An analysis of drought in Italy in the last fifty years

In this paper the authors present a study of drought occurrence over Italy. The NCEP/NCAR reanalysis precipitation rates covering the period from 1948 to 2000 were used to assess drought over the area in question. The analysis is based on the Standardized Precipitation Index (SPI), which has been proposed asan indicator of drought occurrence. The index relieson the knowledge of the precipitation field only, therefore, it may be readily computed from the data. A Principal Component Analysis (PCA) of the resulting fields reveals a downward trend for the index over the area considered. It implies that drought conditions have been in the recent past more frequent and extended, within the region, over larger areas. Moreover, the temporal behaviours of the principal component scores show few long-term periodicities

Long-term response of stratospheric ozone and temperature to solar variability

The long-term variability in stratospheric ozone mass mixing ratio (O3) and temperature (T) from 1979 to 2013 is investigated using the latest reanalysis product delivered by the European Centre for Medium-Range Weather Forecasts (ECMWF), i.e., ERA-Interim. Moreover, using the Mg II index time series for the same time period, the response of the stratosphere to the 11-year Schwabe solar cycle is investigated. Results reveal the following features: (i) upward (downward) trends characterize zonally averaged O3 anomalies in the upper (middle to lower stratosphere) stratosphere, while prevailing downward trends affect the T field. Mg II index data exhibit a weaker 24th solar cycle (though not complete) when compared with the previous two; (ii) correlations between O3 and Mg II, T and Mg II, and O3 and T are consistent with photochemical reactions occurring in the stratosphere and large-scale transport; and (iii) wavelet cross-spectra between O3 and Mg II index show common power for the 11-year period, particularly in tropical regions around 30-50 hPa, and different relative phase in the upper and lower stratosphere. A comprehensive insight into the actual processes accounting for the observed correlation between ozone and solar UV variability would be gained from an improved bias correction of ozone measurements provided by different satellite instruments, and from the observations of the time behavior of the solar spectral irradiance

A precipitation-based regionalization for Western Iran and regional drought variability

Abstract. The spatial distribution of the seasonal and annual precipitation was analyzed in western Iran using data from 140 stations covering the period 1965–2000. Applying the Precipitation Concentration Index (PCI), the intraannual precipitation variability was also studied. Furthermore, nine precipitation-derived parameters were used to regionalize climate in western Iran using principal component analysis and clustering techniques. Results suggest that five spatially homogenous sub-regions can be identified characterized by different precipitation regimes. The spatial pattern of seasonal precipitation seems to be highly controlled by the wide latitudinal extent of the region and by the pronounced orographic relieves, and the time of occurrence of the maximum precipitation varies from spring in the north to winter in the south. The time variability of dry and wet periods in the identified sub-regions was analyzed using the Precipitation Index (PI) and the existence of any long-term trend was tested. Results show that the northern and southern regions of western Iran are characterized by different climatic variability. Furthermore, a negative long-term linear trend in the north and a weak positive trend in the south of the study area have been detected though they are not statistically significan

Multiple jets observed in the summer Northern Hemisphere troposphere

Daily observations of the Northern Hemisphere zonal mean zonal wind during July show an intermittent formation of multiple tropospheric jet streams. In particular, a tropospheric westerly, or easterly, jet occurs at latitudes greater than 75◦N: it co-exists with the mid-latitude jet and is characterized by variability on synoptic time scale. Two sample years are here considered, July 1996 and July 1985, when prevailing westerly and easterly jets occur at high latitudes, respectively. Analysis is consistent with a picture where the physical mechanism, which creates and maintains the polar jet in the summer troposphere, is the baroclinic instability process acting on a westerly, or easterly, background zonal flow. Due to the synoptic variability, monthly mean maps of the zonal mean zonal wind for July may show different jet patterns as a function of the year, depending on the occurrences (number and duration) of westerly or easterly polar jets within each month considered. The study of the inter-annual variability of the zonal mean zonal wind through the principal component analysis indicates, in fact, that three tropospheric jet stream patterns can be distinguished. Consistency of observations with the available theories on the double-jet formation is provided and the impact of polar jet occurrences on surface temperature field is evaluated

Observed drought and wetness trends in Europe: an update

Abstract. Linear and nonlinear trends of drought and wetness are analysed in terms of the gridded Standardized Precipitation Index (SPI) determined from monthly precipitation in Europe (NCEP/NCAR). In characterizing the meteorological and hydrological aspects, the index is computed on a seasonal and on a bi-annual time scale. Two datasets are compared: one from 1949 to 1997 and the other one includes the update of the last decade (to February 2009). The following results are noted: (i) time series of drought and wetness area coverage (number of grid points above/below the severity threshold) show a remarkable linear trend until about the end of the last century, which is reversed in the last (update) decade. This recent trend reversal is an indication of a nonlinear trend, which is more pronounced on the hydrological time scale. (ii) A nonlinear trend analysis is performed based on the time series of the principal component (PC) associated to the first spatial SPI-eigenvector after embedding it in a time delay coordinate system using a sliding window of 70 months (singular spectrum analysis). Nonlinearity appears as a clear feature on the hydrological time scale. (iii) The first spatial EOF-patterns of the shorter and the longer (updated) SPI time series fields show similar structure. An inspection of the SPI time behaviour at selected grid points illustrates the spatial variability of the detected trends

Multiple jets observed in the summer Northern Hemisphere troposphere

Daily observations of the Northern Hemisphere zonal mean zonal wind during July show an intermittent formation of multiple tropospheric jet streams. In particular, a tropospheric westerly, or easterly, jet occurs at latitudes greater than 75◦N: it co-exists with the mid-latitude jet and is characterized by variability on synoptic time scale. Two sample years are here considered, July 1996 and July 1985, when prevailing westerly and easterly jets occur at high latitudes, respectively. Analysis is consistent with a picture where the physical mechanism, which creates and maintains the polar jet in the summer troposphere, is the baroclinic instability process acting on a westerly, or easterly, background zonal flow. Due to the synoptic variability, monthly mean maps of the zonal mean zonal wind for July may show different jet patterns as a function of the year, depending on the occurrences (number and duration) of westerly or easterly polar jets within each month considered. The study of the inter-annual variability of the zonal mean zonal wind through the principal component analysis indicates, in fact, that three tropospheric jet stream patterns can be distinguished. Consistency of observations with the available theories on the double-jet formation is provided and the impact of polar jet occurrences on surface temperature field is evaluated

Sensitivity of cloud radiative forcing to changes of microphysical parameters measured by the CLOUDS mission

CLOUDS (a Cloud and Radiation monitoring satellite) is a study for a satellite mission designed to provide the gross vertical profile, the internal structure, the radiative and the imaging features of clouds. This subject is addressed by several missions designed for process study intent. CLOUDS, instead, is designed for providing data of routine use in long-term NumericalWeather Prediction (NWP) and General Circulation Model (GCM). User requirements have been collected from various sources, and instruments concepts derived to meet those requirements. However, to establish the sensitivity of a GCM to the targeted parameters and confirm the soundness of the specified requirements (mainly accuracy and vertical resolution), special effort had to be placed. The present paper offers a rather complete assessment of the range of usefulness that CLOUDS measurements may have on the radiative calculation. To this purpose, the cloud forcing was computed as a function of cloud parameters by using a radiative model that has been applied in the GCM of the Laboratory for Atmospheres at the NASA Goddard Space Flight Center. The results show that, in most cases, the model response to the addressed cloud parameters is good if the error is within the specified limit. This is better demonstrated for relatively large particle sizes, for ice better than for liquid water, for low optical thickness and for low cloud cover. The model, however, suggests that more stringent requirements would be appropriate when small particles are considered

On non-linear baroclinic adjustment with the stratosphere

The effect of the stratosphere on the baroclinic adjustment of a nonlinear Eady model is presented. The classical linear Eady model has been modified by including an additional layer (the stratosphere), Ekman dissipation at the bottom boundary and a Newtonian cooling at the surface and the tropopause, respectively; non-linearity is introduced by wave-mean flow interaction for a single eddy mode. Results for the rigid-lid case and for small troposphere/stratosphere stratification ratio are compared with those for the linear Eady model with Ekman dissipation at the surface. For these cases model solutions consist of a steady zonal correction and an eddy field with a travelling constant amplitude wave. The equilibrated field, as a function of small stratification ratio, shows that the minimum amplitude of the eddy component raises to a height close to the tropopause (its steering level), denoting that the wave solution becomes vertical evanescent. When realistic values for the static stability in the stratosphere are considered, the zonal correction is no more time independent and reveals a degree of chaotic behaviour, while the eddy field is fully chaotic. Effects of changes in the zonal wind vertical shear and a further decreasing static stability in the stratosphere are also analysed. Results suggest that the minimum amplitude is, in average, higher than the one computed for the classical rigid lid with Ekman dissipation at the surface. Thus, as in the linear Eady model, the stratosphere induces a stabilising effect on the baroclinic dynamics. Finally, the model solutions are compared with the time behaviour of a simplified General Circulation Model

Space-time variability of hydrological drought and wetness in Iran using NCEP/NCAR and GPCC datasets

Abstract. Space-time variability of hydrological drought and wetness over Iran is investigated using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis and the Global Precipitation Climatology Centre (GPCC) dataset for the common period 1948–2007. The aim is to complement previous studies on the detection of long-term trends in drought/wetness time series and on the applicability of reanalysis data for drought monitoring in Iran. Climate conditions of the area are assessed through the Standardized Precipitation Index (SPI) on 24-month time scale, while Principal Component Analysis (PCA) and Varimax rotation are used for investigating drought/wetness variability, and drought regionalization, respectively. Singular Spectrum Analysis (SSA) is applied to the time series of interest to extract the leading nonlinear components and compare them with linear fittings. Differences in drought and wetness area coverage resulting from the two datasets are discussed also in relation to the change occurred in recent years. NCEP/NCAR and GPCC are in good agreement in identifying four sub-regions as principal spatial modes of drought variability. However, the climate variability in each area is not univocally represented by the two datasets: a good agreement is found for southeastern and north-western regions, while noticeable discrepancies occur for central and Caspian sea regions. A comparison with NCEP Reanalysis II for the period 1979–2007, seems to exclude that the discrepancies are merely due to the introduction of satellite data into the reanalysis assimilation schem

GPS radio occultation sounding to support General Circulation Models

An assessment of the suitability of the horizontal and vertical resolution of GPS radio occultation measurements for climate studies is carried out. Simple physical relations are used to estimate the consistency between horizontal and vertical resolutions of radio occultation measurements as compared with those of the existing observing systems. In particular, the horizontal scale of the upper troposphere water vapour is investigated by analysing the variability of the refractivity index using the re-analysis data from NCEP/NCAR. The computation shows that the 300 km horizontal resolution of GPS radio occultation is within the useful range and captures the water vapour variations that are relevant for climatological purposes. Next, focusing the analysis on the requirements of the vertical resolution, we study the sensitivity of a radiative model to changes in the vertical resolution, assessing the impacts of these variations on the atmospheric equilibrium. For this purpose one reference profile and other five with lower vertical resolutions are used to perform the experiment. Results show that the model is sensitive to variations in the vertical sampling, suggesting that high vertical resolution measurements are necessary for an accurate observation of the atmosphere. To further assess the influence of the vertical sampling, the thermal tropopause height dependence on the number of layers considered is studied. Results indicate that the highest vertical resolution is needed for determining the radiative component of the tropopause dynamics