Interannual variations of dust activity in western Iran and their possible mechanisms

Figures & data

Figure 1. Topographic map of the Middle East (gray shaded). Western and southwestern Iran provinces are shown in yellow. The white text denotes mountains.

Figure 2. Map of western and southwestern Iran, with the shaded land cover product of MODIS (MCD12Q1). The stations used for dust observations in this study are shown, with the size of circles indicating the percentage frequency of dust days (DDF) from 1979 to 2015. The monthly mean DDF for each station are also shown, and the most active months are marked in a darker color.

Table 1. Phenomena related to dust according to the World Meteorological Organization (WMO) weather codes.

Synoptic code	Weather Description
06	Widespread dust in suspension in the air, not raised by wind at or near the station at the time of observation.
07	Dust or sand raised by wind at or near the station at the time of observation, but no well-developed dust or sand whirl, and no dust storm or sandstorm seen.
08	Well-developed dust or sand whirl seen at or near the station during the preceding hour or at the time of observation, but no dust storm or sandstorm
09	Dust storm or sandstorm within sight at the time of observation or at the station during the preceding hour
30–32	Slight or moderate dust storm or sandstorm
33–35	Severe dust storm or sandstorm
98	Thunderstorm combined with dust storms or sandstorms at time of observation, thunderstorm at time of observation

Table 2. List of the 10 western Iran stations used in this study.

Station	Latitude (°N)	Longitude (°E)	Elevation (m)
ABADAN	30.37	48.25	7
AHWAZ	31.33	48.67	23
BUSHEHR	28.97	50.82	9
DEZFUL	32.40	48.38	143
HAMEDAN NOZHEH	35.20	48.72	1680
ILAM	33.63	46.43	1337
KERMANSHAH	34.35	47.15	1319
KHORRAMABAD	33.43	48.28	1148
SANANDAJ	35.33	47.00	1373
SHIRAZ	29.53	52.60	1484

Table 3. A summary of datasets used in this study.

	Variables	Time scale	Stations/Resolution	Time period
1	Dust event days	Monthly	10 stations with longer records	1979–2015
2	Precipitation	Monthly	66 synoptic weather stations (including 10 dust stations)	1979–2015
3	Surface wind speed	3-hourly	66 synoptic weather stations	1979–2015
4	Precipitation fromGPCC V7	Monthly	1° × 1° grid	1979–2015
5	Wind velocity from ERA‐Interim	U-wind V-wind (10 m) Monthly (average of 6-hourly)	0.5° × 0.5° grid	1979–2015
6	Soil moisture from ERA-Interim/Land	Monthly (average of 6-hourly)	80 km grid topsoil level (0–7 cm)	1979–2010

Figure 3. (a) Monthly mean DDF, precipitation, average wind speed and maximum wind speed during 1979–2015 and (b) wind speed frequency (WSF) in the months of March to August in western Iran.

Figure 4. Mean monthly precipitation in (a) winter, (b) spring, (c) summer and (d) autumn in western Iran.

Figure 5. Time series of the dust day frequency (DDF) in spring (blue) and summer (red) during 1979–2015, and their time series after the removal of the linear trend (detrend). Horizontal lines represent the mean DDF in spring and summer.

Figure 6. Anomalies (standardized by mean and standard deviation) of the regional mean DDF, precipitation and frequency of strong winds (wind speed more than 11 m/s) in (a) spring and (b) summer during 1979–2015. The correlation coefficients (R) between the DDF and precipitation/wind are also shown with the asterisk denoting a statistical significance at the 0.1 level.

Figure 7. Temporal correlation coefficient between DDF and precipitation and wind speed in spring (a) and summer (b) at the stations. The asterisks denote that the correlation is statistically significant at the 0.1 level. Note that if the correlation coefficient between the DDF and frequency of winds at any intervals (e.g., 1–4 m/s) is larger than that between the DDF and mean wind speed (AveWind), this larger correlation coefficient will be presented.

Figure 8. Temporal correlation coefficient (TCC) between regional mean the DDF in western Iran and concurrent wind speed at 10 m (a, d), precipitation (b, e), and soil moisture (c, f) at each grid box in spring (left column) and summer (right column) during 1979–2015. Wind speed data is from ERA-Interim reanalysis. Precipitation is from GPCC V7 reanalysis, and soil moisture is from ERA-Interim/Land reanalysis. Soil moisture is available only for 1979–2010, and thus, the TCC between the DDF and soil moisture is calculated for 1979–2010. Note only positive correlation coefficients for wind speed and negative correlation coefficients for precipitation and soil moisture are shown. The green vectors denote the regression of surface wind upon DDF. Note only areas with positive regression coefficients of wind speed upon DDF that are statistically significant at the 0.1 level are displayed. Dots in Figure 8(b,c,e,f) denote the statistical significance at 0.1 confidence level (i.e., correlation coefficients >0.275).

Data availability statement

Meteorological data are available on the Islamic Republic of Iran Meteorological Organization (IRIMO) website: https://data.irimo.ir. Reanalysis wind speed data from ERA-Interim and soil moisture data from ERA-Interim/Land data are available at the following website: https://apps.ecmwf.int/datasets. Precipitation dataset from GPCC V7 are available at the following website: https://www.esrl.noaa.gov/psd/data/gridded/data.gpcc.html.