# Icing Detection over East Asia from Geostationary Satellite Data Using Machine Learning Approaches
## 2 Study Area and Data
### Pilot Reports  
Most aero-vehicles provide specific flight logs created by the pilot when they fly through areas of potentially problematic conditions or pre-defined areas. They record the time, position, and external environmental conditions that may cause aviation problems. Such flight logs are called PIREPs. These records provide information on not only basic external conditions (e.g., temperature, wind direction, and wind speed) but also particular events (e.g., turbulence and accretion of ice on the airframe) with the time and location. When ice crystals form on the aircraft body, it is recorded as 'icing' on the PIREP, together with its severity; this has been commonly used as reference data to develop and validate icing detection algorithms [10; 14; 39]. PIREPs are accumulated extensively in the CONUS, with thousands of icing records per year [10; 15]. However, unlike the US, a relatively small number of icing observations  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **COMS MI** & & & **Himawari-8 AHI** & \\ \hline
**Variable** & **Description** & **Resolution** & **Variable** & **Description** & **Resolution** \\ \hline  
are reported in East Asia. In this study, PIREPs from KMA and the Hong Kong Observatory (HKO) were used as reference data to develop icing detection algorithms. Although PIREPs are commonly used as reference data for icing research, they are subjective and thus might provide inaccurate icing information. Furthermore, some PIREPs are reported in a few specific locations designated by governmental agencies. Thus, a quality control (QC) process for PIREPs should be conducted before using them as reference data. QC consists of several rules to verify that an icing PIREP has been obtained in an area corresponding to the physical characteristics of potential icing (e.g., SLW clouds). Table 2 shows the rules used to conduct the quality control of PIREPs using flight information and satellite-derived products. While a total of 54 icing PIREP cases (36 from HKO and 18 from KMA) were originally collected for COMS applications, only 24 PIREP cases were available for Himawari-8 applications. Through quality control, a total of 48 and 20 icing PIREPs were selected to develop icing detection models for COMS and Himawari applications, respectively. In addition, 113 carefully selected non-icing references from KMA PIREPs were used to develop the icing models (Table 3). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Rule**} & \multirow{2}{*}{**Description**} & \multicolumn{3}{c}{**Satellite Sensor**} & \multirow{2}{*}{**Flag**} \\ \cline{3-3} \cline{5-5}  & & **COMS** & & \\ \hline Rule 1 & Flight over a cloud top & FA * \textgreater{} CTH & FA \textgreater{} CTH & Excluded \\ \hline Rule 2 & Flight near an ice cloud & FA \textasciitilteq & \textasciitilteq -CTH & \textasciitilteq - & Excluded \\ \hline Rule 3 & Flight below a thick ice & FA \textasciitilteq -CTH & \textasciitilteq -ICE & \textasciitilteq -ICE & \textasciitilteq -ICE \\ \hline Rule 4 & Flight below a thin ice & FA \textasciitilteq -CTH & \textasciitilteq -CTH & \textasciitilteq -CTH & \textasciitilteq -ICE \\ \hline Rule 5 & Flight near or below a warm cloud & Tb 11 \textasciitilteq -> 270 K & Tb 11 \textasciitilteq -> 270 K & Excluded \\ \hline Rule 6 & Flight near or below a & \textasciitilteq -rigid cloud & Tb 11 \textasciitilteq -< 228 K & Tb 11 \textasciitilteq -< 228 K & Excluded \\ \hline \multicolumn{3}{c}{Final Decision} & The remaining pixels that meet the rules \textgreater{} \textgreater{} \textasciitilteq -> 20\% of the buffer with a radius of 20 km & Valid \\ \hline \hline \end{tabular}  
* FA = Flight Altitude (m); CTH = Cloud Top Height (m); CPH = Cloud Phase; COT = Cloud Optical Thickness; Tb = Brightness temperature (K); R = Reflectance.  
\end{table} 
Table 2: Quality control rules for selecting pilot reports (PIREPs) to be used to develop icing detection models. Six rules were applied to each pixel in the buffer with a radius of 20 km at an icing PIREP location.