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D. R. Jackson, J. Austin, and N. Butchart

Abstract

In this paper results are presented from an improved version of the troposphere–stratosphere configuration of the Met Office Unified Model (UM). The new version incorporates a number of changes, including new radiation and orographic gravity wave parameterization schemes, an interannually varying sea surface temperature and sea ice climatology, and the inclusion of convective momentum transport. The UM climatology is compared with assimilated data and with results from a previous version of the UM. It is shown that the model cold biases in the January winter stratosphere and the January and July summer stratosphere are reduced, chiefly because the new radiation scheme is more accurate. The separation between subtropical and polar night jets in July is also better simulated. In addition, in the current version stratospheric planetary wave amplitudes in southern winter are less than half those in northern winter, which is in much better agreement with observations than the previous model version. Despite these improvements, the model still has a cold bias in the winter polar stratosphere, which suggests that the model representation of gravity wave drag is inadequate. Sensitivity tests were carried out and showed that the improved simulation of the separation of subtropical and polar night jets in July is due both to the different sea ice climatology and to the inclusion of convective momentum transport. The improved simulation of stationary wave amplitudes in July cannot be attributed to an individual model change, although it seems to be related to changed wave propagation and dissipation within the stratosphere rather than changes in the tropospheric forcing.

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D. R. Jackson, J. Methven, and V. D. Pope

Abstract

Recent literature has described a “transition zone” between the average top of deep convection in the Tropics and the stratosphere. Here transport across this zone is investigated using an offline trajectory model. Particles were advected by the resolved winds from the European Centre for Medium-Range Weather Forecasts reanalyses. For each boreal winter clusters of particles were released in the upper troposphere over the four main regions of tropical deep convection (Indonesia, central Pacific, South America, and Africa). Most particles remain in the troposphere, descending on average for every cluster. The horizontal components of 5-day trajectories are strongly influenced by the El Niño–Southern Oscillation (ENSO), but the Lagrangian average descent does not have a clear ENSO signature.

Tropopause crossing locations are first identified by recording events when trajectories from the same release regions cross the World Meteorological Organization lapse rate tropopause. Most crossing events occur 5–15 days after release, and 30-day trajectories are sufficiently long to estimate crossing number densities. In a further two experiments slight excursions across the lapse rate tropopause are differentiated from the drift deeper into the stratosphere by defining the “tropopause zone” as a layer bounded by the average potential temperature of the lapse rate tropopause and the profile temperature minimum. Transport upward across this zone is studied using forward trajectories released from the lower bound and back trajectories arriving at the upper bound. Histograms of particle potential temperature (θ) show marked differences between the transition zone, where there is a slow spread in θ values about a peak that shifts slowly upward, and the troposphere below 350 K. There forward trajectories experience slow radiative cooling interspersed with bursts of convective heating resulting in a well-mixed distribution. In contrast θ histograms for back trajectories arriving in the stratosphere have two distinct peaks just above 300 and 350 K, indicating the sharp change from rapid convective heating in the well-mixed troposphere to slow ascent in the transition zone. Although trajectories slowly cross the tropopause zone throughout the Tropics, all three experiments show that most trajectories reaching the stratosphere from the lower troposphere within 30 days do so over the west Pacific warm pool. This preferred location moves about 30°–50° farther east in an El Niño year (1982/83) and about 30° farther west in a La Niña year (1988/89). These results could have important implications for upper-troposphere–lower-stratosphere pollution and chemistry studies.

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Edward C. D. Pope, David B. Stephenson, and David R. Jackson

Abstract

Categorical probabilistic prediction is widely used for terrestrial and space weather forecasting as well as for other environmental forecasts. One example is a warning system for geomagnetic disturbances caused by space weather, which are often classified on a 10-level scale. The simplest approach assumes that the transition probabilities are stationary in time—the homogeneous Markov chain (HMC). We extend this approach by developing a flexible nonhomogeneous Markov chain (NHMC) model using Bayesian nonparametric estimation to describe the time-varying transition probabilities. The transition probabilities are updated using a modified Bayes’s rule that gradually forgets transitions in the distant past, with a tunable memory parameter. The approaches were tested by making daily geomagnetic state forecasts at lead times of 1–4 days and were verified over the period 2000–19 using the rank probability score (RPS). Both HMC and NHMC models were found to be skillful at all lead times when compared with climatological forecasts. The NHMC forecasts with an optimal memory parameter of ~100 days were found to be substantially more skillful than the HMC forecasts, with an RPS skill for the NHMC of 10.5% and 5.6% for lead times of 1 and 4 days ahead, respectively. The NHMC is thus a viable alternative approach for forecasting geomagnetic disturbances and could provide a new benchmark for producing operational forecasts. The approach is generic and is applicable to other forecasts that include discrete weather regimes or hydrological conditions (e.g., wet and dry days).

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Zachary D. Tessler, Arnold L. Gordon, and Christopher R. Jackson

Abstract

Observations of early stage, large-amplitude, nonlinear internal waves in the Sulu Sea are presented. Water column displacement and velocity profile time series show the passage of two solitary-like waves close to their generation site. Additional observations of the same waves are made as they propagate through the Sulu Sea basin. These waves of depression have an estimated maximum amplitude of 44 m. Observed wave amplitude and background stratification are used to estimate parameters for both a Korteweg–de Vries (K-dV) and a Joseph wave solution. These analytic model solutions are compared with a fully nonlinear model as well. Model wave half-widths bracket the observed wave, with the Joseph model narrower than the K-dV model. The modal structure of the waves change as they transit northward though the Sulu Sea, with higher mode features present in the southern Sulu Sea, which dissipate by the time the waves reach the north. Observed and modeled energies are roughly comparable, with observed potential energy estimated at 6.5 × 107 J m−1, whereas observed kinetic energy is between 4.6 × 107 J m−1 and 1.5 × 108 J m−1, depending on the integration limits. If this energy remains in the Sulu Sea, an average dissipation rate of 10−9 W kg−1 is required over its volume, helping to maintain elevated mixing rates.

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V. D. Pope, J. A. Pamment, D. R. Jackson, and A. Slingo

Abstract

Simulations of the Hadley Centre Atmospheric Climate Model version 3, HadAM3, are used to investigate the impact of increasing vertical resolution on simulated climates. In particular, improvements in the representation of water vapor and temperature in the upper troposphere and lower stratosphere are identified with more accurate advection. Degradations in some aspects of the simulation in the Tropics are identified with undesirable resolution dependencies in the physical parameterizations. The overall improvements in the water vapor and temperature distribution lead to improvements in the clear-sky longwave radiative fluxes with higher vertical resolution.

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S. B. Idso, R. D. Jackson, R. J. Reginato, B. A. Kimball, and F. S. Nakayama

Abstract

Simple albedo measurement may prove useful for sensing surface soil water content and as a research tool in the study of evaporation of water from soil. Intensive concurrent measurements of the albedo and soil water content of a drying bare soil indicate that albedo, normalized for sun zenith angle effects, is a linear function of the soil water content of a very thin surface layer (less than 0.2 cm thick) over a sizeable volumetric water content range (0.00 to 0.18 for an Avondale loam). Albedo is also well correlated with the average soil water content of greater soil thicknesses. Measurements to a depth of 10 cm indicate that the relation is relatively independent of season.

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D. A. Gillette, R. N. Clayton, T. K. Mayeda, M. L. Jackson, and K. Sridhar

Abstract

Tropospheric aerosols from major dust storms (visibility <11 km) originating in cultivated areas of the High Plains in west central Texas and adjacent areas of New Mexico, Oklahoma and Colorado, were sampled by ground-based airturbine samplers with stacks 1 to 6 m high, by membrane filters, by airplane-borne dust samplers and by a static ground-level sampler. The particle size distributions of the aerosol dust obtained by airplane sampling fell mainly between 1 and 30 μm diameter. A bimodal size distribution occurred for the dust from ground samplers, with large concentrations in the 40 to 80 μm range as well as in the 1 to 30 μm range. The concentration of dust 2 to 5 km above the ground, measured by both the filtering and impactor methods, ranged from 0.1 to 0.4 mg m−3 for four intense dust storms in Texas during April of 1972 and 1973. The vertical flux for dust storms over the four-year period ranged from 0.25 × 10−7 to 2.2 × 10−8 g cm−2 s−1.

Oxygen isotopic ratio values of 1 to 10 μm quartz isolated from 17 dusts collected by ground-based samplers ranged from 16.4 to 19.5‰ (mean, 18.35 ± 0.77‰); three dusts from the airplane samplers averaged 18.2 ± 1.1‰. The Texas dusts arose largely from 13 wind-eroding soil mapping units and erodibility classes of sandy to clayey texture in the four states; the δ 18O values of the 1 to 10 μm quartz of these soils averaged 19.55 ± 0.28‰ (reported elsewhere). Abrasion by wind-induced inter-particle impact may have introduced a small amount of coarser quartz into the 1 to 10 μm aerosol fraction. Quartz from the coarser fractions of the dusts had δ 18O values ranging from 16.9 to 13.9‰ with the lower values applying to the preponderantly sand sizes (>53 μm). The fine silt from eroding sandy soils, derived not only from weathering but also possibly from eolian deposition, serves as a reservoir for long-range aerosol minerals, in addition to that from shales.

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A. A. Scaife, D. R. Jackson, R. Swinbank, N. Butchart, H. E. Thornton, M. Keil, and L. Henderson

Abstract

The conditions that lead to the major warming over Antarctica in late September 2002 are examined. In many respects, the warming resembled wave-2 warmings seen in the Northern Hemisphere; the winter cyclonic circulation was split into two smaller cyclones by a large amplitude planetary wave disturbance that appeared to propagate upward from the troposphere. However, in addition to this classic warming mechanism, distinctive stratospheric vacillations occurred throughout the preceding winter months. These vacillations in wave amplitude, Eliassen–Palm fluxes, and zonal-mean zonal winds are examined. By comparison with a numerical model experiment, it is shown that the vacillation is accompanied by a systematic weakening of the westerly winds over the season. This preconditions the Antarctic circulation, and it is argued that it allows anomalously strong vertical propagation of planetary waves from the troposphere into the stratosphere. By contrast, a survey of previous winters shows that stratospheric westerlies usually vary much more gradually, with vacillations only occurring for short periods of time, if at all, in a given winter.

Similar vacillations in a numerical model of the stratosphere only occur if the forcing amplitude is above a certain value. However, the level of winter-mean wave activity entering the stratosphere during 2002 is not unprecedented, and there is still some uncertainty over the cause of the onset and persistence of the vacillation and, ultimately, the major warming.

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Sonia Lasher-Trapp, Shailendra Kumar, Daniel H. Moser, Alan M. Blyth, Jeffrey R. French, Robert C. Jackson, David C. Leon, and David M. Plummer

ABSTRACT

The Convective Precipitation Experiment (COPE) documented the dynamical and microphysical evolution of convection in southwestern England for testing and improving quantitative precipitation forecasting. A strong warm rain process was hypothesized to produce graupel quickly, initiating ice production by rime splintering earlier to increase graupel production and, ultimately, produce heavy rainfall. Here, convection observed on two subsequent days (2 and 3 August 2013) is used to test this hypothesis and illustrate how environmental factors may alter the microphysical progression. The vertical wind shear and cloud droplet number concentrations on 2 August were 2 times those observed on 3 August. Convection on both days produced comparable maximum radar-estimated rain rates, but in situ microphysical measurements indicated much less ice in the clouds on 2 August, despite having maximum cloud tops that were nearly 2 km higher than on 3 August. Idealized 3D numerical simulations of the convection in their respective environments suggest that the relative importance of particular microphysical processes differed. Higher (lower) cloud droplet number concentrations slow (accelerate) the warm rain process as expected, which in turn slows (accelerates) graupel formation. Rime splintering can explain the abundance of ice observed on 3 August, but it was hampered by strong vertical wind shear on 2 August. In the model, the additional ice produced by rime splintering was ineffective in enhancing surface rainfall; strong updrafts on both days lofted supercooled raindrops well above the 0°C level where they froze to become graupel. The results illustrate the complexity of dynamical–microphysical interactions in producing convective rainfall and highlight unresolved issues in understanding and modeling the competing microphysical processes.

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E. Carmack, I. Polyakov, L. Padman, I. Fer, E. Hunke, J. Hutchings, J. Jackson, D. Kelley, R. Kwok, C. Layton, H. Melling, D. Perovich, O. Persson, B. Ruddick, M.-L. Timmermans, J. Toole, T. Ross, S. Vavrus, and P. Winsor

Abstract

The loss of Arctic sea ice has emerged as a leading signal of global warming. This, together with acknowledged impacts on other components of the Earth system, has led to the term “the new Arctic.” Global coupled climate models predict that ice loss will continue through the twenty-first century, with implications for governance, economics, security, and global weather. A wide range in model projections reflects the complex, highly coupled interactions between the polar atmosphere, ocean, and cryosphere, including teleconnections to lower latitudes. This paper summarizes our present understanding of how heat reaches the ice base from the original sources—inflows of Atlantic and Pacific Water, river discharge, and summer sensible heat and shortwave radiative fluxes at the ocean/ice surface—and speculates on how such processes may change in the new Arctic. The complexity of the coupled Arctic system, and the logistic and technological challenges of working in the Arctic Ocean, require a coordinated interdisciplinary and international program that will not only improve understanding of this critical component of global climate but will also provide opportunities to develop human resources with the skills required to tackle related problems in complex climate systems. We propose a research strategy with components that include 1) improved mapping of the upper- and middepth Arctic Ocean, 2) enhanced quantification of important process, 3) expanded long-term monitoring at key heat-flux locations, and 4) development of numerical capabilities that focus on parameterization of heat-flux mechanisms and their interactions.

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