A new version of the RegCM regional climate modeling system, RegCM4, has been recently developed and made available for public use. Compared to previous versions, RegCM4 includes new land surface, planetary boundary layer, and air-sea flux schemes, a mixed convection and tropical band configuration, modifications to the pre-existing radiative transfer and boundary layer schemes, and a full upgrade of the model code towards improved flexibility, portability, and user friendliness. The model can be interactively coupled to a 1D lake model, a simplified aerosol scheme (including organic carbon, black carbon, SO 4 , dust, and sea spray), and a gas phase chemistry module (CBM-Z). After a general description of the model, a series of test experiments are presented over 4 domains prescribed under the CORDEX framework (Africa, South America, East Asia, and Europe) to provide illustrative examples of the model behavior and sensitivities under different climatic regimes. These experiments indicate that, overall, RegCM4 shows an improved performance in several respects compared to previous versions, although further testing by the user community is needed to fully explore its sensitivities and range of applications.

Abstract. The RegCM-CHEM4 is a new online climatechemistry model based on the International Centre for Theoretical Physics (ICTP) regional climate model (RegCM4). Tropospheric gas-phase chemistry is integrated into the climate model using the condensed version of the Carbon Bond Mechanism (CBM-Z; Zaveri and Peters, 1999) with a fast solver based on radical balances. We evaluate the model over continental Europe for two different time scales: (1) an event-based analysis of the ozone episode associated with the heat wave of August 2003 and (2) a climatological analysis of a six-year simulation (2000)(2001)(2002)(2003)(2004)(2005). For the episode analysis, model simulations show good agreement with European Monitoring and Evaluation Programme (EMEP) observations of hourly ozone over different regions in Europe and capture ozone concentrations during and after the summer 2003 heat wave event. For long-term climate simulations, the model captures the seasonal cycle of ozone concentrations with some over prediction of ozone concentrations in non-heat wave summers. Overall, the ozone and ozone precursor evaluation shows the feasibility of using RegCM-CHEM4 for decadal-length simulations of chemistry-climate interactions.

<p><strong>Abstract.</strong> Dust storms are considered to be a natural hazard over the Arabian Peninsula, since they occur all year round with maximum intensity and frequency in Spring and Summer. The Regional Climate Model version 4 (RegCM4) has been used to study the climatology of atmospheric dust over the Arabian Peninsula from 1999 to 2012. This relatively long simulation period samples the meteorological conditions that determine the climatology of mineral dust aerosols over the Arabian Peninsula. The modeled Aerosol Optical Depth (AOD) has been compared against ground-based observations of three Aerosol Robotic Network (AERONET) stations that are distributed over the Arabian Peninsula and daily space based observations from the Multi-angle Imaging SpectroRadiometer (MISR), the Moderate resolution Imaging SpectroRadimeter (MODIS) and Ozone Monitoring Instrument (OMI). The large scale atmospheric circulation and the land surface response that lead to dust uplifting have been analyzed. While the modeled AOD shows that the dust season extends from March to August with two pronounced maxima, one over the northern Arabian Peninsula in March with AOD equal to 0.4 and one over the southern Arabian Peninsula in July with AOD equal to 0.7, the observations show that the dust season extends from April to August with two pronounced maxima, one over the northern Arabian Peninsula in April with AOD equal to 0.5 and one over the southern Arabian Peninsula in July with AOD equal to 0.5. In spring a high pressure dominates the Arabian Peninsula and is responsible for advecting dust from southern and western part of the Arabian Peninsula to northern and eastern part of the Peninsula. Also, fast developed cyclones in northern Arabian Peninsula are responsible for producing strong dust storms over Iraq and Kuwait. However, in summer the main driver of the surface dust emission is the strong northerly wind ("Shamal") that transport dust from the northern Arabian Peninsula toward south parallel to the Arabian Gulf. The AERONET shortwave Top of Atmosphere Radiative Forcing (TOARF) and at the Bottom of Atmosphere Radiative Forcing (BOARF) have been analyzed and compared with the modeled direct radiative forcing of mineral dust aerosol. The annual modeled TOARF and BOARF are &amp;minus;3.3 and −12 W m⁻², respectively. However, the annual observed TOARF and BOARF are significantly different at &amp;minus;10 and −52 W m⁻², respectively. The analysis of observed and modeled TOARF agrees with previous studies in highlighting the need for more accurate specification of surface albedo over the region. Due to the high surface albedo of the central Arabian Peninsula, mineral dust aerosols tend to warm the atmosphere in summer (June–August).</p>

An integrated chemistry-climate model (RegCM4-CHEM) simulates present-day climate, ozone and tropospheric aerosols over Egypt with a focus on northern Africa and the Greater Cairo (GC) region. The densely populated GC region is known for its severe air quality issues driven by high levels of anthropogenic pollution in conjunction with natural sources such as dust, and agricultural burning events. We find that current global emission inventories underestimate key pollutants such as nitrogen oxides and anthropogenic aerosol species. In the GC region, average ground-based observations of the daily July maximum nitrogen dioxide (NO 2 ) are 40 to 60 parts per billion by volume (ppbv) and are about 10 ppbv higher than modeled estimates, likely due to model grid cell resolution, improper boundary layer representation, and poor emissions inventories. Observed July daily maximum ozone concentrations range from 30 ppbv (winter) to 90 ppbv (summer). The model reproduces the seasonal cycle fairly well, but modeled July ozone is underestimated by approximately 10 ppbv and exhibits little interannual variability. For aerosols, springtime dust events dominate the seasonal aerosol cycle. The chemistry-climate model captures the springtime peak aerosol optical depth (AOD) of 0.7 to 1 but is slightly greater than satellite-derived AOD. Observed AOD decreases in the summer and increases again in the fall due to agricultural burning events in the Nile Delta; however, the model underestimates this observed AOD peak in fall, as standard emissions inventories underestimate the extent of this burning and the resulting aerosol emissions. Our comparison of modeled gas and particulate phase atmospheric chemistry in the GC region indicates that improved emissions inventories of mobile sources and other anthropogenic activities, specifically NO x and organic aerosols, are needed to improve air quality simulations in this region.