Estimate of surface direct radiative forcing of desert dust from atmospheric modulation of the aerosol optical depth

Measurements carried out on the island of Lampedusa, in the central Mediterranean, on 7 September 2005, show the occurrence of a quasi-periodic oscillation of aerosol optical depth, column water vapour, and surface irradiance in different spectral bands. The oscillation has a period of about 13 min and is attributed to the propagation of a gravity wave able to modify the vertical structure of the planetary boundary layer, as also confirmed by satellite images. The wave occurred during a Saharan dust event. The oscillation amplitude is about 0.1 for the aerosol optical depth, and about 0.4 cm for the column water vapour. The modulation of the downward surface irradiances is in opposition of phase with respect to aerosol optical depth and water vapour column variations. The perturbation of the downward irradiance produced by the aerosols is determined by comparing the measured irradiances with estimated irradiances at a fixed value of the aerosol optical depth, and by correcting for the effect of the water vapour in the shortwave spectral range. The direct radiative forcing efficiency, i.e., the radiative perturbation of the net surface irradiance produced by a unit of optical depth aerosol layer, is determined at different solar zenith angles as the slope of the irradiance perturbation versus the aerosol optical depth. The estimated direct surface forcing efficiency at about 60° solar zenith angle is −(181 ± 17) W m−2 in the shortwave, and −(83 ± 7) W m−2 in the photosynthetic spectral range. The estimated daily average forcing efficiencies are of about −79 and −46 W m−2 for the shortwave and photosynthetic spectral range, respectively.

Estimate of surface direct radiative forcing of desert dust from atmospheric modulation of the aerosol optical depth

Measurements carried out on the island of Lampedusa, in the central Mediterranean, on 7 September 2005, show the occurrence of a quasi periodic oscillation of aerosol optical depth, column water vapour, and surface irradiance in different spectral bands. The oscillation has a period of about 13 min and is attributed to the propagation of a gravity wave able to modify the vertical structure of the planetary boundary layer. The wave occurred during an event of Saharan dust at Lampedusa. The oscillation amplitude is about 0.1 for the aerosol optical depth, and about 0.4 cm for the column water vapour. The modulation of the downward surface irradiances is in opposition of phase with respect to aerosol optical depth and water vapour column variations. The perturbation to the downward irradiance produced by the aerosols is determined by comparing the measured irradiances with estimated irradiances at a fixed value of the aerosol optical depth, and by correcting for the effect of the water vapour in the shortwave spectral range. The direct radiative forcing efficiency, i.e. the radiative perturbation to the net surface irradiance produced by a unit optical depth aerosol layer, is determined at different solar zenith angles as the slope of the irradiance perturbation versus the aerosol optical depth. The estimated direct surface forcing efficiency at 60° solar zenith angle is −(181 ± 17) W m−2 in the shortwave, and −(83 ± 7) W m−2 in the photosynthetic spectral range. The estimated daily average forcing efficiencies are of about −79 and −46 W m−2 for the shortwave and photosynthetic spectral range, respectively.

Experiments with EOF-Based Perturbation Methods and Their Impact on the CPTEC/INPE Ensemble Prediction System

The impact of modifications of the perturbation method based on empirical orthogonal functions (EOFs) used operationally upon the ensemble prediction system (EPS) at the Center for Weather Prediction and Climate Studies/National Institute for Space Research (CPTEC/INPE) is evaluated. The main changes proposed in this study are to apply the EOF method to perturb the midlatitudes, apply additional perturbations to the surface pressure (P) and specific humidity (Q) fields, and compute regional perturbations over South America. The impact of these modifications in the characteristics of the initial perturbations and in the quality of the EPS forecasts is investigated. The EPS forecasts are evaluated through average statistical scores over the period 15 December 2004–15 February 2005. The statistical scores used in the evaluation are pattern anomaly correlation, root-mean-square error, ensemble spread, Brier skill score, and perturbation versus error correlation analysis (PECA). Results indicate that with the inclusion of perturbations on P and Q, EOF-based perturbations acquire a more baroclinic structure. It is also observed that the simultaneous application of additional perturbations both in the extratropics and to the P and Q fields improves the performance of CPTEC EPS and enhances the quality of forecast perturbations. Moreover, regional EOF-based perturbations computed over South America have positive impact on the ensemble forecasts over the target region.