Dust as a Potential Tracer for the Flow over Different Topographical Shapes Employing MODIS-Terra Observations

The hypothesis, that the magnitude of the Froude number can represent the flow type around an obstacle was examined at several different topographical shapes in dust-laden environments. It was found that in most cases this hypothesis was true. Average of 16 May months AOT data extracted from the MODIS Terra satellite has shown that in case of Froude number less than one, the AOT isolines tend to follow the topographical contours of the mountain peak (the obstacle) along with a minimum AOT near the peak.


Introduction
This paper is an extension of [1] Alpert and Barkan (2015) which investigated the flow types, in this case employing TOMS dust as a tracer, around and above topographical obstacles. Here, the focus is not just on the flow around an ear-circular shaped obstacle, i.e., the Gebel Mara, Sudan, N. Africa, but rather around different topographical shapes.
Here, the Terra MODIS data for all May months, during 2000-2015, 16 years altogether, were employed. Hereafter, the monthly average, not the daily data was used, in order to reduce the noise. The scientific background was described in the previous paper.
NASA's gridded data from various satellite observations. MODIS (Moderate Resolution Imaging Spectroradiometer) is a key instrument aboard the Terra satellite. Terra's orbit around the Earth is timed so that it passes from north to south across the equator in the morning (~10:30 h). The MODIS, aboard the Terra and Aqua satellites, observes the Earth's entire surface every 1 to 2 days, crossing over the equator at 10:30 and 13:30 local time (LT) during the day and at 22:30 and 01:30 LT at night, respectively. The MODIS acquires data in 20 visible and shortwave infrared spectral channels and 16 thermal infrared channels. Among these channels, the 8.6 (Band 29), 11 (Band 31), and 12 μm (Band 32) are widely used for dust detection. The original resolution of the Level-2 MODIS data was 10x10km. While in Level-3, the data was changed into a regular grid with a resolution of 1˚ × 1˚ [2]

Methodology of the Data Processing
For the mountain peaks and of the surrounding area topographical contours (in black) along with May AOT average (16 May months) isolines (in red) and the average wind vectors at the mean height of the peak, were plotted in Figure 1.
The height chosen for the wind vectors was 850 hPa being the nearest standard pressure-level, at the mid-height between the background heights and the mountain peaks.
The AOT values were taken from the Giovanni online visualization and analysis website for the peaks and the upwind site of the surrounding area. The temperature and wind data were obtained from the NCEP/NCAR reanalysis with 2.5 × 2.5 deg. interval. Table 1 summarizes the features of the chosen obstacles.
The airflow around these topographical shapes was investigated with the aid of remotely-sensed MODIS dust. The study was performed for 16 months of May during 2000-2015. May was chosen because it was found to be a dust-rich month.

The May-Average Froude Number Variation
As shown in our preceding study [1] (Alpert and Barkan, 2015) when the Froude number is less than 1, the fluid will tend to flow around the obstacle, while with Froude number exceeding 1 the fluid tends to flow above the obstacle. In Table  (   In the case of the Australia Mt., the explanation is straightforward. This obstacle is relatively low (~450 m) and even in stable atmospheric conditions with a reasonably windy environment, the dust may pass over the obstacle.
In the other two cases (Assir Mts. and Gebel Mara) we can offer a possible explanation: these two sites are located more to the south then the others-around 12 -16N (the Assir mountains are reach the 23N parallel but the northern part is much lower). Due to their more southern location they are in the activity zone of the ITCZ twice a year, at its move to the north and its retreat to the south. This activity causes unstable atmospheric conditions and strong upward flow which drives the dust or other aerosols above the obstacles.
In summary, the 16-y mean Froude number was found less than one in all the investigated peaks, except to the Australia Mts., although in the Gebel Mara and the Assir Mts. The Fr number is closer to one than in the Chibas and Ahagar mountains. Table 3 shows the AOT on the peak and at the upstream foot of the obstacle and the difference between them. Australia Mts. are not shown due to its different topography from the other obstacles.

Discussion
1) In the Chibas Mountains (Figure 1(a)) the AOT behaves exactly according to the theory. Though the topography is complicated, the AOT isolines follow in good proximity the height contours. The lowest values are next to the highest peak though somewhat to the northwest, presumable due to the Coriolis force.
2) In the Ahagar Mountains (Figure 1(b)) with elliptical topography, the AOT isolines, like in the previous case, follow nicely the height contours. The minimum AOT is situated near the highest peak somewhat to the northwest.  Table 2), meaning that the bulk of the aerosols are passing above the peak, the minimum AOT is almost on the peak, a discrepancy which needs further research.

Conclusions
The assumption, based on the Froude number, that the behavior of the flow can be used as an open air hydrodynamic and thermodynamic laboratory, was reaffirmed approximately for different topographical features.
Several peaks with different topographies were examined, by means of averaged AOT data of 16 May months, from the MODIS (Moderate Resolution Imaging Spectroradiometer) on board, the TERRA satellite.
At all the peaks, excluding Australia, the average Froude number was less than one and accordingly the AOT isolines approximately followed the topographical contours of the peaks and the minimum AOT was found near the peak. Only two cases were exceptional: the AOT at the Australian peak behaved like the International Journal of Geosciences other peaks despite that its average Froude number was higher than one. On the other hand, the AOT's behavior at Gebel Mara was exceptional, i.e., the AOT isolines did not follow the topography and the minimum AOT was far from the peak. These outstanding features need further research.