# Evaluating Satellite Sounding Temperature Observations for Cold Air Aloft Detection
## 2 Data and Methods  
This study examines the 2018 northern winter season, from January to March, over a domain that includes Alaska (40-80\({}^{\circ}\) N, 169\({}^{\circ}\) E-131\({}^{\circ}\) W). This time and domain overlaps with the [3] study. MIS detail weather conditions expected to adversely impact air traffic flow. During the 2017-2018 winter season, [3] noted that MIS were issued for CAA for an average for 55% of the time. February 2018 was the most active month of that season, with MIS in effect for 23 days and is thus the focus of our case study presented in Section 3.  
In addition to NUCAPS, conventional radiosondes and Aircraft Meteorological Data Relay (AMDAR) data are often examined alongside NWP models for forecasting CAA [11; 12]. Radiosondes vertically sample the atmosphere every 30 m (a pressure layer thickness of less than 10 hPa) up to 30 km in the atmosphere, depending on local weather conditions. While the vertical resolution exceeds most,  
Figure 1: (a) Illustration of how the tropopause height is lower in the poles. (b) Flights are often diverted when widespread Cold Air Aloft (CAA) is present in the upper troposphere and lower stratosphere, which coincides with commercial flight zones in high latitudes. Air temperatures below \(-65\)\({}^{\circ}\)C can cause water within the jet fuel to freeze and common fuels begins to form wax crystals, which can reduce fuel efficiency or pose a safety hazard.  
if not all, other in situ observing systems, radiosondes are only available over land and do not readily span international borders or flight zones. Furthermore, radiosondes are only routinely launched twice daily at 00 UTC and 12 UTC, although special launches occur in cases of severe weather [13]. In Alaska, radiosondes are launched from 14 sites that are shown in the lower right map in Figure 2. The remainder of the plots show temperature profiles from radiosondes (light grey solid lines) from February to March 2018, where red dots indicate temperatures below 208 K (65 \({}^{\circ}\)C) and therefore indicate CAA events. The data were obtained from [12]. While radiosondes can accurately detect CAA at a specific point in time and space, they are limited in their ability to characterize the spatial extent of CAA events and their launches may not always take place for reasons such as staffing shortage, equipment malfunction, or network outages. While some sites like Anchorage (ANC) have higher launch counts (N = 101, including both 00 UTC and 12 UTC), radiosondes from remote sites like Shemya Island (SYA) can be affected by frost formation upon passing through stratus clouds which cause balloons to burst at lower altitudes (N = 71) [14].  
CAA can also be detected using AMDAR data, which is a collection of measured and derived meteorological parameters (including air temperature) from aircraft sensors. When available, AMDAR data are useful for determining the horizontal extent of CAA. However, the data are irregular because flights do not necessarily follow a consistent schedule. Furthermore, while AMDAR data samples along the flight path of the airplane, and thus provides a greater variety of scenes than radiosondes, AMDAR data still does not adequately cover the \(6.2\times 10^{8}\) km\({}^{2}\) domain that makes up the whole Anchorage CWSU domain [3]. In contrast, the high orbit frequency, and wide swaths of NUCAPS  
Figure 2: Radiosonde temperature profiles from 14 Alaska sites for February through March 2018. A total of 1180 profiles are shown. Radiosondes are released twice daily at 00 UTC and 12 UTC. Red points indicate where CAA is detected. The vertical layer between the two dashed lines in each plot shows the typical transport flight cruising altitudes, assuming a standard atmosphere.  
soundings make it valuable for determining the vertical and spatial extent of CAA events, which make them ideal for augmenting observations from radiosondes and AMDAR data.