Radiative effects of desert dust on weather and climate

Transcription

1 UNIVERSITY OF ATHENS SCHOOL OF PHYSICS, DIVISION OF ENVIRONMENT AND METEOROLOGY ATMOSPHERIC MODELING AND WEATHER FORECASTING GROUP Radiative effects of desert dust on weather and climate Christos Spyrou, George Kallos, Christina Mitsakou and Christina Kalogeri Atmospheric Modeling and Weather Forecasting Group University of Athens - Greece

2 MOTIVATION The presence of desert dust in the atmosphere has considerable impacts The impacts are ranging from modification of the radiative forcing to cloud formation and precipitation. Therefore, perturbations in dust particle production can have impacts on radiative properties, cloud formation and water budget. These links are not one way but there are feedbacks that are critical for both meteorological and climatological scale phenomena. The links and feedbacks become more complicated because of the coexistence of anthropogenically-produced aerosols and chemical transformations.

3 OBJECTIVES This presentation focuses on the Direct Radiative Effects (DRE) of naturally produced particles. More specifically we discuss: Radiative transfer parameterization by utilizing the RRTMG scheme for both short and long wave components in the framework of SKIRON model Algorithm Evaluation based on actual measurements Dust Feedback on Energy Transfer

4 New Features of SKIRON/Dust Model Eight-size particle bin scheme (Zender et al. 2003; Spyrou et al., 2010) New dust source identification and flux production by utilizing additional soil classes and properties (rocky soil, clay amounts) (Reynolds et al., 1999, Spyrou et al., 2010) New dry and wet deposition schemes (Slinn and Slinn, 1980; Kumar et al., 1996; Pandis, 1998) In-cloud scavenging (Pandis, 1998) Utilization of high resolution SST (Spyrou et al., 2010) Aerosol Optical Depth (AOD) calculation (Kaufman et al., 2002 and others) Incorporation of the RRTMG radiative transfer scheme for SW and LW radiation (Iacono et al., 2008, Clough et al., 2005, Mlawer et al., 1997) Aerosol Radiation feedback

5 RRTMG IN SKIRON Pre-Processing Stage Use refractive indexes for dust particles from OPAC (Optical Properties of Aerosols and Clouds) package data (Hess et al., 1998) Calculate Extinction Efficiency of each size bin for each wavelength of RRTM using MIE theory Input Single Scattering Albedo, Asymmetry Parameter and Extinction Efficiency in the host model (SKIRON) Running Stage Dust radiative parameters are used to calculate AOD at every wavelength and input at RRTM along with several other meteorological parameters SKIRON model Radiative feedback included Calculation of Fluxes and Heating/Cooling rates.

6 Dust Storms over N Africa and Impacts on Radiation Balance A dust storm initiated over the lee of the Atlas Mountains on 5 March Model simulation was performed for the period 2-11 March The strong wind flow across the NW Africa that produced the wide front of dust in northern Algeria has been successfully simulated by the SKIRON system.

7 Dust Storms over N Africa and Impacts on Radiation Balance Measurements of the impact of dust transport on the atmospheric radiation balance at Niamey, Niger (13 29'N, 2 10'E) are reported by Slingo et al. (2006). SKIRON/Dust model calculated radiation components by utilizing the GFDL and RRTMG radiative transfer schemes

8 The Case of February 2006 Strong winds transport dust particles over the Mediterranean and towards Greece. At the 24 th Radiosonde data from Helliniko Airport is used to evaluate the radiative feedback.

9 Aerosol & radiation Effects of Dust Particles on Long Wave Fluxes Height (m) Dust Layer Helliniko Airport - 24/02/2006 at 00 UTC During the night the presence of dust particles: Increases the Long Wave Radiation at the surface by ~10 W/m 2. The dust layer emits LW radiation towards the surface, thus heating the ground Irradiances (Watt/m2) Lwdown (NDE) Lwdown (WDE) Lwup (NDE) Lwup (WDE) Heating below the dust layer Cooling inside the layer

10 Aerosol & radiation Effects of Dust Particles on Short Wave Fluxes Height (m) Helliniko Airport - 24/02/2006 at 12 UTC Dust Layer Irradiances (Watt/m2) During the day the presence of dust particles Decreases the Incoming Solar Radiation at the surface by ~100 W/m 2 (shading effect) Decreases the Outgoing Short Wave Radiation at the surface by ~20 W/m 2 The energy absorbed by the dust layer increases its temperature Swdown (NDE) Swdown (WDE) Swup (NDE) Swup (WDE)

11 Aerosol & radiation Lower Tropospheric Heating Due to Presence of Dust Helliniko Radiosonde 24/2/2006 at 12 UTC Increased Temperature inside the dust layer Height (m) Temperature (C) Radiosonde NDE WDE Such heating of the lower troposphere has as an effect the weakening of convection and therefore reduction in precipitation

12 The Case of 6 8 April 2006 Strong 850hPa winds transport dust particles over the Mediterranean and towards Greece Solar Radiation measurements from a pyranometer located at the roof of Crete TEI (data available from AERONET)

13 Aerosol & radiation 1200 Incoming solar radiation is decreased over Crete in the presence of dust. Atmospheric cooling due to radiation reflectance at the upper level of the dust layer leads in cirrus formation. Inc SW Crete WDE NDE Formation of cirrus clouds at the area explains the abrupt decrease of radiation on 7 April /5/2006 0:00 4/6/2006 0:00 4/7/2006 0:00 4/8/2006 0:00 4/9/2006 0:00 4/10/ :00 Date 4/11/ :00 4/12/ :00 4/13/ :00 4/14/ :00 4/15/ :00

14 Reduction of Incoming Solar Radiation SKIRON/Dust model with RRTM radiative transfer scheme for VIS and IR

15 Impact on Outgoing SR (IR) SKIRON/Dust model with RRTM radiative transfer scheme for VIS and IR

16 Atmospheric heating due to dust (column absorption)

17 Aerosol Radiation Feedback on Temperature Distribution 2W Mid-Tropospheric cooling due to reflection Mid-Tropospheric heating due to absorption Low-Tropospheric cooling due to emission and SW reduction from above Near Surface heating due to incoming LW from above and LW trapping

18 Aerosol Radiation Feedback on Temperature Distribution 18E Mid-Tropospheric cooling due to reflection Mid-Tropospheric heating due to absorption Low-Tropospheric cooling due to emission and SW reduction from above Near Surface heating due to incoming LW from above and LW trapping

19 Some Concluding Remarks The presence of dust cloud creates significant perturbations in SW and LW radiation balance: Dust particles reduce the amount of energy at the surface through scattering and absorption of incoming radiation. Redistribution of available energy inside the atmosphere is affecting the radiation balance and cloud formation. The atmospheric absorption is increased throughout the domain due to absorption by the dust cloud. The outgoing radiation from the surface is decreased in land areas with high concentration values, due to surface cooling. In contrast the outgoing radiation over water areas near is increased. This represents the radiation being emitted from the dust cloud very close to the surface. Dust increases the incoming LW radiation (greenhouse effect). The LW effects of desert dust can no longer be ignored as the infer significant changes mostly during nighttime. It causes warming of the mid tropospheric layers that leads in stabilization and possibly precipitation reduction.