Santa Barbara DISORT Atmospheric Radiative Transfer Research Articles

Improvement of the Valencia region ultraviolet index (UVI) forecasting system

The CEAM Foundation (Valencia, Spain) has developed an operational ultraviolet index (UVI) forecasting system based on the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model. The main objective of this system is to provide the general public with a tool to minimize the impact of ultraviolet (UV) radiation, which can cause important human health problems. The system presented in this paper has been developed in collaboration with the Environment Department of the Regional Government of Valencia, and it replaces the one running until 2007. The new system substitutes the previously used Ozone Monitoring Instrument (OMI) observed data with the total ozone column data forecasted from the Global Forecasting System (GFS) model. This has allowed the forecasting period to be increased from only 1 day in the original system to 3 days, with daily updates. The UVI forecast presented herein uses maps to show the hourly daytime evolution of the UV index on selected locations as well as the maximum UVI expected in the area of interest for the following 3 days (D, D+1, and D+2). The locations selected correspond to measurement stations equipped with erythemal radiation instruments. The UVI forecast information, the erythemal radiation experimental data, and other outreach information are supplied to the public through both the CEAM Meteorology and Climatology Program Web page and the Environment Department of the Regional Government of Valencia Web page.

Modeling atmospheric longwave radiation at the surface under cloudless skies

Downward atmospheric longwave radiation (DLR) is an important component of the terrestrial energy budget, strongly related with the greenhouse effect and therefore remarkably affecting the climate. In this study, DLR at the surface has been calculated using a one‐dimensional radiative transfer model, Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART). A sensitivity study has been performed in order to assess the influence of several variables on DLR levels for cloudless conditions. Among these variables, the atmospheric profiles of temperature and water content have been confirmed as the most important. Calculations have been compared with measurements made with pyrgeometers. The study has been applied to data from two European stations, Payerne (Switzerland) and Girona (Spain). For the Payerne case, for which radio soundings were available, calculations show differences with measurements in the range −2.7 ± 3.4 W m−2. For Girona, where no in situ radio soundings are available, soundings taken about 90 km away and atmospheric profiles from a gridded analysis (European Centre for Medium‐Range Weather Forecasts (ECMWF)) were used along with the meteorological information at screen level. In the latter case, differences between modeling and measurements were about 0.3 ± 9.4 W m−2. From our results, it is found that radiative transfer modeling of DLR can produce results that agree with measurements reasonably well even if no in situ radio soundings are available; the use of profiles from the ECMWF analyses does not greatly increase the bias, while the dispersion of differences is only slightly larger than the uncertainty of the measurements. It has also been confirmed that radiative transfer modeling produces better results than previously published simple parameterizations based only on surface measurements.

Investigations of the March 2006 African dust storm using ground‐based column‐integrated high spectral resolution infrared (8–13 μm) and visible aerosol optical thickness measurements: 1. Measurement procedures and results

The infrared (IR) aerosol optical thickness (AOT) spectra of Saharan dust measured during the Portable Infrared Aerosol Transmission Experiment (PIRATE) are reported. Saharan dust optical thickness (extinction) spectra from 8 to 13 μm were obtained using column‐integrated solar transmission measurements in Puerto Rico in July 2005 and Senegal in January and March 2006 (during a dust plume) using a Fourier transform infrared (FTIR) spectrometer. The FTIR measured the solar spectral irradiance in the IR in the presence of Saharan dust, and the AOT was determined by comparing the measured spectra to modeled downwelling spectra without dust for the same atmospheric temperature profile, solar zenith angle, water vapor, and ozone concentrations. The modeled dust‐free spectra are generated using the Santa Barbara Disort Atmospheric Radiative Transfer (SBDART) program. The measured dust AOT is compared with modeled AOT spectra obtained using Mie theory with dust indices of refraction from Volz and Fouquart with assumed lognormal size distributions. When the visible AOT values from nearby Aerosol Robotic Network (AERONET) sensors are compared to the IR AOT values, results from various dust loadings show that the IR dust AOT at 9.5 μm is typically only one third that of the visible (670 nm) dust AOT, but there is some evidence that this ratio could increase for larger dust size distributions. The surface IR dust forcing is determined to be about −0.4 W/m2 by summing the dusty and clear irradiance differences.

Altitude effect in UV radiation during the Evaluation of the Effects of Elevation and Aerosols on the Ultraviolet Radiation 2002 (VELETA‐2002) field campaign

The Evaluation of the Effects of Elevation and Aerosols on the Ultraviolet Radiation 2002 (VELETA‐2002) field campaign was designed to study the influence of aerosols and altitude on solar UV irradiance. The altitude effect (AE) was evaluated for UV irradiance under cloudless conditions by taking spectral and broadband measurements in SE Spain in the summer of 2002 at three nearby sites located at different heights (680 m, 2200 m, and 3398 m). A spectral radiative transfer model (Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART)) was also applied, mainly to evaluate the tropospheric ozone impact on AE. Results are related to the optical properties and air mass origin of the aerosols as determined by back‐trajectory analysis. During the 1‐week observing period of the campaign, there were two main synoptic situations with different air masses (polar maritime and tropical continental air mass associated with a Saharan dust event). The AE showed a high dependency on wavelength, solar zenith angle, and aerosols, although the growth of the mixing layer during the day also caused substantial AE variability. Saharan dust caused an increase in AE, especially in the UVB region and in the erythemal irradiance. In the UVA (320–400 nm) band the AE ranged 6–8% km−1 at noon, while for the UVB (280–320 nm) band it reached 7–11% km−1. The AE for erythemally weighted irradiance ranged from 11 to 14% km−1 between the lowest and highest stations when it was calculated from spectral measurements.

Tropospheric aerosol forcing of climate: a case study for the greater area of Greece

Although greenhouse gas forcing has global significance, the aerosol forcing is regional and seasonal, associated with the much shorter aerosol residence times in the atmosphere, and could become dominant on a regional scale. Several studies indicate that aerosol radiative forcing is among the highest in the world over the Mediterranean in the summer. In this study, the aerosol impact (forcing) on the short‐wave and long‐wave fluxes, as well as the radiative heating rate due to aerosols for different altitudes in the atmosphere over Athens, Greece, was estimated using satellite data and SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer) model. The short‐wave aerosols radiative forcing at the surface in cloud‐free conditions during the period 2000–2001 ranged from 10.8 to 20.1 W m−2 in the winter and from 15.2 to 16.6 W m−2 in the summer. The radiative heating rates near the surface due to aerosols were found to be in the range of 0.2–0.5 K day−1 during the winter period and 0.4 K day−1 during the summer period simultaneous with enhanced heating in the lower troposphere (below 5 km). The long‐wave radiative forcing (clear sky) at the top of the atmosphere induced by aerosols during night‐time was estimated to be only 0.02–0.04 W m−2 and 0.04–0.05 W m−2 for the winter and summer months, respectively.

Aerosol physical properties and Radiative forcing at the outflow region from the Indo‐Gangetic plains during typical clear and hazy periods of wintertime

Results of a campaign mode measurements of column integrated aerosol optical depths, near surface mass concentrations and LIDAR profiles during winter 2004 at Kharagpur located at the vent of the out flow region from the Indo‐Gangetic plains are presented with a view to characterize the atmospheric aerosol radiative forcing during hazy and clear sky conditions. The multispectral optical depths, surface mass concentrations and black carbon are high, while a decrease in Angstrom exponent value was observed during the hazy sky days. During hazy conditions, the LIDAR profiles show a decrease in the mixed layer height leading to confinement and subsidence of aerosols. Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model along with Optical Properties of Aerosols and Clouds (OPAC) are used to perform composite aerosol forcing estimation, incorporating LIDAR derived mixed layer heights. A considerable decrease in estimated single scattering albedo has been observed during hazy sky days. The atmospheric absorption during hazy sky days increased by 75% compared to clear days. Negative forcing was observed both at the top of the atmosphere and at the surface. The forcing estimates of the present study are compared with the similar studies over other locations in the Indo‐Gangetic plains during the campaign period.

Radiative forcing of black carbon over eastern India

Measurements of aerosol Black Carbon using a 7 channel Aethalometer at Visakhapatnam, a coastal tropical station on the east coast of India are used to study the temporal variation of surface BC mass concentration. The surface BC mass concentrations show a significant diurnal variation which is seasonally dependant. Analysis using the multi spectral data indicates that the BC mass does not show significant absorption due to non‐BC aerosol species which indicates that the surface BC mass is dominated by fossil fuel sources with no indication of any other strong anthropogenic source. The Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model was used in conjunction with Optical Properties of Aerosols and Clouds (OPAC) to estimate the BC radiative forcing. The results show large negative surface forcing during winter (−35.78 W/m2), moderate during summer (−16.8 W/m2) and lower forcing during monsoon (−9.9 W/m2) and post monsoon (−2.81 W/m2). The forcing at the top of the atmosphere is positive for all the seasons.

A comparison between modeled and measured clear‐sky radiative shortwave fluxes in Arctic environments, with special emphasis on diffuse radiation

The ability of the Santa Barbara Disort Atmospheric Radiative Transfer (SBDART) model to predict clear‐sky diffuse and direct normal broadband shortwave irradiances is investigated. Model calculations of these quantities are compared with data from the Atmospheric Radiation Measurement program's Southern Great Plains (SGP) and North Slope of Alaska (NSA) sites. The model tends to consistently underestimate the direct normal irradiances at both sites by about 1%. In regard to clear‐sky diffuse irradiance, the model overestimates this quantity at the SGP site in a manner similar to what has been observed in other studies [Halthore and Schwartz, 2000]. The difference between the diffuse SBDART calculations and Halthore and Schwartz's MODTRAN calculations is very small, thus demonstrating that SBDART performs similarly to MODTRAN. SBDART is then applied to the NSA site, and here it is found that the discrepancy between the model calculations and corrected diffuse measurements (corrected for daytime offsets) [Dutton et al., 2001] is 0.4 W/m2 when averaged over the 12 cases considered here. Two cases of diffuse measurements from a shaded “black and white” pyranometer are also compared with the calculations and the discrepancy is again minimal. Thus, it appears as if the “diffuse discrepancy” that exists at the SGP site does not exist at the NSA sites. We cannot yet explain why the model predicts diffuse radiation well at one site but not at the other.