One of the first large cloud systems ever observed on Titan was a stationary event at the southern pole that lasted almost two full Titan days. Its stationary nature and large extent are puzzling ...given that low‐level winds should transport clouds eastward, pointing to a mechanism such as atmospheric waves propagating against the mean flow. We use a composite of 47 large convective events across 15 Titan years of simulations from the Titan Atmospheric Model to show that Rossby waves trigger polar convection—which halts the waves and produces stationary precipitation—and then communicate its impact globally. In the aftermath of the convection, forced waves undergo a complicated evolution, including cross‐equatorial propagation and tropical‐extratropical interaction. The resulting global impact from convection implies its detectability anywhere on Titan, both via surface measurements of pressure and temperature and through remote observation of the outgoing longwave radiation, which increases by ∼0.5% globally.
Plain Language Summary
Saturn's moon Titan hosts a methane hydrologic cycle with occasional large cloud events that are sometimes stationary and last for up to 30 days at a time. These events have previously been speculated to be caused by convective thunderstorms, but for the first time, we show that their formation is reliant on the interaction between a particular type of high‐latitude atmospheric wave, a Rossby wave, and instability caused by increased surface heating during the summer. Convectively forced growth of the Rossby wave accounts for the lack of movement as waves in the summer hemisphere interact with waves that have been forced in the winter hemisphere. The resulting global impact from the forced convection may be detected both from Earth as changes in outgoing longwave radiation and on the surface, which may have relevance for the Dragonfly mission.
Key Points
Development of convection on Titan involves the coordinated interaction of Rossby waves with convective instability
Convection temporarily halts the movement of waves, which may explain stationary cloud features in the middle and high latitudes
Global impacts imply that observables like surface pressure and outgoing longwave radiation may reveal instances of convection
The impact of methane convection on the circulation of Titan is investigated in the Titan Atmospheric Model (TAM), using a simplified Betts–Miller (SBM) moist convection parameterization scheme. We ...vary the reference relative humidity (RHSBM) and relaxation timescale of convection (τ) parameters of the SBM scheme. Titan’s atmosphere is mostly insensitive to changes in τ, but convective instability and precipitation are highly impacted by changes in RHSBM. Convection behavior changes from infrequent (<1 per Titan year), intense events at summer solstice that quickly encompass the entire globe at low RHSBM to near-continuous precipitation at the poles during summer at high RHSBM (85%). The intermediate regime (RHSBM=70%–80%) consists of frequent events (∼10 per Titan year) of moderate intensity that are limited in meridional extent to their respective hemisphere. Using results from the Titan Regional Atmospheric Modeling System (TRAMS) and observations, we tune the parameters of the SBM parameterization with optimum values of RH=80% and τ=28800 s. We present a simulated decadal climatology that qualitatively matches observations of Titan’s humidity and cloud activity and generally resembles previous results with TAM. Comparing this simulation to one without moist convection demonstrates that convection strengthens the meridional circulation, warms the mid-levels and cools the surface at the poles, and magnifies zonal-mean global moisture anomalies.
•Changing parameters in a simplified convective parameterization alters the nature of convection in the Titan Atmospheric Model (TAM).•Using results from a cloud-resolving model and observations, we tune the scheme in TAM.•The resulting decadal climatology broadly matches observations of humidity and cloud activity.•Moist convection warms 1400–600 hPa, cools the surface, and strengthens the ascending branch of the solstitial Hadley cell.
•This paper presents the first wind measurements in an active dune field on Mars.•Daytime upslope/nighttime downslope flows dominate in winter on the slopes of Aeolis Mons.•The mid-morning and ...evening wind rotates rapidly and generally clockwise between these directions.•Wind blocking and wind speed distribution broadening occurs in the lee of a dune.•Mesoscale modeling captures the general pattern of wind speed and direction.
A high density of REMS wind measurements were collected in three science investigations during MSL's Bagnold Dunes Campaign, which took place over ∼80 sols around southern winter solstice (Ls∼90°) and constituted the first in situ analysis of the environmental conditions, morphology, structure, and composition of an active dune field on Mars. The Wind Characterization Investigation was designed to fully characterize the near-surface wind field just outside the dunes and confirmed the primarily upslope/downslope flow expected from theory and modeling of the circulation on the slopes of Aeolis Mons in this season. The basic pattern of winds is ‘upslope’ (from the northwest, heading up Aeolis Mons) during the daytime (∼09:00–17:00 or 18:00) and ‘downslope’ (from the southeast, heading down Aeolis Mons) at night (∼20:00 to some time before 08:00). Between these times the wind rotates largely clockwise, giving generally westerly winds mid-morning and easterly winds in the early evening. The timings of these direction changes are relatively consistent from sol to sol; however, the wind direction and speed at any given time shows considerable intersol variability. This pattern and timing is similar to predictions from the MarsWRF numerical model, run at a resolution of ∼490m in this region, although the model predicts the upslope winds to have a stronger component from the E than the W, misses a wind speed peak at ∼09:00, and under-predicts the strength of daytime wind speeds by ∼2–4m/s. The Namib Dune Lee Investigation reveals ‘blocking’ of northerly winds by the dune, leaving primarily a westerly component to the daytime winds, and also shows a broadening of the 1Hz wind speed distribution likely associated with lee turbulence. The Namib Dune Side Investigation measured primarily daytime winds at the side of the same dune, in support of aeolian change detection experiments designed to put limits on the saltation threshold, and also appears to show the influence of the dune body on the local flow, though less clearly than in the lee. Using a vertical grid with lower resolution near the surface reduces the relative strength of nighttime winds predicted by MarsWRF and produces a peak in wind speed at ∼09:00, improving the match to the observed diurnal variation of wind speed, albeit with an offset in magnitude. The annual wind field predicted using this grid also provides a far better match to observations of aeolian dune morphology and motion in the Bagnold Dunes. However, the lower overall wind speeds than observed and disagreement with the observed wind direction at ∼09:00 suggest that the problem has not been solved and that alternative boundary layer mixing schemes should be explored which may result in more mixing of momentum down to the near-surface from higher layers. These results demonstrate a strong need for in situ wind data to constrain the setup and assumptions used in numerical models, so that they may be used with more confidence to predict the circulation at other times and locations on Mars.
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Abstract Changes in the vertical and meridional temperature gradients of the atmosphere drive competing influences on storm-track activity. We apply local eddy energetics to the ERA5, JRA-55, ...MERRA-2, and NCEP-2 reanalyses during 1980–2020 to determine the locations, magnitudes, and trends of the energy transfer mechanisms for synoptic-scale eddies. Eddy kinetic energy (EKE) increases more rapidly in the Southern Hemisphere at all altitudes and seasons, with larger increases during austral winter and spring. In the Northern Hemisphere, increases occur within the Atlantic and Pacific storm tracks at pressures below 300 hPa but only during boreal winter and spring and confined within a narrow zonal band; EKE decreases during boreal summer and fall. Most EKE changes correspond with trends in baroclinic energy conversion upstream of storm tracks and appear to align with increases in the growth rate of the most unstable baroclinic mode. Barotropic energy conversion of EKE to the mean flow becomes locally more intense downstream of the storm tracks. Conversion of EKE to long-period eddies plays a minor role averaged over a hemisphere but can be important locally. The primary strengthening pathway for removal of EKE is a combination of surface friction and viscous dissipation. The increased baroclinic conversion in the Southern Hemisphere appears related to upper-level tropical temperature increases. In the Northern Hemisphere, increased baroclinic conversion is enabled by a combination of increased vertical heat fluxes and a region of temperature increases within 30°–60°N. Significance Statement Traveling atmospheric disturbances arrange into storm tracks that determine the weather in the midlatitudes. Storm tracks are evolving in time due to anthropogenic warming; however, the location and strength of temperature changes compete for influence on the storm tracks. A framework to quantify the mechanisms of generation of kinetic energy contained by eddies pinpoints the extent of storm-track evolution. Storm tracks generally strengthen across the planet but have increased the most in the Southern Hemisphere. Strengthening in the Northern Hemisphere is limited to the winter in a narrow latitudinal band, because of warming in the Arctic that reduces the primary instability that drives eddies. The locations of northern warming and storm-track strengthening suggest a role for tropical dynamics.
Atmospheric rivers (ARs) are filamentary conduits of intense moisture transport crucial for water delivery to mid‐latitude coastal regions. How ARs have responded to extratropical climate variability ...remains poorly understood despite ARs being features of the extratropical atmosphere. Here, using “Last Millennium” simulations, we characterize the role of annular modes of extratropical variability on ARs and moisture transport. We find that positive (negative) phases of the annular modes intensify (weaken) and weaken (intensify) ARs over the subpolar and subtropical latitudes, respectively, with up to ∼20–25 mm/month associated changes in precipitation. Importantly, the annular modes comprise the primary mode of AR variability over the last ∼1,000 years. We also separately examine the annular modes' influence on storm track activity and find it distinct from that on ARs and moisture transport, despite the storm tracks being associated with ARs and overlapping with strong moisture transport. Lastly, our results provide a robust paleoclimate baseline from which to contextualize projected 21st century AR intensification.
Plain Language Summary
Atmospheric rivers (ARs) are filamentary structures of intense water vapor transport commonly found in the extratropical atmosphere. ARs are a crucial component of the Earth's general circulation: ARs account for up to 90% of poleward moisture transport and are a reliable (and sometimes extreme) source of precipitation for midlatitude coastal regions around the world. Although ARs operate primarily in the extratropics, how ARs respond to extratropical climate variability remains poorly understood. In this study, we use millennium‐long climate model simulations to examine how annular modes of climate variability—the dominant mode of climate variability in the extratropics in both the Northern and Southern Hemispheres—affect ARs. We find that phases of the annular modes induce strong north–south displacements in AR activity, with up to ∼20–25 mm/month associated changes in precipitation. These AR changes are also found in the observational record, indicating that our model results are representative of real‐world influences. Our results provide a robust baseline of natural AR variability from which to contextualize projected 21st century AR intensification.
Key Points
Mean north–south displacements in zonal winds induced by annular modes are the primary driver of atmospheric river (AR) variability over the last millennium
The Northern Annular Mode exerts global influences on ARs, but the Southern Annular Mode's influence is mostly confined to the Southern Hemisphere
AR intensification from severe global warming rivals in magnitude the most prominent natural changes to AR activity
Physical activity (PA) guidelines recommend that PA be accumulated in bouts of 10 minutes or more in duration. Recently, researchers have sought to better understand how participants in PA ...interventions increase their activity. Participants can increase their daily PA by increasing the number of PA bouts per day while keeping the duration of the bouts constant; they can keep the number of bouts constant but increase the duration of each bout; or participants can increase both the number of bouts and their duration. We propose a novel joint modeling framework for modeling PA bouts and their duration over time. Our joint model is comprised of two sub-models: a mixed-effects Poisson hurdle sub-model for the number of bouts per day and a mixed-effects location scale gamma regression sub-model to characterize the duration of the bouts and their variance. The model allows us to estimate how daily PA bouts and their duration vary together over the course of an intervention and by treatment condition and is specifically designed to capture the unique distributional features of bouted PA as measured by accelerometer: frequent measurements, zero-inflated bouts, and skewed bout durations. We apply our methods to the Make Better Choices study, a longitudinal lifestyle intervention trial to increase PA. We perform a simulation study to evaluate how well our model is able to estimate relationships between outcomes.
Using Perseverance (Mars 2020) and Mars Science Laboratory (MSL) measurements, we obtain planetary waves with a period of 1.5–30 sols in the 1.5 m temperature, winds (Mars 2020), surface pressure, ...and relative humidity. Planetary waves emerge and propagate latitudinally across all variables. Short‐period waves peak at periods of ∼2, 3, and 4.5 sols, confirming previous detections of waves, and wave amplitudes agree at Mars 2020 and MSL for all variables. The simultaneous detection of waves within Gale and Jezero craters in multiple variables indicates that planetary‐scale dynamics influences many facets of local weather throughout the year. Multiple waves are correlated or anti‐correlated between MSL and Mars 2020, suggesting waves originate in both hemispheres. Further, waves at one site do not always lead the other, suggesting a combination of baroclinic and barotropic processes as wave sources, determined from the phasing of the temperature and winds in Jezero compared to the Ensemble Mars Atmospheric Reanalysis System.
Plain Language Summary
Atmospheric waves on Mars can initiate large dust storms, so understanding their properties is critical for the safety of missions. The surface pressure, temperature, relative humidity, and winds of Gale and Jezero craters are analyzed by Mars Science Laboratory (MSL) and Mars 2020 to quantify the signal of planetary waves over the first few months of the Mars 2020 mission. Mars 2020 and MSL detect waves in all variables with frequencies similar to observations from orbit and elsewhere on the surface. Comparing waves from Mars 2020, MSL, and orbit will provide needed context for these waves to enable predictions of dust storms.
Key Points
Both rovers observe 1.5–30 sol waves in multiple variables during northern spring with amplitudes agreeing with a reanalysis
Mars Science Laboratory (MSL) and Mars 2020 observations alternatively correlate and anti‐correlate, pointing to planetary waves originating in different hemispheres
North‐south propagating waves vacillate between first occurring at MSL or Mars 2020, suggesting waves grow from multiple types of instabilities
Martian gravity waves (GW) greatly impact the atmospheric circulation and formation of clouds, but many GW observations of the lower atmosphere are confined to specific orientations and wavelengths, ...leaving many gaps in the continuum of waves. To overcome the issue, we analyze eight Mars years of data during the season of Ls = 120°–150° from Band 10 (14.9 µm) of the Thermal Emission Imaging System (THEMIS), sensitive to GWs at ∼25 km altitude. All horizontal orientations at wavelengths below ∼40 km are detectable, with the potential to detect north‐south oriented GWs up to ∼1,000 km in length. Most THEMIS observations have brightness temperature variances compatible with GW disturbances. Intense GW activity concentrates poleward of 60°S, with normalized magnitudes up to 10−4 K2 K−2; activity decreases toward the equator and remains low throughout the northern hemisphere. The interannual intensity of GWs varies by latitude within three regimes of GW length: short (<20 km), medium (20–100 km), and long (>100 km). Gravity wave orientations that are detectable in all directions do not favor a single direction; that is, Martian GWs at 25 km altitude are isotropic. Finally, interannual variability seems to emerge from interactions with dust storms, orography, and planetary waves.
Plain Language Summary
Small‐scale disturbances in the atmosphere—called gravity waves (GW)—transport momentum and energy throughout the atmosphere, so understanding their strength and direction is important for understanding the circulation of Mars's atmosphere. We calculate wave occurrence within a data set previously unused in the documentation of GWs. The Thermal Emission Imaging System allows for calculation of waves in all horizontal directions 25 km above the surface. We find that GWs are strongest during the middle of northern hemisphere summer in the southern hemisphere and do not propagate in any preferred direction. Three regimes of waves that vary in latitude and/or year are found with short, medium, and long wavelengths. Finally, waves can be formed by winds blowing over and around mountains and craters, larger types of waves, or dust events.
Key Points
Thermal Emission Imaging System Band‐10 detects gravity waves (GW) at 25 km altitude that are stronger in the southern hemisphere and weaker in the northern hemisphere
GW orientations are generally isotropic, with no preferred average orientation, but orientations are modulated by topography
Variability in Martian GWs appears in three wave length regimes: short (<20 km), medium (20–100 km), and long (>100 km)
Gravity waves are one way Mars’s lower atmospheric weather can affect the circulation and even composition of Mars’s middle and upper atmosphere. A recent study showed how on-planet observations near ...the center of the 15 micron CO2band by the A3 channel(635–665 cm−1)of the Mars Climate Sounder on board Mars Reconnaissance Orbiter (MRO-MCS) could sense horizontally short, vertically broad gravity waves at≈25 km above the surface by looking at small-scale radiance variability in temperature-sensitive channels. This approach is extended here to two additional channels closer to the wings of the 15 micron CO2band,A1 (595–615 cm−1) and A2 (615–645 cm−1), to sense gravity waves throughout the lower atmosphere. Using information from all three channels demonstrates that gravity wave activity in Mars’s lowermost atmosphere is dominated by orographic sources, particularly over the extremely rough terrain of Valles Marineris. Much of this orographic population is either trapped or filtered in the lowest two scale heights, such that variations in filtering and non-orographic sources shape the gravity wave population observed at 25 km above the surface. During global dust storms, however, gravity wave activity in the first scale height decreases by approximately a factor of two, yet trapping/filtering of what activity remains in the tropics substantially weakens. Exceptionally high radiance variability at night in the tropics during the less dusty part of the year is the result of observing mesospheric clouds rather than gravity waves.
The deep convective cloud–environment feedback loop is likely important to Titan's global methane, energy, and momentum cycles, just as it is for Earth's global water, energy, and momentum budgets. ...General circulation models of Titan's atmosphere are unable to explicitly simulate deep convection and must instead parameterize the impact of this important subgrid-scale phenomenon on the model-resolved atmospheric state. The goal of this study is to better quantify through cloud resolving modeling the effects of deep convective methane storms on their environment and to feed that information forward to improve parameterizations in global models. Dozens of atmospheric profiles unstable with respect to deep moist convection are extracted from the global Titan Atmospheric Model (TAM) and used to initialize the cloud-resolving Titan Regional Atmospheric Modeling System (TRAMS). Mean profiles of heating/cooling and moistening/drying of the large-scale environment in TRAMS indicate that Titan's deep convection forces the environment in a manner analogous to Earth: Large-scale subsidence of the environmental air warms and dries the environment, but clouds can also moisten the environment through the detrainment and evaporation of condensate near cloud top. Relative humidity profiles and characteristic convective time scales are derived to guide the tuning of the deep convective parameterization implemented in TAM, as described in a companion paper. The triggering of convection, the dry convective mixing of the planetary boundary layer, and the entrainment of environmental air into rising air parcels are found to be critical to determining whether a deep convective cloud will form. Only profiles with relatively large convective available potential energy (CAPE) and well mixed planetary boundary layers with high relative humidity were found to produce storms. Environments with low-level thermal inversions and planetary boundary layers with low relative humidity or rapidly decreasing moisture with height failed to generate deep convection in TRAMS despite positive CAPE.
•Cloud resolving simulations are initialized with soundings from a global atmospheric model of Titan.•The impact of deep convection on the large-scale environment is diagnosed.•Titan's large-scale environmental response to deep convection is analogous to Earth.•Tunable parameters for a deep convective parameterization in global models are derived.
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