Sidharth Misra

The distribution of ammonia on Jupiter from a preliminary inversion of Juno Microwave Radiometer data

The Juno microwave radiometer measured the thermal emission from Jupiters atmosphere from the cloud tops at about 1 bar to as deep as a hundred bars of pressure during its first flyby over Jupiter (PJ1). The nadir brightness temperatures show that the Equatorial Zone is likely to be an ideal adiabat, which allows a determination of the deep ammonia abundance in the range 362^(+33)_(-33) ppm. The combination of Markov chain Monte Carlo method and Tikhonov regularization is studied to invert Jupiters global ammonia distribution assuming a prescribed temperature profile. The result shows (1) that ammonia is depleted globally down to 50–60 bars except within a few degrees of the equator, (2) the North Equatorial Belt is more depleted in ammonia than elsewhere, and (3) the ammonia concentration shows a slight inversion starting from about 7 bars to 2 bars. These results are robust regardless of the choice of water abundance.

Validation of SMAP soil moisture for the SMAPVEX15 field campaign using a hyper‐resolution model

Accurate global mapping of soil moisture is the goal of the Soil Moisture Active Passive (SMAP) mission, which is expected to improve the estimation of water, energy, and carbon exchanges between the land and the atmosphere. Like other satellite products, the SMAP soil moisture retrievals need to be validated, with the validation relying heavily on in situ measurements. However, a one-to-one comparison is ill advised due to the spatial mismatch of the large SMAP footprint (∼40 km) and the point scale in situ measurements. This study uses a recently developed hyper-resolution land surface model—HydroBlocks—as a tool to upscale in situ soil moisture measurements for the SMAPVEX15 (SMAP Validation Experiment 2015) field campaign during 2–18 August 2015. Calibrated against in situ observation, HydroBlocks shows a satisfactory Kling-Gupta efficiency (KGE) of 0.817 and RMSE of 0.019 m3/m3 for the calibration period. These results indicate that HydroBlocks can be used to upscale in situ measurements for this site. Different from previous studies, here in situ measurements are upscaled using a land surface model without bias correction. The upscaled soil moisture is then used to evaluate SMAP (passive) soil moisture products. The comparison of the upscaled network to SMAP shows that the retrievals are generally able to capture the areal-averaged soil moisture temporal variations. However, SMAP appears to be oversensitive to summer precipitation. We expect these findings can be used to improve the SMAP soil moisture product and thus facilitate its usage in studying the water, energy, and carbon cycles.

Spatial Downscaling of SMAP Soil Moisture Using MODIS Land Surface Temperature and NDVI During SMAPVEX15

The Soil Moisture Active Passive (SMAP) mission provides a global surface soil moisture (SM) product at 36-km resolution from its L-band radiometer. While the coarse resolution is satisfactory to many applications, there are also a lot of applications which would benefit from a higher resolution SM product. The SMAP radiometer-based SM product was downscaled to 1 km using Moderate Resolution Imaging Spectroradiometer (MODIS) data and validated against airborne data from the Passive Active L-band System instrument. The downscaling approach uses MODIS land surface temperature and normalized difference vegetation index to construct soil evaporative efficiency, which is used to downscale the SMAP SM. The algorithm was applied to one SMAP pixel during the SMAP Validation Experiment 2015 (SMAPVEX15) in a semiarid study area for validation of the approach. SMAPVEX15 offers a unique data set for testing SM downscaling algorithms. The results indicated reasonable skill (root-mean-square difference of 0.053 m3/m3 for 1-km resolution and 0.037 m3/m3 for 3-km resolution) in resolving high-resolution SM features within the coarse-scale pixel. The success benefits from the fact that the surface temperature in this region is controlled by soil evaporation, the topographical variation within the chosen pixel area is relatively moderate, and the vegetation density is relatively low over most parts of the pixel. The analysis showed that the combination of the SMAP and MODIS data under these conditions can result in a high-resolution SM product with an accuracy suitable for many applications.

L-Band Radio-Frequency Interference Observations During the SMAP Validation Experiment 2012

Radio-frequency interference (RFI) observations for L-band microwave radiometry during the SMAP Validation Experiment 2012 (SMAPVEX12) airborne campaign are reported in this paper. The soil moisture measurement campaign was conducted in summer 2012 near Winnipeg, MB, Canada, with additional RFI flights over Denver, CO, USA. The Passive Active L-Band sensor (PALS) radiometer of the Jet Propulsion Laboratory was used with a full-bandwidth direct sampling digital backend to measure and store predetection data that is fully resolved in time and frequency. Overviews of SMAPVEX12 and the receiver and digital backend used to collect data are presented, along with the data processing techniques used for RFI detection. Properties of the observed RFI are examined and compared with the results of previous studies. Finally, implications of the results are explained considering current missions such as NASAs Soil Moisture Active Passive Mission.

SMAP RFI mitigation algorithm performance characterization using airborne high-rate direct-sampled SMAPVEX 2012 data

The SMAP RFI detecting digital backend performance is characterized using real-environment L-band RFI data from the SMAPVEX 2012 campaign. Various types of RFI signals are extracted from the airborne campaign dataset and fed to the SMAP radiometer using an Arbitrary Waveform Generator (AWG). The backend detection performance is tested, and missed-detections are further investigated. Initial results indicate RFI detection performance for the SMAP digital backend is acceptable.

Prevalent lightning sferics at 600 megahertz near Jupiter’s poles

Lightning has been detected on Jupiter by all visiting spacecraft through night-side optical imaging and whistler (lightning-generated radio waves) signatures1–6. Jovian lightning is thought to be generated in the mixed-phase (liquid–ice) region of convective water clouds through a charge-separation process between condensed liquid water and water-ice particles, similar to that of terrestrial (cloud-to-cloud) lightning7–9. Unlike terrestrial lightning, which emits broadly over the radio spectrum up to gigahertz frequencies10,11, lightning on Jupiter has been detected only at kilohertz frequencies, despite a search for signals in the megahertz range12. Strong ionospheric attenuation or a lightning discharge much slower than that on Earth have been suggested as possible explanations for this discrepancy13,14. Here we report observations of Jovian lightning sferics (broadband electromagnetic impulses) at 600 megahertz from the Microwave Radiometer15 onboard the Juno spacecraft. These detections imply that Jovian lightning discharges are not distinct from terrestrial lightning, as previously thought. In the first eight orbits of Juno, we detected 377 lightning sferics from pole to pole. We found lightning to be prevalent in the polar regions, absent near the equator, and most frequent in the northern hemisphere, at latitudes higher than 40 degrees north. Because the distribution of lightning is a proxy for moist convective activity, which is thought to be an important source of outward energy transport from the interior of the planet16,17, increased convection towards the poles could indicate an outward internal heat flux that is preferentially weighted towards the poles9,16,18. The distribution of moist convection is important for understanding the composition, general circulation and energy transport on Jupiter.Observations of broadband emission from lightning on Jupiter at 600 megahertz show a lightning discharge mechanism similar to that of terrestrial lightning and indicate increased moist convection near Jupiter’s poles.

Characterizing Drifts in Spaceborne L-Band Radiometers Using Stable Reference Regions: Application to the Aquarius Mission

The global measurement of sea-surface salinity and more precise measurements of soil moisture from space has been enabled by L-band observations from the Soil Moisture and Ocean Salinity (SMOS) mission, the Aquarius mission, and the Soil Moisture Active Passive (SMAP) mission. These measurements are key components of the global water cycle and the ability to resolve climate scale variations in these variables is extremely valuable for improved understanding of how the hydrologic cycle varies in a changing climate system. It is therefore imperative that the radiometer systems making these measurements be calibrated to remove any spurious instrument temporal drifts. In this paper, models are developed over Antarctica and rainforest regions to track the gain and offset drifts in spaceborne L-band radiometer brightness-temperature (TB) measurements. The Antarctica region is found to be best for tracking small variations (0.1 K over 10 days) for vertically polarized observations near the Brewster angle. The rainforest regions are found to be best for tracking longer term variations (>60 days) in all channels, due to larger uncertainty in the surface temperature knowledge, and excellent for tracking shorter term variations between channels (<;10 days). These reference regions were used to separate a long-term gain drift and a quasi-monthly offset variation in the Aquarius radiometer. These observations eventually led to the characterization of the root cause for the drift and are contributing to improved correction methods based on models for the hardware behavior.

Enabling the Extraction of Climate-Scale Temporal Salinity Variations from Aquarius: An Instrument Based Long-Term Radiometer Drift Correction

All channels of the Aquarius radiometer were observed to have calibration instability consisting of a drift in the antenna temperature during the first couple of months of the mission and pseudo-periodic oscillations of the antenna temperature over the mission life. For the version 4 Aquarius processing, both of these anomalies were corrected by removing a time variable bias in the Aquarius measurements relative to a seven-day global average from a salinity model. In order to accurately track long-term variation of salinity on climate scales it is necessary to decouple Aquarius radiometric calibration from ocean salinity models. In this paper, a new technique is used to investigate the nature of anomalies using nonocean vicarious external sources such as Antarctic ice or Amazonian rain forests. Two completely different solutions are developed to correct the pseudo-periodic oscillations as well as the drift of the Aquarius radiometers, decoupling the Aquarius measurements from salinity model.

Implementation of a flexible wide-band on-board radio frequency interference mitigating digital back-end radiometer system

Summary fom only given. Recent passive space-borne microwave observing systems operating below 40GHz have shown an increase in the amount of man-made interference corrupting incoming natural thermal emissions (McKague et al., 2010 IGARSS). Many radiometer systems operate in bands (e.g. 18.7GHz) that are shared with space to ground-transmissions. Other space-borne systems (e.g. Aquarius, Soil Moisture Ocean Salinity - SMOS) operate in protected radio bands to avoid Radio Frequency Interference (RFI). Measurements from these missions have shown that RFI still persists even in protected bands. The RFI environment has forced many radiometer systems to operate in narrower bands than usual. This directly impacts the radiometric noise and instrument design, which in turn impacts the necessary fidelity required for retrieving the EDRs. Based on these issues, there is a need for developing wideband microwave radiometer systems that can co-exist with a harsh RFI environment. The following talk will present the work undertaken by the Jet Propulsion Laboratory, to develop an agile wideband digital backend system that can operate in and adapt to any RFI environment. The digital backend system needs to be capable of implementing a flexible digital signal processing system that can detect and mitigate RFI contaminated spectrum regions. The goal of the digital backend is to incorporate all necessary processing in the backend to take a corrupted spectrum and produce a single RFI mitigated output value with a minimal data rate. The first portion of the talk will focus on the intermediate RFI detection algorithms that were compared and contrasted with each other in terms of algorithm performance and backend implementability. The algorithms are compared with respect to various RFI parameters such as duty-cycle, power, spectral width, number of sources etc. We utilize innovative evaluation techniques and performance metrics to compare the different algorithms. The algorithms are also tested using real airborne data measured during the Soil Moisture Active/Passive Validation Experiment (SMAPVEX) field campaign of 2012. An optimal version of the kurtosis detection algorithm and an innovative “squrtosis” algorithm with cross-frequency is implemented. A brief description of these algorithms will be presented. The final aspect of the talk will focus on the firmware implementation of the above algorithms. The algorithms are implemented on a Reconfigurable Open Architecture Computing Hardware (ROACH) -2, a Xilinx Virtex 6 stand-alone FPGA board. We will present results on the initial implementation as well as initial results based on lab-generated RFI signals. Further work based on the obtained results will also be discussed.

Intercalibration of Jason-3 advanced microwave radiometer through GPM core and constellation satellite instruments

The advanced microwave radiometer (AMR) is a critical payload on the recently launched Jason-3 mission, designed to provide the electrical range delay of the radar altimeter signal due to tropospheric water vapor and cloud liquid water [1]. The errors in the wet tropospheric path delay measurements have a direct impact on the record of global mean sea level (GMSL) and could lead to uncertainty in derived trends if spurious drifts in the radiometer are not accounted for. Therefore, it is imperative to quantify and correct radiometer calibration drift, enabling producing of a high-quality stable record of wet tropospheric path delay for use in the development of GMSL.