M. L. Cancillo

Monthly Modes of Variation of Precipitation over the Iberian Peninsula

Abstract An attempt is made to find the main monthly modes of variation of precipitation over the Iberian Peninsula. The modes of variation of precipitation were derived from principal component analysis. The dataset used consists of records of monthly precipitation from 40 meteorological stations over 74 yr (1919–92). The stations are spatially representative of most of the Iberian Peninsula. To take into account the seasonality of precipitation over the Iberian Peninsula, one analysis was performed separately for each calendar month. The modes of variation resulting from the different analyses were compared and clustered in groups according to their loading patterns. Seven main patterns were found to be of importance during various months of the year. These seven patterns explained more than 75% of the variance of the precipitation field from December to April. However, less than 20% of the total variance is explained for July and August. It is concluded that, depending on the month or season of interes...

An Improved Outdoor Calibration Procedure for Broadband Ultraviolet Radiometers

This article aims at improving the broadband ultraviolet radiometers calibration methodology. For this goal, three broadband radiometers are calibrated against a spectrophotometer of reference. Three different one‐step calibration models are tested: ratio, first order and second order. The latter is proposed in order to adequately reproduce the high dependence on the solar zenith angle shown by the other two models and, therefore, to improve the calibration performance at high solar elevations. The proposed new second‐order model requires no additional information and, thus, keeps the operational character of the one‐step methodology. The models are compared in terms of their root mean square error and the most qualified is subsequently validated by comparing its predictions with the spectrophotometer measurements within an independent validation data subset. Results show that the best calibration is achieved by the second‐order model, with a mean bias error and mean absolute bias error lower than 2.2 and 6.7%, respectively.

Application of an analytical formula for UV Index reconstructions for two locations in Southwestern Spain

This paper focuses on the application of a simple analytical parameterization to the filling of the Ultraviolet Index (UVI) data gaps, and the reconstruction of past UVI values at Badajoz and Caceres (Southwestern Spain). The empirical model involves three independent variables: the solar zenith angle, the total ozone column and the clearness index. Regarding the first application, daily UVI was estimated for more than 30 days when UV measurements were not available in 2007. For these cases, the missing UVI data were replaced by estimated values, thus affecting the UVI annual mean and median. Regarding the second application, the reconstruction of past UVI time-series (1950–2000) is performed only for clear-sky cases (cloud and aerosol free conditions) using the COST 726 total ozone climatology. The linear UVI trends for two periods (1957–1978 and 1979–2000) are calculated for summer months using linear least squares fits. Both locations show statistically significant UVI trends for the most recent period 1979–2000, with values of +4.4 ± 1.6% per decade for Badajoz, and +4.9 ± 1.8% per decade for Caceres. This result is mainly driven by the ozone decline at northern mid-latitudes during this period. No significant trend is found for the other analysed period.

The Empirically Corrected EP-TOMS Total Ozone Data Against Brewer Measurements at El Arenosillo (Southwestern Spain)

This paper focuses on the validation of the empirically corrected total ozone column (TOC) data provided by the Earth Probe Total Ozone Mapping Spectrometer (EP-TOMS) using ground-based measurements recorded by a well-calibrated Brewer spectroradiometer located at El Arenosillo (Spain). In addition, satellite TOC observations derived from the Ozone Monitoring Instrument (OMI) with the TOMS algorithm are also used in this paper. The agreement between EP-TOMS TOC data and Brewer measurements is excellent (R2 ~ 0.92) even for the period 2000-2005 when a higher EP-TOMS instrument degradation occurred. Despite its low magnitude, the EP-TOMS-Brewer relative differences depend on the solar zenith angle (SZA), showing a clear seasonal cycle with amplitude between ±2% and ±4%. Conversely, OMI-Brewer relative differences show a constant negative value around -1% with no significant dependence on SZA. No significant dependence on the ground-based to satellite-based differences with respect to the EP-TOMS scene or to the OMI crosstrack position is observed for either satellite retrieval algorithm. Finally, TOC, estimated by the two satellite instruments, have also been compared, showing a good agreement (R2 ~ 0.88). Overall, we conclude that the empirical correction of the EP-TOMS data record provides a reprocessed set of high quality. However, EP-TOMS data after year 2000 should not be used in calculations of global-ozone trending due to remaining errors in the data set and because it is no longer an independent data set.

Characterization of atmospheric turbulence by dynamical systems techniques

We describe a dimensional analysis for different time series of the vertical component of wind velocity from half-hourly sonic anemometermeasurements. The goal is a characterization of the atmospheric turbulence fromthe point of view of Dynamical Systems Theory, based on the correlation dimension of the strange attractor. Our results suggest that the convective turbulence correlation dimension (values ≈ 6) is lowerthan the mechanical one (values ≈ 7–9).

Empirical evaluation of a simple analytical formula for the ultraviolet index.

This paper focuses on the estimation of the UV Index (UVI) for all sky conditions using a simple analytical parameterization involving three independent variables: the solar zenith angle, the total ozone column and the clearness index. Measurements of the UVI made at Badajoz and Cáceres (Southwestern Spain) from January 2006 to December 2007 are used to estimate optimal fitting parameters for the model formula, while measurements from January to December 2008 are used to show that the formula‐based estimations have mean absolute errors lower than 6% and R2ca 0.99.

Pyranometer Thermal Offset: Measurement and Analysis

AbstractThe reliable estimation of the radiative forcing and trends in radiation requires very accurate measurements of global and diffuse solar irradiance at the earth’s surface. To improve measurement accuracy, error sources such as the pyranometer thermal offset should be thoroughly evaluated. This study focuses on the measurement and analysis of this effect in a widely used type of pyranometer. For this aim, a methodology based on capping the pyranometer has been used and different criteria for determining the thermal offset have been applied and compared. The thermal offset of unventilated pyranometers for global and diffuse irradiance has been measured under a wide range of cloud, ambient temperature, wind speed, and radiation conditions. Significant differences in absolute values and variability have been observed between daytime and nighttime, advising against correcting the thermal offset effect based only on nighttime values. Notable differences in the thermal offset between cloudy and cloud-fre...

Correction of Angular Response Error in Brewer UV Irradiance Measurements

Abstract Ultraviolet spectral irradiance measured by spectroradiometers usually presents high deviations from the ideal angular response due to imperfections in the entrance optics. In this paper a methodology to correct the angular error in the global UV spectral measurements of a Brewer MKIII spectroradiometer under all weather conditions is presented. This methodology calculates the global correction factor as a function of three variables: the direct irradiance correction factor, the diffuse irradiance correction factor, and a factor depending on the direct-to-global irradiance ratio. This work contributes to better measuring the UV radiation by improving the parameterization of the clouds effects. Depending mainly on wavelength, solar zenith angle, and cloud optical thickness, the angular correction obtained ranges from 2% to 9%. The accuracy of this correction is limited by the uncertainties in the measured angular response and in the ratio of direct to global radiation. The original and the correct...

AN APPROXIMATE METHOD OF CALCULATING THE HEAT TRANSFER COEFFICIENT IN METAL BARS

A least‐squares method is used to determine the heat transfer coefficient H of a solid copper bar in air at constant temperature. For this purpose, twelve steady states and their corresponding cooling curves were measured with temperature excesses of the metal over the surrounding air in the range of 11–74°C and were compared to those calculated using a mathematical model to solve the equation for the heat flow in the bar. The model reduces an experimental double exponential law to an overall single exponential and gives similar behavior, within 10% of statistical uncertainty, for H in the steady state and in cooling. The values obtained in this study are in qualitative agreement with the values given in the literature under similar experimental conditions, but there it is not specified how they are obtained in solids.

Correcting Daytime Thermal Offset in Unventilated Pyranometers

AbstractA main source of error in solar radiation measurements is the thermal offset inherent to pyranometers. Despite acknowledgment of its importance, its correction has been widely ignored for several decades. This neglect may have caused a generalized underestimation in solar radiation measurements. This study focuses on the correction of this error in solar irradiance measurements. For this aim a plethora of correction models built as a linear combination of several environmental variables related to the ambient temperature and to the incoming radiation were proposed. The models are fitted to experimental measurements obtained during capping events and, finally, their performance is evaluated and compared. The main results indicate that models with only one independent variable moderately correct the thermal offset error. These simple models are useful when no additional instrumentation other than the pyranometer is available. On the other hand, the more complex models show the best performance, with...