The effect of dust on the feedback of some climatic factors from Ilam province

Introduction
Climate factors such as temperature, humidity, and precipitation play an important role on environments. Aerosol is one of the factors influencing on the climate system. Approximately 40% of aerosols in troposphere are dust. Aerosols and dust particles can affect the equilibrium energy of earth radiative, dynamic clouds, and their microphysics, both directly and indirectly impacts (Nabat et al., 2015). Rosenfeld (2000), used some satellite data, examined the processes of rainfall formation in Australian urban and industrial areas. The results indicated that the effective radius of cloud droplets have reduced as result of industrial and anthropogenic aerosols and then, coalescences droplets declined. These leaded to diminishing formation of raindrops and suppressed rainfall. Koehler et al. (2010) applied laboratory studies to give information about effect of temperature changes on several types of dust. Dust activated at less relative humidity for heterogeneous ice nucleus when temperature was higher. Additionally, those particles covered with secondary organic aerosols in lower temperatures require higher relative humidity for ice nucleus. Nabat et al. (2015) and Gu et al. (2016) studied direct and semi-direct aerosols effect on regional climate. They showed that the effect of dust on rainfall varies and can enhance or even suppress it. Absorbtion and scattering of shortwave by dust, cause the air columns to be heat. As a result, the strong vertical updraft movements are being created. On the other hand, forced radiative can dwindle surface temperature and weakens updrafts which lead to constant air. These processes can change rainfalls. Using observations with lidar in a region of China from 2010 to 2013, Wu and Yi (2017) investigated interaction of aerosols and moisture layers in cloud. Their results suggested that ice nucleus is growing with increased relative humidity thus evaluate rainfall. Ilam province is located in the western part of Iran and affected by dust storms. Hence, dust impacts were studied upon some climate factors such as temperature, humidity, cloudiness, and precipitation of Ilam province by using meteorological data from autumn and winter 2000-2013.

Materials & Methods
Ilam province is located in the western part of the Zagros chain mountain at Iran, from 31 ˚58' to 34 ˚15' northern latitude and 45 ˚24' to 48 ˚10' eastern longitude (Fig. 1).


Fig. 1. Location of Ilam province in Iran showing the synoptic and hydrology stations.

In this work, temperature, humid relatively, cloudiness, and rainfall information gathered from Iran Meteorological Organization in daily scale from 2000 to 2013 for Ilam province. Precipitation data collected from hydrology station of the Ministry of Energy as well. Statistical variance analysis in SPSS software was done to study relation among dust, temperature, relative humidity, and cloudiness. Because of the high volume of observation, MATLAB software was used to separate variables on scheduled days included dusty days as well as the days before and after. The stations were selected as control stations were those stations which had the highest correlation coefficient of daily precipitation with the target stations (Fig. 1). A regression model is used to forecast the target rainfalls by controlling rainfalls as a function of dust impact. The statistical ratio is used to evaluate climate change projections and operational cloud seeding programs on streamflow or rainfall at target stations (Gabriel., 2002; Silverman., 2010). The statistical ratio for historical regression was calculated target stations with observed and predicted rainfalls. The Monte Carlo permutation test was conducted for evaluation of dust impacts on rainfalls (Silverman., 2010).

Results
The results of statistical test presented in fig. 1 include temperature, relative humidity, and cloudiness in dusty days with the days before and after it for Ilam, Eyvan, and Dehloran.
Cloudiness
Relative Humidity (%)
Temperature (ºC)


Fig. 1. Comparison of modified temperature (ɪ), humidity (ɪɪ) and cloudiness (ɪɪɪ) on dusty days with days before and after it
a: two days ago b: one day ago c: dusty d: two days later c: on day later

Temperature variations had the lowest records in the dusty days of Ilam, Eyvan, and Dehloran with values of 10.5ºC, 10.4ºC and 19.8ºC, respectively (Fig. 1. I).
Statistical comparison of relative humidity values during these days showed that the highest relative humidity of each station was related to the days that dust occurs (Fig. 1. II). Average of relative humidity for dusty days were 54.4%, 46.9%, and 43.8%, respectively for Ilam, Eyvan, and Dehloran. Studied cloud cover for stations showed that cloudiness had the highest value for days with dust. Values of cloudiness were 3.4, 3.1, and 3 for Ilam, Eyvan, and Dehloran on dusty days, respectively.

Table 1. Monte-Carlo permutation test for statistical ratio index for historical regression target stations

90% confidence intervals significant level Statistical Ratio Station
confidence lower confidence upper
0.68 1.28 0.05 0.98 Ilam
0.5 1.1 0.04 0.8 Eyvan
0.4 1 0.03 0.7 Dehloran

Table 1 shows a statistical evaluation of dust impacts on rainfalls by using Monte Carlo Permutation test. The statistical ratios for historical regression were 0.98, 0.8, and 0.7, respectively for Ilam, Eyvan, and Dehloran. Obtained statistical ratios are less than one which shows negative effect of dust on rainfalls. However, the negative effects of dust were difference on rainfalls for studied stations.

Conclusions
Results of temperature variable showed that they decreased on dusty days. Atmospheric aerosols, such as dust, play a significant role on the radiative budget of the earth-atmosphere system. Temperature declines due to downdraft reduction shortwave flux during dust storm (Wu & Yi., 2017). Stations temperature decreased for Ilam, Eyvan, and Dehloran when dust occurred and is similar to results of Nabat et al., 2015 and Wu & Yi., 2017. Result of variance analysis of studied days showed relative humidity enhancement on dusty days in all station in this work. The latent heat is altered on earth surface by changing radiative energy under dust effect, which changes specific humidity (Dessens & Bücher., 1995). Relative humidity increased in Ilam, Eyvan, and Dehloran that agrees with Gu et al., 2016 and some other studies. Gu et al., 2016 reported that if temperature becomes less and specific humidity is constant, saturation humidity is reducing and then relative humidity is increasing. Cloudiness of studied stations showed that it increased on dusty days. One of the most important mechanisms for the cloud’s formation is sufficient relative humidity (Weare et al., 1995) which can influence on cloud cover (Wu & Yi., 2017). Changed temperature by dust impacts can vary the relative humidity. Dust effect on solar radiation at the upper atmosphere and earth surface, therefore temperature may change. These processes alter relative humidity and cloud cover. Here, cloudiness enhanced on the dusty days. These results match with McFiggans et al., 2006 that showed rising humidity may increase cloud cover due to temperature reduction.
The results of Monte Carlo of permutation analysis showed that dust had negative effect on station rainfalls. In this study, by comparing statistical ratio of each station with its relative humidity it can come to an end that effect of dust on rainfall variations are under influence of relative humidity. Interaction of water vapor and mineral dust can lead to cloud condensation nuclei and ice nucleus (Wu & Yi., 2017). The relative humidity can strongly control the surface precipitation rate (Ackerman et al., 2004). If relative humidity isn’t enough during dust storm, precipitation reduces or suppresses (Rosenfeld., 2000). Ilam station had 0.98 statistical ratio which had 54.4% relative humidity that dust had the lowest negative effect on the rainfalls. On the other hand, Dehloran station had 0.7 statistical ratio with 43.8% relative humidity. Dust effects had the highest negative effect on the rainfall of Dehloran’s station in the studied stations. These results accord with some studies that suggest suppression of precipitation on storm dust because of cloud condensation and ice nuclei in insufficient relative humidity (Wu & Yi., 2017).