ENTICE Satellite Orbital Simulator to Study Ice Clouds

Clouds play a significant role in the Earth's energy balance and hydrologic cycle through their effects on radiation and precipitation, and therefore are crucial for life on Earth. Earth's NexT‐generation ICE mission (ENTICE) is proposed to measure diurnally resolved global vertical profiles of cloud ice particle size, ice water content, and in‐cloud humidity and temperature combining a 94 GHz cloud radar and multi‐frequency sub‐millimeter (sub‐mm) microwave radiometers from space. The scientific objective of ENTICE is to identify the important processes by which anvil clouds evolve and interact with ambient thermodynamic conditions to advance our fundamental understanding of clouds and reduce uncertainties in cloud climate feedback. Whether such a science objective could be achieved depends on the orbital sampling characteristics of the mission. In this study, ENTICE sampling statistics are simulated with more than 1 billion (109) profiles and using five different scanning methods in a 400 km altitude precession orbit with an inclination of 65°: nadir, forward pointing, side scanning, and conical scanning for the radiometers, and nadir pointing for the radar. These statistics can then be used to help with future mission planning efforts. Using the GEOS‐5 model atmosphere (produced by nature‐run at 7‐km and 30‐min resolution), the simulator calculates sampling statistics related to cloud types and overpass times with enhancement from radar. The wide swath of ENTICE radiometers by conical and side scanning methods ensures ample high cloud samples (1.3 × 109 samples) gathered by ENTICE over its 2‐year mission for different types of clouds with sufficient sampling over the diurnal cycles. Sampling differences between radar and radiometers at nadir shows the combination of radar and radiometers will allow for measurements of cloud vertical profiles. Therefore, our results show that the designed orbit sampling of ENTICE is sufficient to fulfill the mission science goals.

• A satellite orbital simulator for cloud sampling using passive radiometer and radar from space is developed for (Earth Next Generation Ice Mission) ENTICE • The simulator simulates four different sampling methods while flying in a model atmosphere at 400 km altitude precession orbit with an inclination of 65°• The results demonstrate the designed orbit sampling of ENTICE generate sufficient data to fulfill the mission science goals microphysical properties and the thermodynamic background conditions are necessary, which is still lacking in the current satellite observations (Jiang et al., , 2019. One source of climate model uncertainty on anvil clouds stems from ice cloud particle size and fall speed parameterizations. Differences in ice cloud particle size and fall speed parameterizations result in large differences in upper tropospheric cloudiness, precipitation, diurnal cycle of convection (Elsaesser et al., 2017;Li et al., 2015), and cloud radiative effects (Hourdin et al., 2017;Muri et al., 2014). Wang et al. (2020) show that the uncertainties of climate sensitivity can be reduced by a factor of 2 by reducing ice particle size uncertainty by 25%. The Goddard Institute for Space Studies (GISS) model simulated ice water content (IWC) magnitude and distribution significantly improved when a new ice particle size was incorporated based on field campaign data (Elsaesser et al., 2017).
Therefore, it is necessary to provide better observations of ice cloud vertical profiles. Spaceborne instruments such as radar and radiometers have been used to study the properties of ice clouds like this before. This is due to their ability to cover large areas of Earth for long periods of time (Wang et al., 2019). The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and CloudSat in NASA's A-Train satellites have led to a much more complete understanding of vertical structures in clouds and precipitation (Stephens et al., 2018;Winker et al., 2010). However, limitations in CloudSat and CALIPSO still exist, especially on measuring the vertical profiles of particle sizes and in-cloud temperature and humidity. The cloud particle size retrieved from these instruments are highly subject to prior information based on empirical relationships between IWC and temperature from a limited number of field campaigns. Various validation studies have found more than a factor of 2 uncertainties in these measurements of IWC (Jiang et al., 2012;Stein et al., 2011;Wu et al., 2008). As A-Train satellites are at the end of their lifetime (Stephens et al., 2018) and the Aerosol, Cloud, and Convection, and Precipitation missions are still in planning, SmallSat and CubeSat missions will provide an opportunity to fill the observational void with low cost.
Earth's NexT-generation ICE mission (ENTICE) is proposed to enhance our understanding of the radiative effects and climate feedbacks of ice cloud microphysical properties (Jiang et al., 2019) by providing vertical profiles of ice particle size, IWC, in-cloud humidity, and temperature ENTICE will take advantage of a low-cost multi-frequency sub-mm microwave radiometers and a 94 GHz cloud radar. To sample the diurnal cycle with a near-global coverage, ENTICE proposes to fly at an approximately 400 km altitude in a 65° inclined orbit. However, simulation studies are necessary to investigate whether ENTICE will return sufficient samples to meet its scientific goals.
In this study, a satellite orbiter simulator was used to determine the number of samples of different types of ice clouds the ENTICE mission would obtain and how well diurnal cycles will be sampled. Specifically, four scanning modes of radiometers were tested in this simulation and the sampling rate of nadir-pointing radar is evaluated. The four scanning modes were nadir, forward pointing, side scanning, and conical scanning ( Figure 1). These modes were chosen based on previous satellite observation methods (Fu et al., 2020) and evaluated based on ENTICE sensitivity to IWC and ice water path (IWP) (Jiang et al., , 2019 using nature runs generated by the GEOS-5 Nature Run, Ganymed Release (Putman et al., 2014). Cloud types are determined following the convention of International Satellite Cloud Climatology Project (Rossow & Schiffer, 1991), and IWP is used to further separate the ice cloud into different categories (Jiang et al., 2015). Sampling statistics was determined as the frequency of ENTICE observations with respect to different cloud types and local time. Diurnal sampling rate is studied over two regions of interest: the Tropical Western Pacific (TWP) and the tropical Western Atlantic (TWA).

Positional Data
The first step in the simulation is to generate the positional data of all the scanning methods outlined above. This is achieved by running an analytical two-body force model simulation of the satellite as it orbits the Earth. For ENTICE, the simulation uses the orbit characteristics of the Global Precipitation Mission (GPM) orbit given in Table 1 (Matsui et al., 2013). These parameters are input into Kepler's equation which is solved using a Newton's method algorithm. Then the orbital elements are converted into an Earth Centered Inertial (ECI) Frame.

Writing -original draft: Kyle Johnson
Writing -review & editing: Jonathan H. Jiang Using the position of the satellite in the ECI Frame, the simulator finds the latitude and longitude of the satellite at different local times. It then also finds the latitude and longitude for the location each scanning method is observing. For the forward pointing scan, the software rotates the position vector forward 45° in the satellites reference frame by first converting the angle to the ECI frame and then calculating the direction cosine matrix (DCM) to rotate the ECI position vector. In this case it rotates around the satellite's normalized angular momentum vector. Then the simulator finds the latitude and longitude of the rotated position vector. There is about a minute time difference between the forward pointing and nadir scanning methods. For side scanning, depending on the time, the simulator will rotate between 0° and 55° either to the right or left. The simulation software follows the same procedure as the forward pointing scan except it varies what the angle it rotates by depending on the time and is rotating the ECI position vector around the satellite's normalized velocity vector. This in turn generates a swath width of about 1,200 km. The conical scan simply combines the two above rotations into one motion. First the simulator rotates the ECI position vector forward and then to the side depending on the time. Figure 1 provides a visual depiction of each scanning method. The side and cone scans create a swath, while the nadir and forward pointing scans do not.
In the simulation, the nadir and forward pointing scans are sampled at 10 times per second. The side and conical scans complete their swath every time the nadir and forward scans take a sample, so they sample at 100 times per second. The simulation then plots these ground tracks for reference on an equirectangular projection map of the Earth. Figure 2 shows the ground tracks of the center of satellite fields of view from each scanning method. The side and cone modes complete one swath after the nadir and forward pointing modes complete one observation.

Nature Runs and ENTICE Instruments
The clouds and atmosphere from GEOS-5 Nature Run Ganymed Release is used in this study (Putman et al., 2014). The simulation is run at spatial and temporal resolutions of 7 km and 30 min respectively. The model atmosphere is sampled every second in the simulation. Then, each scanning method is scaled by the sampling rate discussed above, assuming that the clouds in the model atmosphere are randomly distributed. We use the cloud distribution from the model and neglect the change of clouds at the same geographical location within 30 min. For the diurnal sampling study, statistics are generated for eight 3-hr local time intervals: 0000, 0300, 0600, 0900, 1,200, 1,500, 1,800, and 2,100. For simulated observations made at high scan angles, the IWC values from multiple model grid boxes along the path of observation are used.
Four cloud path scanning methods are simulated for radiometers. The radar is only simulated on the nadir scanning method similar to CloudSat (Heymsfield et al., 2014). To account for the sensitivity of radiometers used in ENTICE (Jiang et al., , 2019, the pressure level of the first cloud layer in a column with IWC values above the threshold of 1 × 10 −3 g/m 3 is recorded as cloud top and occurrence of clouds detected by radiometers. These thresholds are between the detection limit of CALIPSO and CloudSat (Heymsfield et al., 2014) and consistent with the sensitivity of the ENTICE instrument. Extremely small IWC values below 1 × 10 −8 kg/kg produced by GEOS-5 Nature Run was discarded from this analysis.
First the sampling by ISCCP cloud types is investigated. High altitude clouds are defined as any clouds above 412.5 hPa. Medium altitude clouds are ote. This orbit is based on the GPM orbit.

Table 1
The Orbital Elements of the Orbit Used in the Simulation defined as any cloud above 675 hPa and below 412.5 hPa. Low altitude clouds are any clouds below 675 hPa. A clear sky is a column with no clouds (i.e., IWP <1 × 10 −8 kg/kg).
The radar instrument records all pressure levels of clouds in a column with IWC above a certain threshold. The radar threshold is determined using the formula shown in Equation 1 (Hong et al., 2008).
for this simulation we have assumed that minimum detection threshold for the radar is −20 dBz (Heymsfield et al., 2003), which corresponds to an IWC value of 6.3221 × 10 −3 g/m 3 . This is in line with the ENTICE radar instrument design.

Diurnal Cycles
The variation of sampling frequency with respect to local time is investigated for two regions of interest: TWP region (330°E to 350°E in longitude and 12°N to 12°S in latitude), and TWA (82°E to 117°E in longitude and 10°N to 30°N in latitude). These two regions were selected due to the high presence of high-altitude clouds (Brogniez & Kirstetter, 2020;Hartmann & Berry, 2017). In addition, the TWA has a significant impact on the weather and climate of the United States. Different ice cloud types represented by IWP values are analyzed. Figure 3 shows the maps of ground tracks produced by each of the scanning methods over 24 hr. With 1 day of observations, the side and conical scans provide a near-global coverage. It also confirms the simulation is accurately reproducing the different scanning methods. This is part of the first step in generating the positional data as described in the methodology and data section. The orbit used in this simulation is the GPM satellite orbit. This orbit was chosen as the inclination of the satellite would help the instruments study the impact of diurnal cycles on ice cloud formation, duration, and coverage. Other satellite orbits like Earthcare, do not provide appropriate coverage of local times to study diurnal cycles. The inclination is too high and puts the satellite in a sun synchronous orbit (Illingworth et al., 2015), causing the satellite to pass over the area of interest at the same local time each day. Orbits like the international space station (ISS) on the other hand, do not provide enough spatial coverage of Earth. The inclination of the ISS is 52° (Thirsk et al., 2009), much lower than GPM's. The GPM orbit allow for a compromise between these two extremes. On top of that, the side and conical scanning modes lead to a much higher coverage of Earth on a shorter time scale. By sweeping a swath, the satellite can cover more ground than if it just had a nadir or forward pointing sensor. To illustrate the sampling of vertical profiles, Figure 4 shows a curtain plot for a short section of 250 s under the nadir scan mode for both radiometers and radar. The contours show the cloud fields simulated by the nature run (i.e., the truth data). The red and green symbols show the cloud altitudes measured by radiometer and radar, respectively. As discussed, previously, radiometers have higher sensitivity to column integrated cloud properties and limited sensitivity to vertical profiles. Therefore, only the cloud top that reach the threshold of radiometer sensitivity is recorded. For radar, all vertical layers that are above the −20dBz detection threshold are recorded. Jiang et al. (2019) have shown by combining information from both radar and radiometers, vertical profiles of cloud microphysical properties with high accuracy and vertical resolution could be achieved. Figure 5 compares the total counts of observations for different cloud types using various scanning method over an observing operation period of 1 year. The conical and side scans return much larger sample sizes on high clouds than the other scanning methods. More importantly, Figure 5 shows that ENTICE with a 2-year mission time would provide sufficient samples for different types of clouds.

Results
Focusing on high altitude ice cloud only, Figure 6 shows the seasonal variation of samplings seen by the radar and nadir viewing radiometer over 1 year. With just one month of data, nadir observations provide ice cloud samples  10.1029/2021EA002145 6 of 9 on a 10 7 order of magnitude. The reason the number of samples is higher for the radar compared to the radiometer is due to its ability to identify multiple clouds within a layer. Figure 7 shows the number of high clouds seen by each radiometer scanning method over the course of 1 year. There is not a significant difference between the cone and side scan or between the nadir and forward scan. Figures 8 and 9 show ENTICE sample size for ice clouds with different ranges of IWP values in the two different regions TWA and TWP respectively. These regions were chosen not only for the large amount of high clouds present (Brogniez & Kirstetter, 2020;Hartmann & Berry, 2017), but also because the TWA has a significant pattern of moderate deep convective core occurrence and the TWP is known for its frequently occurring mesoscale convective systems (Houze et all). Four IWP bins are used here: 10 −12 to 1 g/m 2 , 1-10 g/m 2 , 10-100 g/m 2 , and >100 g/m 2 . IWP values less than 0.01 were discarded from this analysis. The results are grouped based on the local time of the observation using 3-hr intervals. The revisit rate was 9.5 hr on average over 1 month. It varied between 11.5 and 1.5 hr. With 1 month of observations, large number of samples are obtained over the full diurnal cycle for both regions. Therefore, sufficient observations for different types of ice clouds will be obtained by ENTICE over the 2-year mission, which allows researchers to study the effect diurnal cycles of ice clouds.

Conclusions and Discussions
A satellite orbit simulator is used in this study to investigate the sampling frequency of the ENTICE mission. It used four different scanning methods for the simulated radiometer: nadir, forward pointing, side, and conical and nadir pointing for the radar. The radar instrument is able to penetrate deeper into the cloud layer than the radiometer and study cloud vertical profiles. The sampling rate for nadir and forward pointing radiometer was 10 times per second and 100 times per second for side and conical. The sampling rate for the radar was 10 times per second. The side and conical scanning methods create wide swaths that provide daily near-global coverage. These scanning methods used IWC and IWP to evaluate their effectiveness. Our results show that the number of high-altitude ice cloud samples ENTICE will return over a year mission is on the order of 1 × 10 9 with either the side scanning or conical scanning methods.
Not only that, based on the analysis of local sampling throughout the day, this data will be diurnally resolved and provide key insights into the effect diurnal cycle's role in the formation and evolution of ice clouds in the atmosphere. The IWP data gathered over a year-long simulation also demonstrate the valuable insights ENTICE will gain regarding diurnal cycles. This first of its kind data will lead to better modeling of diurnal cycles and better models of our atmosphere.  We have shown this paper that ENTICE will be able to gather valuable scientific information that will provide key insights into Earth's changing climate. ENTICE will provide sufficient samples to fulfill its science mission goals. By retrieving vertical profiles of ice particle size, IWC, in-cloud humidity, and temperature ENTICE will enhance our understanding of the radiative effects and climate feedbacks of ice cloud microphysical properties. The data it gathers will reduce uncertainties in climate and weather modeling.

Data Availability Statement
The data underlying this article are available in the article. The model atmosphere used in this simulation was produced by the Global Measuring and Assimilation Office at Goddard Space Flight Center (https://gmao.gsfc. nasa.gov/global_mesoscale/7km-G5NR/). All data and codes to generate the simulated data that are used in this study can be freely downloaded at https://opendap.nccs.nasa.gov/dods/OSSE/G5NR/Ganymed/7km/0.0625_deg/ inst/inst30mn_3d_QI_Nv.info. For additional questions regarding the data sharing, please contact the corresponding author at Jonathan.H.Jiang@jpl.nasa.gov.