Storm Decay Research Articles

Modeling the Martian dust cycle 2. Multiannual radiatively active dust transport simulations

Multiannual dust transport simulations have been performed using a Mars general circulation model containing a dust transport scheme which responds to changes in the atmospheric state. If the dust transport is “radiatively active,” the atmospheric state also responds to changes in the dust distribution. This paper examines the suspended dust distribution obtained using different lifting parameterizations, including an analysis of dust storms produced spontaneously during these simulations. The lifting mechanisms selected are lifting by (1) near‐surface wind stress and (2) convective vortices known as dust devils. Each mechanism is separated into two types of parameterization: threshold‐sensitive and ‐insensitive. The latter produce largely unrealistic annual dust cycles and storms, and no significant interannual variability. The threshold‐sensitive parameterizations produce more realistic annual and interannual behavior, as well as storms with similarities to observed events, thus providing insight into how real Martian dust storms may develop. Simulations for which dust devil lifting dominates are too dusty during northern summer. This suggests either that a removal mechanism (such as dust scavenging by water ice) reduces opacities at this time or that dust devils are not the primary mechanism for storm production. Simulations for which near‐surface wind stress lifting dominates produce the observed low opacities during northern spring/summer, yet appear unable to produce realistic global storms without storm decay being prevented by the occurrence of large‐scale positive feedbacks on further lifting. Simulated dust levels are generally linked closely to the seasonal state of the atmosphere, and no simulation produces the observed amount of interannual variability.

Linear transition ripple migration and wave orbital velocity skewness: Observations

Field observations were made in 3–4 m water depth of linear transition ripple geometry and migration using a high‐resolution laser‐video bed profiling system and acoustic scanning sensors during both the growth and decay phases of an autumn storm event. Linear transition ripples are long‐crested, low‐steepness bedforms of the anorbital ripple type and were observed to occur here at relatively high wave energies just below the flatbed threshold, with wavelengths of 8.5±0.5 cm and heights of 0.3±0.1 cm. The maximum observed migration rate was 0.7 cm/min. Migration was offshore during storm growth and onshore during storm decay. The observed ripple migration velocities were highly correlated (r2 > 0.7) with nearbed wave orbital velocity skewness in both cross‐shore directions. During storm growth the incident wave spectrum was bimodal and the orbital velocity skewness was negative. During storm decay the wave spectrum was unimodal and the velocity skewness was positive. Bispectral analysis shows that the main contribution to negative velocity skewness during storm growth was due to a difference interaction between the two principal (sea and swell) components of the bimodal velocity spectrum. Positive skewness during storm decay was due to self‐self interaction of the narrowband residual swell. The negative velocity skewness observed during storm growth is consistent with prediction by a two‐frequency second‐order wave theory.

Lightning evolution related to radar-derived microphysics in the 21 July 1998 EULINOX supercell storm

Results of a combined analysis of data from a C-band polarimetric Doppler radar and a 3D VHF interferometric lightning mapping system, as obtained during the European Lightning Nitrogen Oxides project (EULINOX) field campaign, are presented. For 21 July 1998, the lightning data from a supercell thunderstorm weakly indicate a tendency for a bi-level vertical distribution of lightning VHF emissions around the −15°C and −30°C temperature levels. Also, in some parts of the clouds, evidence is found for the presence of a lower positive charge center near the freezing level. However, where strong vertical motions prevail, VHF emissions are not organized in horizontal layers but in oblique or vertical regions. Correlation of VHF signals with radar quantities shows that in the growing storm, peak VHF activity is low and related to reflectivity factors around 30 dBZ, while after the mature stage, the peak VHF activity is about three times larger. The highest density of VHF signals is now found near reflectivity factors of 45 dBZ. A polarimetric hydrometeor classification indicates that during storm development, most lightning activity occurs where graupel and, secondarily, snow and small dry hail are present. In the decaying phase of the supercell hailstorm, however, most lightning VHF emissions stem from the region with hail and heavy rain. Furthermore, while the VHF signal frequency per cubic kilometer in the graupel and rain regions remains nearly constant throughout the supercell life cycle, the signal frequency in the hail region rises during storm decay.

Global precipitation statistics from dual‐frequency TOPEX altimetry

TOPEX is a nadir‐pointing dual‐frequency radar altimeter that has been in orbit for more than 6 years. Empirical methods, based on the close correlation between the C band and Ku band backscatter, yield an estimate of the rain rate. The principal rain bands of the Intertropical Convergence Zone (ITCZ) are shown to be bounded by “marine deserts” receiving only one hundredth of the rainfall of the ITCZ. Analysis of the entire data set reveals variations in the frequency of oceanic precipitation over a range of timescales. For most regions the diurnal cycle of precipitation has a minimum likelihood at evening local time, with maximum being early morning for the tropics and northern oceans, but late afternoon for the Southern Ocean. On the seasonal cycle, the Pacific ITCZ reaches its southernmost limit during March‐April, and is then nearly always accompanied by a secondary but weaker band an equal distance to the south of the equator. Finally, on the interannual timescale the 1997–1998 El Niño is seen to cause a broadening of the Pacific ITCZ and its translation to the south and east, with increased global levels of rainfall rather than just a geographical redistribution. High‐resolution sampling shows that the length scale of rain events varies not only latitudinally, but also longitudinally according to whether they lie in regions of genesis or decay of storms. Simultaneous wind and wave information from TOPEX indicates that the rainfall in midlatitudes occurs preferentially with high winds, but in the tropics is associated with swell. The long‐term near‐global single‐instrument data set from TOPEX is thus complementary to those from other sensors.

Mapping Mariner 9 Dust Opacities

We present the results from a study of the spatial and temporal 9-μm opacity variation of the Martian 1971 global dust storm. T. Z. Martin (1986,Icarus66, 2–21) designed a procedure to calculate 9-μm opacities from the Viking IR Thermal Mapper data set. We have convolved the Mariner 9 IRIS data with Viking IRTM filter functions in order to use Martin's approach. We find a mean opacity of 0.32 and a modal opacity of 0.21. The resulting opacities are mapped and binned by solar longitude in order to investigate the structure of the 1971 dust storm. There is a diminishing spatial variation in opacity as the storm decays. The highest opacities occur in the southern tropics while the lowest opacities consistently appear in the southern polar region and near the equator. Hellas is relatively clear at the peak of the storm but becomes dustier than the rest of the planet as the storm dissipates. This behavior may be typical of Hellas and Argyre during global dust storms. In contrast, the Tharsis plateau is relatively dusty early in the mission but clears quickly. We have also calculated storm decay constants for different regions. The storm has a decay constant of 67 days in the 20°–30° S zone, 42 days in the 55°–65° S zone, and 46 days on the Tharsis plateau.

Storm-generated, hummocky stratification on the outer-Scotian Shelf

Hummocky megaripples occur on Sable Island Bank, Scotian Shelf. Submersible observations show that the megaripples form during winter storms and are subsequently obliterated through bioturbation and fair-weather reworking. The megaripples of this study were underlain by a storm bed composed of: (A) a basal scoured and infilled gravel lag facies; (B) low-angle tangential crossbedding in gravel to coarse sand; (C) anisotropic hummocky stratification in medium sand; and (D) wave ripple cross-lamination in medium/fine sand. This sequence forms a tempestite bed created by a winter storm during our sampling program. Numerical simulation of bed conditions during the storm suggests that the hummocky megaripples and hummocky stratification formed together during late stages of storm decay from conditions of sheet flow. Near-bed wave motion during deposition exceeded steady currents by an order of magnitude.

Equilibrium slopes and cross-shore velocity asymmetries in a storm-dominated, barred nearshore system

The near-bed (0.10 m elevation), horizontal velocities were measured across a two-barred, lacustrine shoreface during a storm and reveal a spatially coherent but temporally variable pattern in the low-order moments of the velocity field. Large cross-shore velocity asymmetries indicated: 1. (1) Spatially extensive and temporally persistent offshore mean flows (up to 0.18 m s −1 across the landward slope of the linear outer bar) reflecting an undertow driven by a wave-induced setup. 2. (2) An onshore directed skewness opposing the mean flow, of wind wave origin and persistent under propagating bores in the inner bar system long after this asymmetry had disappeared from the outer bar. Extensive sediment reactivation with near-zero net sediment flux over the whole of the upper shoreface indicated a sediment prism in a steady state (volumetrically) over the storm cycle. Uniform depths of sediment reactivation with zero bed elevation change over the storm cycle revealed local slopes in a steady state in the outer linear bar. However, time integration of the near-bed velocity field at this location indicated an offshore net flux of water, while sedimentary structures indicated bedform migration (and thus at least bedload) almost uniformly onshore when ripples and megaripples were present. The sediment transport giving rise to the local equilibrium cannot be explained therefore on the basis of the mean properties of the near-bed velocity vector, but requires knowledge of the time-varying, instantaneous sediment transport vector. Onshore migration of the inner bar correlated well with the large and persistent onshore skewness of the velocity distribution in the inner surf zone associated with translatory surf bores throughout the storm decay period. Low-frequency oscillations were only significant close to the shoreline, but may have influenced the development of a sinuous three-dimensional inner bar form as it migrated onshore.

Numerical simulations of the decay of Martian global dust storms

The decay of Martian global (great) dust storms is investigated. One‐dimensional (vertical, static atmosphere) and two‐dimensional (latitude‐height, steady state circulation) simulations carried out with an aerosol transport‐microphysical model indicate that atmospheric motions play a significant role in the observed decay of global dust storms. Spacecraft observations (Mariner 9, Viking) of the 1971 and the two 1977 planet‐encircling dust storms have provided suggestions about some characterisrics of storm decay. Specifically, the dust particle size distribution is inferred to have remained essentially unchanged for particles with radii between 1 and 10 μm during decay of the 1971 storm, and surface visible opacity declined quasi‐exponentially with time in northern mid‐latitudes during the decay of the two 1977 storms. The results from this investigation indicate that two‐ and three‐dimensional dynamical processes play a significant role in the observed decay features of Martian global dust storms. The most important processes are the lofting of dust by vertical motions in the dust source region of the southern hemisphere subtropics and a continuing advective resupply of atmospheric dust into the dust sink regions of the northern hemisphere. Our work has implications for Viking data analyses and future Mars observer observations and requires that the particle size distribution be treated as a time and latitude dependent quantity.

A Numerical Study of the Effect of a Mountain Range on a Landfalling Tropical Cyclone

A triply-nested, movable mesh model was used to study the effects of a mountain range on a landfalling tropical cyclone embedded in an easterly flow of ∼10 m s−1. The integration domain consisted of a 37° wide and 45° long channel, with an innermost mesh resolution of 1/6°. An idealized mountain range with maximum height of ∼958 meters was placed parallel to the shoreline. The mountain range, which spanned 19° in the north–south direction and 5° in the east–west direction, was centered in the middle of the channel. Results obtained were compared with a previous landfall simulation, performed without the effect of the mountain range included. In particular, comparison was made of the total storm rainfall, maximum wind distribution and storm decay rate. It was found that the storm filled much more rapidly in the simulation run with the mountain included. The mountain range affected the decay rate through reduction in the supply of latent and kinetic energy into the storm circulation during, as well as after, passage of the storm over the mountain. It was found that a low-level, warm and dry region was produced where the storm winds descended the mountain slope. In order to better isolate the effect of the mountain on the basic easterly flow, a supplemental integration was performed for the flow without the storm. It revealed that the mountain range caused a significant change in the basic flow over the mountain as well as up to several hundred kilometers downstream and extending considerably above the mountain top. A low-level southerly jet was observed to the west of the mountain base.

Lagrangian sediment motion in a crescentic nearshore bar under storm-induced waves and currents

A fluorescent tracer experiment, in conjunction with morphological and sedimentological data, demonstrates the Lagrangian sediment flux induced by storm waves and currents in a permanently submerged, outer crescentic nearshore bar system. The steady state bar form (height = 2.6 m, wavelength = 390 m) is maintained in the presence of landward sediment advection under asymmetric oscillatory flow during storm buildup and decay and seaward advection under rip-cell flows at the peak of the storm. The seaward displacement of the bar crest in the areas of the convex seaward crescent reflects transport associated with the rip current, which, though variable in its location through time, is never located over the landward projecting horns. The storm studied had a recurrence interval of approximately 1 month and reworked upwards of 16% of the bar sediments. The sediment flux indentified is therefore associated with an event of frequent occurrence and is most likely the control on both bar form and dynamics of the bar system.

Prediction of development of geomagnetic storms using the solar wind-magnetosphere energy coupling function ϵ

It is shown that by monitoring time variations of the solar wind-magnetosphere energy coupling function ϵ( t) upstream of the solar wind, one should be able to predict fairly accurately the growth and decay of individual magnetospheric substorms and storms.

Properties and effects of dust particles suspended in the Martian atmosphere

Direct measurements of the optical depth above the two Viking landers are reported for a period covering the summer, fall, and winter seasons in the northern hemisphere, a time period during which two global dust storms occurred. The optical depth had a value of about 1 just before the onset of each storm; it increased very rapidly, on a time scale of a few days, to peak values of about 3 and 6 with the arrival of the first and second storms, respectively; and it steadily decreased shortly thereafter (≳ few days to few weeks) for both storms, with the decay occurring more rapidly during the initial period of decay. We have also carried out further analyses of observations of the sky brightness made with the lander cameras during the summer season to obtain improved estimates of other dust particle parameters, including the cross section weighted mean particle radius, several shape factors, and the imaginary indices of refraction. These results have been used to define the radiative properties of the suspended dust particles at solar wavelengths. Particle properties inferred by Toon et al. (1977) from their study of the Mariner 9 Iris data were employed to define the radiative properties of the dust at thermal wavelengths. In an effort to understand the effect of dust content on atmospheric temperatures and winds, we have incorporated the derived radiative properties of the dust into a 1 D radiative convective model. When only radiation and thermal convection are taken into account, these calculations fail to reproduce the temperature structure determined during the descent of the landers to the surface. However, satisfactory agreement is achieved when allowance is made for the effects of vertical motions induced by large scale atmospheric dynamics. The sign and magnitude of the required vertical velocities are crudely consistent with those found by Pollack et al. (1976a) in their general circulation calculations. The diurnal temperature variations predicted by the 1 D calculations for the observed optical depths are also in crude agreement with values inferred from orbiter and lander measurements. The 1 D model predicts that the diurnal temperature change and daily mean temperature, averaged over the entire atmospheric vertical column, steadily increase as the optical depth of the dust increases to a value of several, and then subsequently change little. Thus, for small and moderate values of optical depth, there is a positive feedback between atmospheric dust content and both tidal and seasonal winds, but little feedback occurs for very large values of optical depth. A negative feedback exists between dust content and thermally driven topographic winds. The occurrence of these feedback effects are supported by several Viking observations. It is suggested that they play a role in the generation of certain types of local dust storms, in the development of local dust storms into global ones, and in the decay of global storms. At present, dust is being preferentially deposited in the north polar region due to scavanging of dust by CO2 ice precipitation. Using the results of this paper and those of Pollack et al. (1977), we obtain an estimate of the sedimentation rate of dust and water ice in the polar regions. These results imply a time scale of about 105 years to form individual laminae in the polar laminated terrain, a result consistent with astronomical theories of their origin. Analogous calculations imply that equatorial and mid‐latitude regions are being eroded at a rate of about 7 m per million years.

The Role of Surface Divergence and Vorticity in the Life Cycle of Convective Rainfall. Part I: Observation and Analysis

Abstract The role of the surface velocity fields in the formation, maintenance and decay of convective storms is examined using approximately 90 days of measurements in a densely instrumented network (660 km2) in south Florida. The results show statistically strong cause and effect relationships between surface convergence and onset of rain, storm intensity and duration. Short-term prediction of the onset of rain and the amount of rain produced proves possible. The surface fields of divergence provide an estimate of storm mass and moisture transports. The size of the surface area of convergence, by governing the supply of moisture, plays a controlling role in storm intensity. Large storms are efficient (72%), in terms of moisture supplied to rain produced, compared to smaller storms (37%). Within the confines of the experiment network, weak storms are in near mass balance, while inflow greatly exceeds outflow in the intense storm. The near mass balance of the weak storm suggests cloud-to-subcloud layer in...

The geomagnetic storm of April 17-18, 1965

Growth and decay of geomagnetic storms of April 17-18, 1965 studied using data from 88 ground stations, Explorer XXVI and Vela satellites