# Construction of Nighttime Cloud Layer Height and Classification of Cloud Types
## 2 Sensors and Data  
In this study, we utilized data from CALIOP, CPR, and MODIS. The three satellites flew in this order within seconds to minutes of each other before the two active sensors exited the A-Train and lowered to the C-Train orbit in 2018. Collocation of the active and the passive sensors in the A-Train constellation provides opportunities to get synergistic insights and make improvements on the retrieval algorithms [28].  
CPR, the 94 GHz nadir-looking radar on board the CloudSat, and CALIOP, the two-wavelength polarization-sensitive lidar on board the CALIPSO, worked together to provide complete vertical profiles of atmosphere for features such as clouds and aerosols. Unfortunately, CloudSat had a battery anomaly on 17 April 2011, and only daytime data collection was resumed later in the year. In this work, we used 2B-CLDCLASS-lidar product released by CloudSat Data Processing Center before the battery anomaly, which combines radar and lidar measurements to provide a more complete cloud vertical structure and classify clouds into eight classes. These classes are abbreviated as stratus (St), stratus cumulus (Sc), cumulus (Cu, including cumulus congestus), nimobstratus (Ns), altocumulus (Ac), altocratus (As), deep convective (cumulonimbus, DC), or high (cirrus and cirrostratus, Ci) clouds. Data are available for download from Data Processing Center website: [[http://www.cloudsat.cira.colostate.edu](http://www.cloudsat.cira.colostate.edu)]([http://www.cloudsat.cira.colostate.edu](http://www.cloudsat.cira.colostate.edu)).  
MODIS instrument with its 36 channels spanning visible to thermal wavelengths makes continuous observations of the Earth at near daily frequency. It retrieves both physical and radiative cloud properties using combined infrared and visible techniques at day and infrared only at night. The current Collection 6 (C6) refinements of operational cloud top properties algorithms are improved from a previous version based on improvements in the spectral response functions for the bands used in CO\({}_{2}\) slicing algorithm and long-term comparison with CALIPSO measurements [10]. In this study, geographic product MYD03, calibrated radiances product MYD021 KM, and cloud properties product MYD06 were used. Specifically, IR measurements in bands 27, 29, 31, 32, and 35 were used, which have bandwidths 6.535-6.895, 8.400-8.700, 10.780-11.280, 11.770-12.270, and 13.785-14.085 \(\upmu\)m, respectively. Data are available for download at MODIS website: [[https://ladsweb.modaps.eosdis.nasa.gov/](https://ladsweb.modaps.eosdis.nasa.gov/)]([https://ladsweb.modaps.eosdis.nasa.gov/](https://ladsweb.modaps.eosdis.nasa.gov/)).  
CPR profiles have a footprint size of approximately 1.3 km \(\times\) 1.7 km, while all MODIS products used in this work have a resolution of 1 km \(\times\) 1 km. Each CPR profile along its ground track was matched with a MODIS pixel determined by having the smallest sum of the squared absolute errors and square relative errors between geodetic latitudes and longitudes of these pixels. This orbit registration process was adopted from [PERSON] and [PERSON] [29] and the reference therein.