Passive microwave sensors provide critical observations for initialization and validation of regional and global weather forecasting models. Although much progress has been made in global climate modeling, extreme weather events are still misrepresented in parameterizations, largely due to a lack of sufficient observations. At the same time, passive microwave sensors perform Earth observation exclusively from low Earth orbit (LEO), so their limited number leads to substantial temporal sampling gaps from the tropics and sub-tropics to the mid-latitudes, where cyclones and other precipitating storms cause the greatest damage to life and property.
The accuracy, precision, and long-term stability of TEMPEST-D microwave radiometer operation on a 6U CubeSat throughout the three-year mission demonstrated the potential for substantial enhancement of temporal observations from the tropics to the mid-latitudes. Future constellations of passive microwave sensors on small satellites in LEO are expected to provide greatly enhanced temporal observations for atmospheric sounding and remote sensing of clouds and precipitation.
Passive microwave temperature and water vapor sounding of the Earth’s atmosphere provides one of the most valuable quantitative contributions to weather prediction and is a key factor in initializing and validating climate models. Recent advances in the capabilities and robustness of small satellite components and systems provide an opportunity for NOAA, EUMETSAT and other operational agencies to explore the value of launching passive microwave sounder/imagers and complementary instruments on small spacecraft, including CubeSats, for relatively small investments. This provides the potential for deployment of microwave sounder constellations in low-Earth orbit (LEO) to substantially shorten revisit times. In this context, the first CubeSat-based multi-frequency microwave sounder to provide global data over a substantial period is the Temporal Experiment for Storms and Tropical Systems Demonstration (TEMPEST-D) mission. This mission was designed to demonstrate on-orbit capabilities of a five-frequency millimeter-wave radiometer to enable a future constellations of 6U CubeSats with low-mass, low-power millimeter-wave sensors to observe changes in convection and water vapor vertical profiles with revisit times on the order of minutes instead of hours. TEMPEST millimeter-wave radiometers provide observations at five frequencies from 87-181 GHz, with spatial resolution ranging from 12.5-25 km. To demonstrate technology necessary for deployment and operation of a CubeSat constellation of microwave sounders, the TEMPEST-D satellite was launched on May 21, 2018 from NASA Wallops to the ISS and successfully deployed into a 404-km orbit at 51.6° inclination on July 13, 2018. Now more than two years and nine months into its mission, the TEMPEST-D radiometer continues to provide science-quality data. The TEMPEST-D mission met all of its Level-1 requirements within the first 90 days of operations and achieved TRL 9 for both instrument and spacecraft systems. Validation of the TEMPEST-D brightness temperatures was performed over 50 days during a 13-month period through comparisons with GPM/GMI and MHS on NOAA-19, MetOp-A, MetOp-B and MetOp-C satellites. Results demonstrated calibration accuracy of TEMPEST-D within 1 K and stability within 0.6 K, as well as no evidence of any significant changes over time or with instrument temperature. TEMPEST-D brightness temperatures have been used to demonstrate data assimilation into NOAA numerical weather prediction models as well as atmospheric science parameter retrievals. In summary, on-orbit results show that TEMPEST-D is a very well-calibrated, highly stable radiometer, indistinguishable in performance from larger, more expensive operational sensors. Over its mission lifetime of nearly three years, TEMPEST-D has demonstrated the feasibility of deployment of a constellation of microwave sounders on CubeSats for relatively low cost and short timeline for implementation. A recently-completed CSU study, funded by NOAA, showed the potential for a CubeSat constellation of TEMPEST-based microwave sounders to perform temperature and moisture profiling with shorter refresh times. The InP HEMT low-noise amplifier technology developed for TEMPEST-D receivers for moisture profiling using 87-181 GHz frequencies can be enhanced by adding receivers with temperature profiling frequencies from 114-118 GHz range. The NOAA study demonstrated that a TEMPEST-based constellation of less than 12 CubeSats has the potential to greatly improve revisit times of current polar-orbiting operational microwave sensors.
This Conference Presentation, “Millimeter-wave sounder/imager on a CubeSat providing global atmospheric science observations for more than two years on orbit: Temporal Experiment for Storms and Tropical Systems-Demonstration (TEMPEST-D) Mission,” was recorded at SPIE Optics + Photonics 2021 held in San Diego, California, United States.
The Temporal Experiment for Storms and Tropical Systems Demonstration (TEMPEST-D) mission is the first CubeSat-based multi-frequency microwave sounder to provide global data over a sustained period. The mission was designed to demonstrate on-orbit capabilities of a five-frequency millimeter-wave radiometer for a complete TEMPEST mission using a closely-spaced train of eight 6U CubeSats with identical low-mass, low-power millimeter-wave sensors to sample rapid changes in convection and surrounding water vapor every 3-4 minutes for up to 30 minutes. The TEMPEST-D satellite was launched on May 21, 2018 from NASA Wallops to the ISS and was successfully deployed on July 13, 2018, into a 400-km orbit at 51.6° inclination. The TEMPEST-D sensor has been operating nearly continuously since its first light data on September 5, 2018. On-orbit results indicate that TEMPEST-D is a very well-calibrated, highly stable radiometer, indistinguishable in performance from larger operational sensors.
To improve understanding of rapid, dynamic evolution of convective cloud and precipitation processes as well as the surrounding water vapor environment, we require fine time-resolution multi-frequency microwave sounding observations capable of penetrating inside the storm where the microphysical processes leading to precipitation occur. To address this critical observational need, the Temporal Experiment for Storms and Tropical Systems (TEMPEST) mission deploys a train of 6U CubeSats carrying identical low-mass, low-power millimeter-wave radiometers to sample rapid changes in convection and water vapor every 3-4 minutes for up to 30 minutes. These millimeter-wave radiometers observe at five frequencies from 87 to 181 GHz. By rapidly sampling the life cycle of convection, TEMPEST fills a critical observational gap and complements existing and future satellite missions.
To demonstrate global, well-calibrated radiometric measurements to meet the needs of TEMPEST, the TEMPEST Technology Demonstration (TEMPEST-D) mission satellite was launched on May 21, 2018 on Orbital ATK’s CRS-9 mission to the ISS and deployed into a 400-km altitude and 51.6° inclination orbit by NanoRacks on July 13, 2018. TEMPEST-D has met all mission requirements on schedule and within budget. After achieving first light on September 5, 2018, the TEMPEST-D mission has achieved TRL 7 for both the instrument and spacecraft systems. TEMPEST-D brightness temperatures have been cross-calibrated with those of four NASA, NOAA and EUMETSAT reference sensors observing at similar frequencies. Results demonstrate that the TEMPEST-D on-orbit instrument is a very well-calibrated and stable radiometer with very low noise, rivaling that of much larger, more expensive operational instruments.
The TEMPEST-D in-space technology demonstration mission will reduce the risk, cost and development time of a future constellation of 6U-Class nanosatellites observing the temporal evolution of clouds to the onset of precipitation. TEMPEST-D provides passive millimeter-wave observations using a compact instrument that fits well within the 6U-Class nanosatellite architecture. TEMPEST millimeter-wave radiometers with five frequencies from 89 GHz to 182 GHz penetrate into the cloud to observe key changes as precipitation begins or as ice accumulates inside the storm. A full TEMPEST constellation mission would enable study of the conditions controlling the transition from non-precipitating to precipitating clouds using high-temporal resolution observations. Knowledge of cloud processes is essential to our understanding of climate change. Uncertainties in the representation of key processes that govern the formation and dissipation of clouds and, in turn, control the global water and energy budgets lead to substantially different predictions of future climate in current models. For the full TEMPEST constellation mission, five identical 6U-Class nanosatellites would be deployed in the same orbital plane with 5-minute spacing, initially at 450 km altitude and 51° inclination. A one-year mission would capture 3 million observations of precipitation, including at least 100,000 deep convective events. Passive drag-adjusting maneuvers would separate the five CubeSats in the same orbital plane, similar to those planned for NASA’s CYGNSS mission scheduled for launch in October 2016. TEMPEST-D was selected by NASA’s CubeSat Launch Initiative (CSLI) in February 2015 and TEMPEST-D manifested for a March 2018 launch on Firefly Space Systems Alpha.
Based on the comparison of TMI measurements and the AWS rain rates, characteristics of rain systems over the Korean
Peninsula during summer were examined. It was found that the TMI brightness temperatures (TBs) at high frequency (85
GHz) are generally higher than those in GPROF database for the same rain rate. With these characteristics kept in mind,
cloud resolving model simulations were performed by modifying intercept values in the Marshall-Palmer size
distribution relationship. The intercept values of graupel and snow were assumed to twice the default value (snow: 1.0 ×
108 m-4, Graupel: 4.0 × 106 m-4).
Microwave brightness temperatures at passive microwave radiometer frequencies and vertical reflectivity at precipitation
radar frequency are calculated by repeated application of the one-dimensional version of Eddington's approximation1.
The hydrometeor profiles were selected by comparing with TRMM PR reflectivities. By adding those selected profiles to
the convective profiles of GPROF database, we modified database for the rain retrieval over the Korean Peninsula.
Rainfall retrievals by using the new database show that quality is significantly improved, indicating the need of
algorithm locally more suitable.
Global rainfall is the primary distributor of latent heat through atmospheric circulation. This important atmospheric parameter can only be measured reliably from space. The on-going Tropical Rainfall Measuring Mission (TRMM) is the first space based mission dedicated to advance our understanding of tropical precipitation patterns and their implications on global climate and its change. The Precipitation Radar (PR) aboard the satellite is the first radar ever flown in space and has provided exciting, new data on the 3-D rain structures for a variety of scientific applications. The continuous success of TRMM has led to new development of the next generation of spaceborne satellites and sensors for global rainfall and hydrological parameter measurements. From science and cost efficiency prospective, these new sensing instruments are expected to provide enhanced capabilities and reduced consumption on the spacecraft resources. At NASA, the Earth Science Enterprise has strengthened its investment on instrument technologies to help achieving these two main goals and to obtain the best science values from the new earth science instruments. It is with this spirit that a notional instrument concept, using a dual-frequency rain radar with a deployable 5-meter electronically-scanned membrane antenna and real-time digital signal processing, is developed. This new system, the Second Generation Precipitation Radar (PR-2), has the potential of offering greatly enhanced performance accuracy while using only a fraction of the mass of the current TRMM PR. During the last two years, several of the technology items associated with this notional instrument have also been prototyped. In this paper, the science rationales, the instrument design concept, and the technology status for the PR-2 notional system will be presented.
Recent attempts to determine the response of the tropical hydrological cycle to climate forcing have been hampered by large discrepancies in the variability of available long-term satellite rainfall datasets over seasonal to interannual time scales. From investigations of regional and time-dependent changes in the structure of precipitation systems it is apparent that variations in precipitation systems due to differences in the meteorological regimes produce biases in satellite retrievals. This can have severe consequences for studies of climate variability since changes in these "climate regime" biases over seasonal or interannual time scales can produce large errors in the observed variability.
Using data from the Tropical Rainfall Measuring Mission (TRMM) we have attempted to identify and understand how changes in the structure of precipitation systems result in biases in various single sensor satellite retrievals. Evidence of differences in the relative content of ice versus liquid water, systematic changes in the height of the liquid water column, and changes in the relationship of low-level liquid water content to surface rainfall have been found, which can impact passive microwave rainfall retrievals. In addition, evidence of significant changes in the mean drop diameter between these regions may lead to significant biases in retrievals from the precipitation radar. Although TRMM has produced improvements in estimates of zonal mean rainfall, we must address this issue of time-dependent systematic biases associated with climate regimes if we wish to develop more accurate retrievals for either regional applications or global mean estimates suitable for studies of climate variability.
The planned Global Precipitation Mission (GPM) consists of a core satellite carrying a state-of-the-art dual frequency precipitation radar and a passive microwave radiometer. In addition, the GPM concept uses a constellation of satellites carrying passive microwave radiometers in order to achieve three hourly rainfall sampling. This constellation consists of radiometers on operational satellites such as the current SSM/I series, as well as some that are planned specifically as part of the GPM mission. As such, GPM is both a satellite "mission", as well as a concept designed to combine the many international assets into a coherent framework. In order to achieve this conceptual benefit, however, it is imperative that we develop algorithms and error models that allow a coherent description of rainfall to emerge from wide ranging sets of sensor capabilities. This paper will discuss work being performed to develop such a framework for the algorithms.
Recent improvements in a method for remotely sensing precipitation and latent heating distributions based upon satellite-borne, passive microwave radiometer observations are summarized. In applications to synthetic data, estimated rainfall rates at sensor footprint-scale (14 km) are subject to significant random errors, but these errors are substantially reduced by spatial averaging. After spatial-averaging, rain rate and latent heating profile estimates exhibit biases that arise from a lack of specificity in the information contained in the microwave radiance data.
The retrieval method is applied to observations from the Tropical Rainfall Measuring Mission Microwave Radiometer (TMI). Retrieved instantaneous precipitation and heating distributions show general self-consistency and delineate plausible storm structures in an application to TMI observations of a mesoscale convective system over the tropical North Atlantic. Well-known climatological distributions of rainfall are reproduced by global, monthly-mean TMI precipitation estimates from July 2000. Zonal-mean heating profiles in the Tropics from the same period exhibit a primary maximum of heating near 7 km altitude and a secondary peak near 3 km, while at higher latitudes in the Southern Hemisphere, a vertical structure with heating aloft and cooling at lower altitudes is derived.
Rainfall retrieved from space-borne instruments has been accepted as reliable and accurate by a majority of the atmospheric community. One of the Tropical Rainfall Measuring Mission (TRMM) facility rain algorithms is the passive microwave-based rain retrieval algorithm (2A-12). In order to introduce latent heating as a product of 2A-12, many improvements have been made to the current Version 5 algorithm. This paper shows how these modifications impact retrieved surface rainfall rate and latent heating estimates. Comparisons indicate that the error statistics for the prototype Version 6 2A-12 are similar to those of Version 5 at footprint-scale and 30-km resolution; however, consistent latent heating vertical profiles are now obtainable. Preliminary comparisons to dual-doppler radar-based estimates show similar heating structures, but further study will be required to establish the general credibility of 2A-12 latent heating estimates.
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