Observing System Microwave Limb Sounder Research Articles

Long‐Term Study of Quasi‐16‐day Waves Based on ERA5 Reanalysis Data and EOS MLS Observations From 2005 to 2020

AbstractThis study uses the latest ERA5 reanalysis data and Earth Observing System microwave limb sounder (MLS) observations to investigate the climatological characteristics of quasi‐16‐day waves (Q16DWs) with different zonal wavenumbers. The fitted temperature amplitudes deduced from ERA5 reanalysis data and from MLS observations are highly consistent with each other, even in fine spatial‐temporal structures. The westward‐propagating wave with wavenumber 1 (W1) and the eastward‐propagating waves with wavenumbers 1 (E1) and 2 (E2) are identified as the most dominant propagating components according to the statistical analyses of their fitted temperature and wind amplitudes. All three waves exhibit significant intra‐annual and interannual variations. Their amplitudes in winter/spring and autumn/summer are strong, moderate, or weak. The W1 wave is more intense in boreal winter than in austral winter, while the E2 wave is the opposite, and the E1 wave is equally strong in boreal and austral winters. Their interannual variabilities measured by the ratios of peak amplitudes in temperature, zonal wind, and meridional wind range from 1.64 to 2.15, indicating significant variability. The presence of intense wave activities is always accompanied by a positive squared refractive index. We suggest that the squared refractive index contributes greatly to the global distribution characteristics of the Q16DWs. The W1 wave in the troposphere is more likely to travel upward and reach the stratosphere and mesosphere, while the intense E1 and E2 waves in the stratosphere and mesosphere may come from the upper atmosphere or be excited in situ.

The 27 day solar rotational effect on mesospheric nighttime OH and O3 observations induced by geomagnetic activity.

Observations performed by the Earth Observing System Microwave Limb Sounder instrument on board the Aura satellite from 2004 to 2009 (2004 to 2014) were used to investigate the 27 day solar rotational cycle in mesospheric OH (O3) and the physical connection to geomagnetic activity. Data analysis was focused on nighttime measurements at geomagnetic latitudes connected to the outer radiation belts (55°N/S–75°N/S). The applied superposed epoch analysis reveals a distinct 27 day solar rotational signal in OH and O3 during winter in both hemispheres at altitudes >70 km. The OH response is positive and in‐phase with the respective geomagnetic activity signal, lasting for 1–2 days. In contrast, the O3 feedback is negative, delayed by 1 day, and is present up to 4 days afterward. Largest OH (O3) peaks are found at ~75 km, exceeding the 95% significance level and the measurement noise of <2% (<0.5%), while reaching variations of +14% (−7%) with respect to their corresponding background. OH at 75 km is observed to respond to particle precipitation only after a certain threshold of geomagnetic activity is exceeded, depending on the respective OH background. The relation between OH and O3 at 75 km in both hemispheres is found to be nonlinear. In particular, OH has a strong impact on O3 for relatively weak geomagnetic disturbances and accompanying small absolute OH variations (<0.04 ppb). In contrast, catalytic O3 depletion is seen to slow down for stronger geomagnetic variations and OH anomalies (0.04–0.13 ppb), revealing small variations around −0.11 ppm.

Circulation changes in the stratosphere and mesosphere during the stratospheric sudden warming event in January 2009

In this study, general circulation changes are investigated during the stratospheric sudden warming event in January 2009 by the use of Aura Earth Observing System Microwave Limb Sounder observations to show dynamical coupling of stratosphere and mesosphere. It is revealed that the reversal from westerly winds to easterly winds occurred earlier in the mesosphere in comparison with the stratopause level and suggested that the earlier reversal in the mesosphere may be caused by large-scale waves formed in the lower mesosphere due to barotropic and/or baroclinic instability there. Such instability is considered to be brought about by the intensification of the westerly jet with double maxima in the polar night stratosphere and subtropical mesosphere before the warming occurrence. Furthermore, it is shown that an independent meridional circulation cell might be formed by the earlier reversal of zonal winds in the mesosphere.

Sub-millimeter observations of the terrestrial atmosphere during an Earth flyby of the MIRO sounder on the Rosetta spacecraft

Sub-millimeter spectra recorded by the MIRO sounder aboard the Rosetta spacecraft have been used at the time of an Earth flyby (November 2007) to check the consistency and validity of the instrumental data. High-resolution spectroscopic data were recorded in 8 channels in the vicinity of the strong water line at 557GHz, and in a broad band continuum channel at 570GHz. An atmospheric radiative transfer code (ARTS) and standard terrestrial atmospheres have been used to simulate the expected observational results. Differences with the MIRO spectra suggest an anomaly in the behavior of four spectroscopic channels. Further technical investigations have shown that a large part of the anomalies are associated with an instability of one of the amplifiers. The quality of the MIRO data has been further tested by inverting the spectra with an atmospheric inversion tool (Qpack) in order to derive a mesospheric temperature profile. The retrieved profile is in good agreement with the one inferred from the Earth Observing System Microwave Limb Sounder (EOS-MLS). This work illustrates the interest of validating instruments aboard planetary or cometary spacecraft by using data acquired during Earth flybys.

The effect of recombination and attachment on meteor radar diffusion coefficient profiles

Estimates of the ambipolar diffusion coefficient producedusing meteor radar echo decay times display an increasing trend below 80–85 km, which is inconsistent with a diffusion‐only theory of the evolution of meteor trails. Data from the 33 MHz meteor radar at King Sejong Station, Antarctica, have been compared with observations from the Aura Earth Observing System Microwave Limb Sounder satellite instrument. It has been found that the height at which the diffusion coefficient gradient reverses follows the height of a constant neutral atmospheric density surface. Numerical simulations of meteor trail diffusion including dissociative recombination with atmospheric ions and three‐body attachment of free electrons to neutral molecules indicate that three‐body attachment is responsible for the distortion of meteor radar diffusion coefficient profiles at heights below 90 km, including the gradient reversal below 80–85 km. Further investigation has revealed that meteor trails with low initial electron line density produce decay times more consistent with a diffusion‐only model of meteor trail evolution.

Evaluation of Whole Atmosphere Community Climate Model simulations of ozone during Arctic winter 2004–2005

The work presented here evaluates polar stratospheric ozone simulations from the Whole Atmosphere Community Climate Model (WACCM) for the Arctic winter of 2004–2005. We use the Specified Dynamics version of WACCM (SD‐WACCM), in which temperatures and winds are nudged to meteorological assimilation analysis results. Model simulations of ozone and related constituents generally compare well to observations from the Earth Observing System Microwave Limb Sounder (MLS). At most times, modeled ozone agrees with MLS data to within ~10%. However, a systematic high bias in ozone in the model of ~18% is found in the lowermost stratosphere in March. We attribute most of this ozone bias to too little heterogeneous processing of halogens late in the winter. We suggest that the model under‐predicts ClONO2 early in the winter, which leads to less heterogeneous processing and too little activated chlorine. Model HCl could also be overestimated due to an underestimation of HCl uptake into supercooled ternary solution (STS) particles. In late winter, the model overestimates gas‐phase HNO3, and thus NOy, which leads to an over‐prediction of ClONO2 (under‐prediction of activated chlorine). A sensitivity study, in which temperatures for heterogeneous chemistry reactions were reduced by 1.5 K, shows significant improvement of modeled ozone. Chemical ozone loss is inferred from the MLS observations using the pseudo‐passive subtraction approach. The inferred ozone loss using this method is in agreement with or less than previous independent results for the Arctic winter of 2004–2005, reaching 1.0 ppmv on average and up to 1.6 ppmv locally in the polar vortex.

Variability of mesopause temperature derived from two independent methods using meteor radar and its comparison with SABER and EOS MLS and a collocated multi-wavelength dayglow photometer over an equatorial station, Thumba (8.5° N, 76.5° E)

Two independent methods for deriving mesopause temperature using meteor radar installed at an equatorial station, Thumba (8.5° N, 76.5° E), are discussed in this article. This meteor radar-derived mesopause temperature is then compared with two different types of spaceborne measurement, namely (i) Sounding the Atmosphere using Broadband Emission Radiometry (SABER) and (ii) the Earth Observing System Microwave Limb Sounder (EOS MLS), and a collocated multi-wavelength dayglow photometer (DGPM). The meteor radar-derived temperature is in fairly good agreement with all the three measurement techniques, with an uncertainty of ±10°. This study focuses on a detailed evaluation and inter-comparison of mesopause temperature derived from different measurement techniques. An attempt is also made to compare the suitability of these observations to study planetary waves and other oscillation activities in the mesospheric region.

Estimation of Arctic ozone loss in winter 2004/05 based on assimilation of EOS MLS and SBUV/2 observations

AbstractIn this paper we present a new technique for the estimation of ozone loss in the stratospheric polar vortex based on the assimilation of Earth Observing System Microwave Limb Sounder (EOS MLS) and Solar Backscatter Ultraviolet radiometer (SBUV/2) ozone observations in the Met Office data assimilation system. We focus on the northern winter of 2004/05, which was exceptionally cold in the Arctic stratosphere, with associated large ozone depletion due to heterogeneous chemistry. Our ozone loss estimate, which was calculated for the 1 February to 10 March 2005 period, peaks at 450 K (approximately 17–18 km), and is 0.6 ppmv at that isentropic level (our loss estimate for the vortex core only was somewhat higher (1.0 ppmv) and indicates uncertainties related to mixing at the vortex edge). This value is similar to or smaller than results from other studies, which estimate ozone loss in this period to be in the 0.6–1.2 ppmv range. When combined with results from other studies that estimate ozone loss occurring outside our assimilation period, we obtain an estimate of 0.8–1.2 ppmv for ozone loss from early January to early March 2005.We find a second maximum in ozone loss for the 1 February–10 March period near 650 K (approximately 25 km) of around 0.4 ppmv. This is a lower figure than found in other studies, but ozone loss is actually much stronger at this level outside the vortex in a low‐ozone pocket in the Aleutian anticyclone, likely due to the NOx catalytic cycle. Our results show that the ozone data assimilation method we have used to estimate ozone loss is very promising, and can lead to potentially more accurate ozone‐loss estimates than other methods. © Royal Meteorological Society and Crown Copyright, 2008.

A study of ozone depletion in the 2004/2005 Arctic winter based on data from Odin/SMR and Aura/MLS

Ozone depletion in the colder than average 2004/2005 Arctic polar vortex is mapped and quantified using ozone profiles from two limb sounding satellite instruments, the Earth Observing System Microwave Limb Sounder (Aura/MLS) and the Odin Sub‐Millimetre Radiometer (Odin/SMR). Profiles of chemically inert nitrous oxide (N2O) are used to trace vertical transport during the winter. Two methods are used for estimating the vortex average ozone losses north of 67° equivalent latitude. In a first step, the time evolution of ozone mixing ratios is described on N2O isopleths. Maximum ozone depletion is found on the 100 ppbv and 150 ppbv N2O isopleths (located in the 430–460 K potential temperature range in mid‐March 2005) where vortex average ozone depletion totalled 1.0–1.1 ppmv for Aura/MLS and 0.7–0.9 ppmv for Odin/SMR. Second, ozone profiles from Aura/MLS and Odin/SMR are assimilated into the DIAMOND isentropic transport model. Ozone depletion is estimated by comparing assimilated fields to ozone fields passively transported from 1 January. On the 450 K potential temperature level, the Aura/MLS ozone fields indicate 0.9–1.3 ppmv vortex‐averaged ozone depletion while the Odin/SMR fields indicate 0.6–0.9 ppmv depletion. The uncertainty depends mainly on the rates of cross‐isentropic transport used in the study. The ozone depletion estimates in this study are lower than previously published estimates. The discrepancies to some studies can be attributed to the more adequate treatment of an ozone poor region that is found in the central polar vortex in the early winter.

Use of Canadian Quick covariances in the Met Office data assimilation system

AbstractIn this paper, we describe the use of Canadian Quick (CQ) covariances in the Met Office assimilation system. These covariances have two particular advantages over other methods (such as the National Meteorological Center (NMC) method). First, they are calculated from a single long forecast run, rather than a series of short forecasts, and thus are much quicker to produce. Second, in cases where the vertical range of the assimilation system increases, they can be used immediately.Here, we compare the performance of CQ and NMC covariances in a troposphere/stratosphere configuration of the Met Office assimilation system. The forecast model used has 50 levels from the surface to 63 km. In general, the performance of the two covariances is similar. However, it is clear that a consequence of the bootstrapping approach that was used to develop the NMC covariances is noisy patterns in the error covariances which adversely affects mean errors, particularly in the winter middle and high latitudes above the 10 hPa level. In addition, the NMC covariances show evidence of gravity waves, which appear to have been generated spuriously by the 3D‐Var assimilation due to lack of dynamical balance in the 3D‐Var analyses. Such signals are absent in the trials where CQ covariances are used.The CQ method was also used to generate covariances for expanded versions of the model which spans the surface to around 80–84 km. Trial results are generally in good agreement with Earth Observing System Microwave Limb Sounder (EOS MLS) correlative measurements. Through this work, the CQ approach is proving to be a very effective tool for developing and testing new models which will be used to provide operational weather forecasts at the Met Office. The results show that the CQ approach is a quick and effective alternative to the NMC method, and can produce similar or sometimes better results. It is a particularly useful tool in the development of new assimilation systems. © Crown Copyright 2008 Reproduced with the permission of Her Majesty's St ationery Office. Published by J ohn Wiley &amp; Sons, Ltd

Short-period planetary waves in the Antarctic middle atmosphere

Planetary waves with periods between two and four days in the middle atmosphere over Antarctica are characterized using one year of data from the medium-frequency spaced antenna (MFSA) radars at Scott Base, Rothera, and Davis. In order to investigate the origin of the observed waves, the ground-based data are complemented by temperature measurements from the Earth Observing System Microwave Limb Sounder (EOS MLS) instrument on the Aura satellite as well as wind velocity data from the United Kingdom Met. Office (UKMO) stratospheric assimilation. Observed characteristics of waves with a period of approximately two days in summer are consistent with the quasi-two-day wave (QTDW) generally found after the summer solstice at low- and mid-latitudes. The Scott Base observations of the QTDW presented here are the highest-latitude ground-based observations of this wave to date. Waves with preferred periods of two and four days occur in bursts throughout the winter with maximum activity in June, July, and August. The mean of the two- and four-day wave amplitudes is relatively constant, suggesting constant wave forcing. When several waves with different periods occur at the same time, they often have similar phase velocities, supporting suggestions that they are quasi-non-dispersive. In 2005, a “warmpool” lasts from late July to late August. An alternative interpretation of this phenomenon is the presence of a structure propagating with the background wind. Consideration of the role of vertical shear (baroclinic instabilities) and horizontal shear (barotropic instabilities) of the zonal wind suggests that instabilities are likely to play a role in the forcing of the two- and four-day waves, which are near-resonant modes and thus supported by the atmosphere.

Low level of stratospheric ozone near the Jharia coal field in India

The Indian reserve of coking coal is mainly located in the Jharia coal field in Jharkhand. Although air pollution due to oxides and dioxides of carbon, nitrogen and sulphur is reported to have increased in this area due to large-scale opencast mining and coal fires, no significant study on the possible impact of coal fires on the stratospheric ozone concentration has been reported so far. The possible impact of coal fires, which have been burning for more than 90 years on the current stratospheric ozone concentration has been investigated using satellite based data obtained from Upper Atmospheric Research Satellite (UARS MLS), Earth Observing System Microwave Limb Sounder (EOS MLS) and Ozone Monitoring Instrument (OMI) in this paper. The stratospheric ozone values for the years 1992–2007, in the 28–36 km altitude range near Jharia and places to its north are found to be consistently lower than those of places lying to its south (up to a radius of 1000 km around Jharia) by 4.0–20%. This low stratospheric ozone level around Jharia is being observed and reported for the first time. However, due to lack of systematic ground-based measurements of tropospheric ozone and vertical ozone profiles at Jharia and other far off places in different directions, it is difficult to conclude strongly on the existence of a relationship between pollution from coal fires and stratospheric ozone depletion.

Assimilation of EOS MLS ozone observations in the Met Office data‐assimilation system

AbstractIn this paper the impact of Earth Observing System Microwave Limb Sounder (EOS MLS) observations on ozone analyses is investigated using the Met Office data‐assimilation system. EOS MLS was launched in 2004, and produces high‐quality ozone observations in the upper troposphere and stratosphere at high vertical and horizontal resolution. The experiments shown here are run using 3D‐Var and a forecast model that has 50 levels—from the surface to approximately 63 km—and a horizontal resolution of 2.5° latitude by 3.75° longitude. Most of the experiments are run for the period January–February 2005.The chief impact of assimilating EOS MLS data is a reduction of the mean analysis error, and of the standard deviation, in the lower stratosphere. Compared with control simulations, mean errors drop by 5%–25% in the Southern‐Hemisphere extratropics, by around 10% in the Northern‐Hemisphere extratropics, and by around 50% (with respect to ozonesondes) in the region of the tropical upper troposphere and lower stratosphere.Further investigation shows that the improved ozone analyses in the Southern Hemisphere largely result from a much better representation of summertime low‐ozone events. These events have been documented in the northern summer stratosphere, but never before in the southern summer stratosphere. In the Northern Hemisphere the addition of EOS MLS to the assimilation system is shown to lead to a better representation of winter polar ozone depletion. At low latitudes, it appears that errors in the background model transport fields often lead to errors in the ozone analysis, but that the addition of high‐density, good‐quality EOS MLS data alleviates much of this error. It is noted that the EOS MLS data have a more beneficial impact on ozone analyses in this region than do data from the Michelson Interferometer for Passive Atmospheric Sounding, which has a similar resolution and observational error to EOS MLS. © Crown Copyright 2007. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley &amp; Sons, Ltd.

Estimating biases and error variances through the comparison of coincident satellite measurements

A framework for the statistical comparison of six coincident remote sounding measurements is presented, which distinguishes between additive and multiplicative biases. The relationship between multiplicative bias and error variance is explored, and three methods are proposed for producing sets of values for three comparison variables: the multiplicative bias, and the error variance for each of two instruments. We illustrate and compare the three methods through the comparison of coincident measurements of the relatively long‐lived stratospheric species O3, N2O, and HNO3 from two independent measurement sets: version 2.2 retrievals (with updated O3) from the Atmospheric Chemistry Experiment‐Fourier transform spectrometer onboard SCISAT‐1, and version 1.51 retrievals from the Earth Observing System Microwave Limb Sounder onboard Aura. We find that multiplicative bias between the two measurement sets, compared on a common vertical grid, is significant at some heights for O3 and N2O, and for all heights tested for HNO3. The most realistic estimates of measurement error are produced by a method which incorporates a third correlative data set into the analysis. Using this method, estimated error standard deviations (SDs) are comparable between the two instruments for O3 measurements, and are less than 10% of the mean measurement value between approximately 100 and 1 hPa. ACE N2O measurements are consistent with a 10% error SD at all heights tested, although the uncertainty of the estimates is large at heights above 5 hPa. Estimated MLS N2O error SDs are comparable with those for ACE in the lower stratosphere, but increase steeply with height. For HNO3, estimated error SDs are approximately 10% between 70 and 10 hPa for both instruments. At heights above 10 hPa and below 100 hPa, estimated ACE errors are significantly smaller than those for MLS.

Retrieval of Tangent Pressures From EOS–MLS Radiances Using a Neural Network for Use in an Assimilation Scheme

Limb sounding instruments provide high vertical resolution data on the temperature and composition of the atmosphere. Their data are, therefore, valuable for assimilating into general circulation models of the atmosphere. Direct assimilation of radiances from limb sounders is more complex in practice than from nadir sounders due to the need to know the tangent pressures of the measurements. This paper discusses the practical implications of tangent pressures in direct radiance assimilation of limb sounding radiances and demonstrates that a neural network can be used to find these tangent pressures for the Earth Observing System Microwave Limb Sounder with a root mean-square error of sigma=50 m, which is comparable with that in traditional retrieval techniques

Quantifying Arctic ozone loss during the 2004–2005 winter using satellite observations and a chemical transport model

During the last decade, much attention has been placed on quantifying and modeling Arctic stratospheric O3 loss. At issue in particular is the reliability of models for simulating the loss under variable dynamical conditions in the Arctic region. This paper describes inferred O3 loss calculations for the 2004–2005 Arctic winter using data from four solar occultation satellite instruments, as well as the Earth Observing System Microwave Limb Sounder (EOS MLS). O3 loss is quantified with the “Chemical Transport Model (CTM) passive subtraction” approach, using a passive O3 tracer field from the SLIMCAT CTM. The 2004–2005 Arctic winter was moderately active dynamically, but was still one of the coldest Arctic winters on record, with prime conditions for O3 loss. Loss estimates inferred from all of the different satellite instruments peaked in mid‐March at 450 K between 2–2.3 ppmv, slightly less than similar estimations for the cold 1999–2000 winter. The SLIMCAT CTM was also used to simulate O3 for the 2004–2005 winter. In March, near 450 K, the model O3 was 0.3 ppmv (∼10–15%) lower than the observations, leading to a maximum O3 loss that was 10–15% larger than that inferred from observations, using the passive subtraction approach. Modeled loss maximized around the same time as that inferred from observations. Although some discrepancies between the observed and modeled O3 remain, the level of agreement presented here shows that the SLIMCAT CTM was able to satisfactorily simulate O3 and polar O3 loss during the dynamically active 2004–2005 Arctic winter.

EOS MLS observations of dehydration in the 2004–2005 polar winters

Observations of water vapour and temperature from the Earth Observing System Microwave Limb Sounder are used to study dehydration in the 2004 and 2005 polar vortices. Significant differences were found for the Antarctic winters, with the 2005 vortex colder and more extensively dehydrated. For the 2005 winter water vapour reductions were observed from mid‐June, coincident with a decrease in temperatures, extending vertically between ∼12–21 km. Reductions of up to ∼3 ppmv in water vapour were recorded a month later. Permanent dehydration was apparent between ∼15–20 km, where 3 months later the temperature recovery was not followed by a recovery in water vapour. The 2004–2005 Arctic winter was unusually cold, but only one single event of depleted water vapour at the end of January was linked to ice formation. For this event, a reduction of up to ∼0.5 ppmv was observed over Spitsbergen between ∼12–20 km.

The Earth observing system microwave limb sounder (EOS MLS) on the aura Satellite

The Earth Observing System Microwave Limb Sounder measures several atmospheric chemical species (OH, HO/sub 2/, H/sub 2/O, O/sub 3/, HCl, ClO, HOCl, BrO, HNO/sub 3/, N/sub 2/O, CO, HCN, CH/sub 3/CN, volcanic SO/sub 2/), cloud ice, temperature, and geopotential height to improve our understanding of stratospheric ozone chemistry, the interaction of composition and climate, and pollution in the upper troposphere. All measurements are made simultaneously and continuously, during both day and night. The instrument uses heterodyne radiometers that observe thermal emission from the atmospheric limb in broad spectral regions centered near 118, 190, 240, and 640 GHz, and 2.5 THz. It was launched July 15, 2004 on the National Aeronautics and Space Administration's Aura satellite and started full-up science operations on August 13, 2004. An atmospheric limb scan and radiometric calibration for all bands are performed routinely every 25 s. Vertical profiles are retrieved every 165 km along the suborbital track, covering 82/spl deg/S to 82/spl deg/N latitudes on each orbit. Instrument performance to date has been excellent; data have been made publicly available; and initial science results have been obtained.

Retrieval algorithms for the EOS Microwave limb sounder (MLS)

The retrieval algorithms for the Earth Observing System Microwave Limb Sounder (MLS) on the Aura spacecraft, launched on July 15, 2004, are described. These algorithms are used to produce estimates of geophysical parameters such as vertical profiles of atmospheric temperature and composition (Level 2 data) from the calibrated MLS observations of microwave limb radiance (Level 1 data). The MLS algorithms are based on the standard optimal estimation approach, a weighted nonlinear least squares optimization with a priori constraints. New aspects include adaptation to a two-dimensional system, and an approach to the issues of retrieval phasing and error propagation that differs from that taken for previous similar instruments. Important new aspects of the software that implements these algorithms are also described, along with the algorithm configuration for the version 1.5 dataset. Some examples are shown from MLS in-orbit observations.

EOS MLS cloud ice measurements and cloudy-sky radiative transfer model

A cloud ice retrieval technique is described here using measurements at frequencies near 118, 190, 240, and 640 GHz and 2.5 THz from the Earth Observing System Microwave Limb Sounder on the NASA Aura satellite. Measurement principles, methods for cloud detection, and radiative transfer models for retrieving cloud properties are discussed. The 240-GHz data from high-tangent heights are used to retrieve ice water content at pressures <215 hPa, and the 118-, 190-, 240-, and 640-GHz radiances from low-tangent heights are used to retrieve ice water paths with different penetration depths. Some early Microwave Limb Sounder (MLS) results are highlighted, and the observed cloud signatures are consistent with the expectation from model simulations, in general. The simultaneous measurements from MLS 240 and 640 GHz radiometers contain useful information on particle sizes. There are significant cloud-induced radiances at 2.5 THz, despite strong attenuation from the atmosphere. Cloud-scattering signatures are polarized at 122 GHz, but the polarization differences are typically less than 10% of the total cloud-induced radiance.