, but they provide a global view. However, radar-based quantitative estimates of solid precipitation are still a challenge due to the variability of the microphysical, geometrical, and electrical features of snow particles. Estimations of snowfall rate are usually accomplished using empirical, long-term relationships between the equivalent radar reflectivity factor (Ze) and the liquid-equivalent snowfall rate (SR). Nevertheless, very few relationships take advantage of the direct estimation of the microphysical characteristics of snowflakes. In this work, we used a K-band vertically pointing radar collocated with a laser disdrometer to develop Ze-SR relationships as a function of snow classification. The two instruments were located at the Italian Antarctic Station Mario Zucchelli. The K-band radar probes the low-level atmospheric layers, recording power spectra at 32 vertical range gates. It was set at a high vertical resolution (35 m), with the first trusted range gate at a height of only 100 m. The disdrometer was able to provide information on the particle size distribution just below the trusted radar gate. Snow particles were classified into six categories (aggregate, dendrite aggregate, plate aggregate, pristine, dendrite pristine, plate pristine). The method was applied to the snowfall events of the Antarctic summer seasons of 2018–2019 and 2019–2020, with a total of 23,566 min of precipitation, 15.3% of which was recognized as showing aggregate features, 33.3% dendrite aggregate, 7.3% plates aggregate, 12.5% pristine, 24% dendrite pristine, and 7.6% plate pristine. Applying the appropriate Ze-SR relationship in each snow category, we calculated a total of 87 mm water equivalent, differing from the total found by applying a unique Ze-SR. Our estimates were also benchmarked against a colocated Alter-shielded weighing gauge, resulting in a difference of 3% in the analyzed periods.
Abstract. The paper presents the first 2-years of continuous surface ozone (O3) observations and systematic assessment of the influence of stratospheric intrusions (SI) at the Nepal Climate Observatory at Pyramid (NCO-P; 27°57' N, 86°48' E), located in the Southern Himalayas at 5079 m a.s.l. Continuous O3 monitoring has been carried out at this GAW-WMO station in the framework of the Ev-K2-CNR SHARE and UNEP ABC projects since March 2006. Over the period March 2006–February 2008, an average O3 value of 49±12 ppbv (±1δ) was recorded, with a large annual cycle characterized by a maximum during the pre-monsoon (61±9 ppbv) and a minimum during the monsoon (39±10 ppbv). In general, the average O3 diurnal cycles had different shapes in the different seasons, suggesting an important interaction between the synoptic-scale circulation and the local mountain wind regime. Short-term O3 behaviour in the middle/lower troposphere (e.g. at the altitude level of NCO-P) can be significantly affected by deep SI which, representing the most important natural input for tropospheric O3, can also influence the regional atmosphere radiative forcing. To identify days possibly influenced by SI at the NCO-P, analyses were performed on in-situ observations (O3 and meteorological parameters), total column O3 data from OMI satellite and air-mass potential vorticity provided by the LAGRANTO back-trajectory model. In particular, a specially designed statistical methodology was applied to the time series of the observed and modelled stratospheric tracers. On this basis, during the 2-year investigation, 14.1% of analysed days were found to be affected by SI. The SI frequency showed a clear seasonal cycle, with minimum during the summer monsoon (1.2%) and higher values during the rest of the year (21.5%). As suggested by the LAGRANTO analysis, the position of the subtropical jet stream could play an important role in determining the occurrence of deep SI transport on the Southern Himalayas. In order to estimate the fraction of O3 due to air-mass transport from the stratosphere at the NCO-P, the 30 min O3 concentrations recorded during the detected SI days were analysed. In particular, in-situ relative humidity and black carbon observations were used to exclude influence from wet and polluted air-masses transported by up-valley breezes. This analysis led to the conclusion that during SI O3 significantly increased by 27.1% (+13 ppbv) with respect to periods not affected by such events. Moreover, the integral contribution of SI (O3S) to O3 at the NCO-P was also calculated, showing that 13.7% of O3 recorded at the measurement site could be attributed to SI. On a seasonal basis, the lowest SI contributions were found during the summer monsoon (less than 0.1%), while the highest were found during the winter period (24.2%). These results indicated that, during non-monsoon periods, high O3 levels could affect NCO-P during SI, thus influencing the variability of tropospheric O3 over the Southern Himalayas. Being a powerful regional greenhouse gas, these results indicate that the evaluation of the current and future regional climate cannot be assessed without properly taking into account the influence of SI to tropospheric O3 in this important area.
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