# NPP-VIIRS DNB Daily Data in Natural Disaster Assessment: Evidence from Selected Case Studies
## 2 Materials and Methods
### Methods
#### 2.3.1 Data Preprocessing  
NPP-VIIRS DNB captures light from city lights, fishing boats, fire, gas flares, lighting, auroras, volcanic magma, as well as reflected moonlight from clouds, snow cover, and the ground [10]. The data quality is affected by stray light and the position of the pixels in the swath [65]. To correctly detect human-related light changes after natural disasters, preprocessing work was required. For our selected case study events, we performed lunar radiance removal, cloud clearing, and edge-of-swath pixel removal as the other conditions such as fishing boats, gas flares, lighting, auroras, volcanic magma, and stray light were not in effect. Figure 2a shows a DNB image free from the impact of these factors. Figure 2b-e shows DNB images affected by moonlight, cloud, moonlight and cloud, and at the edge of the swath, respectively. The methods we applied to achieve this are described below. For cloud-free observations, DNB nighttime imagery is a composition of light emissions and the reflected moonlight from cloud tops and land surfaces [36; 37; 66]. The DNB can capture moonlight when the moon is above the horizon at the satellite overpass time for about 15 days in a lunar cycle (about 29.5 days) [8]. The DNB radiance of a place under full moon conditions could be three times larger than the radiance under a noon moon condition [67]. Therefore, the lunar radiance should be removed to determine the true light radiance. Figure 2: National Polar-orbiting Partnership Visible Infrared Imaging Radiometer Suite (NPP-VIIRS) Day /Night Band (DNB) images under different conditions: (**a**) no moonlight or cloud, (**b**) moonlit, (**c**) cloudy, (**d**) moonlit and cloudy, and (**e**) edge-of-swath. The amount of moonlight reflected from the earth's surface depends on the lunar phase (ranging from new to full moon), elevation, azimuth, and the surface reflectance. [PERSON] [37] introduced a method to compute the lunar radiance observed by DNB sensors with the assistance of a Top of Atmosphere (TOA) spectral lunar irradiance model (MT2009) developed by [PERSON] and [PERSON] [68]. We adapted this method by using 8-day MODIS surface reflectance instead of TOA reflectance to quantify the lunar radiance contribution to produce the lunar-corrected nighttime light data. To compute the lunar radiance (\(L_{m}\)), as observed by the DNB sensor, we used:  
\[L_{m}=\frac{E_{m}}{\pi}\rho\cos(\theta_{m}) \tag{1}\]  
where \(\theta_{m}\) is the lunar zenith angle provided by NPP-VIIRS DNB SDR, \(E_{m}\) is the downwelling TOA sensor response function-weighted lunar irradiance derived from the MT2009 model, and \(\rho\) is the band averaged surface reflectance and is estimated using Band 1, 2, and 4 channels of MOD09A1. The true light radiance is the initial DNB radiance subtracting the lunar radiance \(L_{m}\). Clouds have a complex effect on DNB images according to the opacity of the cloud and the intensity of the moonlight. In no-lunar-illumination conditions, clouds can block the city light below them and blur the surrounding lights (Figure 2c). However, in high-lunar illumination conditions, clouds not only dim the city light but also reflect moonlight (Figure 2d). The Summed Cloud Cover (SCC) from the NPP-VIIRS CCL data have been proven to have good ability to detect clouds [69]. SCC is the cloud fraction summed over all layers and has a range from 0 to 1. A pixel with an SCC equal to 0 indicates that the pixel is clear of cloud. A strict criterion for SCC values smaller than 0.01 was used to identify and exclude the cloud-contaminated pixels. The DNB covers the earth with a swath width of 3044 km. To achieve a near-constant 742 m sample, the swath was aggregated into 32 zones on each side of the nadir via a scan-angle-dependent aggregation strategy [70]. The last four aggregation zones (zone 29-32) at the edge of the scan have a visible increase in noise [36] (Figure 2e), so that pixels in these zones were discarded.