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Yu-An Chen
and
Chun-Chieh Wu

Abstract

The interaction between Typhoon Nepartak (2016) and the upper-tropospheric cold low (UTCL) is simulated to better understand the impact of UTCL on the structural and intensity change of tropical cyclones (TCs). An experiment without UTCL is also performed to highlight the quantitative impacts of UTCL. Furthermore, idealized sensitivity experiments are carried out to further investigate the specific TC-UTCL configurations leading to different interactions. It is shown that a TC interacting with the UTCL is associated with a more axisymmetric inner-core structure and an earlier rapid intensification. Three plausible mechanisms related to the causality between a UTCL and the intensity change of TC are addressed. First, the lower energy expenditure on outflow expansion leads to higher net heat energy and intensification rate. Second, the external eddy forcing reinforces the secondary circulation and promotes further TC development. Ultimately, the shear-induced downward and radial ventilation of the low-entropy air is unexpectedly reduced despite the presence of UTCL, leading to stronger inner-core convections in the upshear quadrants. In general, the TC-UTCL interaction process of Nepartak is favorable for TC intensification owing to the additional positive effect and the reduced negative effect. In addition, results from sensitivity experiments indicate that the most favorable interaction would occur when the UTCL is located to the north or northwest of the TC at a stable and proper distance of about one Rossby radius of deformation of the UTCL.

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Vinzent Klaus
,
Harald Rieder
, and
Rudolf Kaltenböck

Abstract

Data from a dual-polarized, solid-state X-band radar and an operational C-band weather radar are used for high-resolution analyses of two hailstorms in the Vienna region. The combination of both radars provides rapid-update (1 min) polarimetric data paired with wind field data of a dual-Doppler analysis. This is the first time that such an advanced setup is used to examine severe storm dynamics at the Eastern Alpine fringe, where the influence of local topography is particularly challenging for thunderstorm prediction. We investigate two storms transitioning from the pre-Alps into the Vienna basin with different characteristics: (1) A rapidly evolving multicell storm producing large hail (5 cm), with observations of an intense ZDR column preceding hail formation and the rapid development of multiple pulses of hail; (2) a cold pool-driven squall line with small hail, for which we find that the updraft location inhibited the formation of larger hailstones. For both cases, we analyzed the evolution of different ZDR column metrics as well as updraft speed and size and found that (i) the 90th percentile of ZDR within the ZDR column was highest for the cell later producing large hail; (ii) the peak 90th percentile of ZDR preceded large hailfall by 20 minutes and highest updraft size and speed by 10 minutes; (iii) sudden drops of the 90th percentile of ZH within the ZDR column indicated imminent hailfall.

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Thomas Mazzetti
,
Bart Geerts
, and
Lulin Xue

Abstract

This study evaluates an operational glaciogenic cloud seeding program using ground-based generators of silver iodide (AgI), with a total of 190 seeded storms over 10 cold seasons, using the Weather Research and Forecasting Weather Modification (WRF-WxMod®) scheme at 900 m grid spacing. This study examines both the quantitative change in precipitation and the ambient and cloud conditions impacting seeding efficacy. An ensemble approach is used, with differing model boundary conditions, ice nucleation physics, concentrations of cloud condensation nuclei, and boundary layer schemes. This is intended to provide an envelope of uncertainty of natural clouds and seeding impacts. The simulations are validated against radiosonde, snow gauge, and microwave radiometer observations, and the seeding impact is inferred from simulations with/without AgI seeding. The seeding-induced precipitation enhancement (“yield”) varies greatly between storms. A small portion of the cases produces the majority of the yield. Overall, the precipitation in the target area (the Wind River Range in Wyoming, USA) increased by 1.10 ± 0.13% in the 10 years of operational seeding. This rather low fractional increase is related to the frequent seeding at unsuitable times, primarily because of low-level flow blocking. The flow and cloud structure for select cases is examined, to provide a better insight into the variability of yield. Cases with unblocked surface flow and abundant cloud liquid water tend to be the most productive. The technique presented here readily can be adapted to evaluate the seeding impact of other long-term glaciogenic seeding operations, and to improve their operational efficiency.

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Johannes M. L. Dahl
and
Jannick Fischer

Abstract

The authors explore the dynamical origins of rotation of a mature tornado-like vortex using an idealized numerical simulation of a supercell thunderstorm. Using 30 minute forward parcel trajectories that terminate at the base of the tornado-like vortex (TLV), the vorticity dynamics are analyzed for n = 7 parcels. Aside from the integration of the individual terms of the traditional vorticity equation, an alternative formulation of the vorticity equation and its integral, here referred to as vorticity source decomposition, is employed. This formulation is derived on the basis of Truesdell’s “basic vorticity formula,” which is obtained by first formulating the vorticity in material (Lagrangian) coordinates, and then obtaining the components relative to the fixed spatial (Eulerian) basis via a coordinate transformation. The analysis highlights surface drag as the most reliable vorticity source for the rotation at the base of the vortex for the analyzed parcels. Moreover, the vorticity source decomposition exposes the importance of small amounts of vorticity produced baroclinically, which may become significant after sufficient stretching occurs. Further, it is shown that ambient vorticity, upon being rearranged as the trajectories pass through the storm, may for some parcels directly contribute to the rotation of the tornado-like vortex. Finally, the role of diffusion is addressed using analytical solutions of the steady Burgers-Rott vortex, highlighting that diffusion cannot aid in maintaining the vortex core.

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Camilla W. Stjern
,
Piers M. Forster
,
Hailing Jia
,
Caroline Jouan
,
Matthew R. Kasoar
,
Gunnar Myhre
,
Dirk Olivié
,
Johannes Quaas
,
Bjørn H. Samset
,
Maria Sand
,
Toshihiko Takemura
,
Apostolos Voulgarakis
, and
Christopher D. Wells

Abstract

The climate system responds to changes in the amount of atmospheric greenhouse gases or aerosols through rapid processes, triggered within hours and days, and through slower processes, where the full response may only be seen after centuries. In this paper we aim to elucidate the mechanisms operating on timescales of hours to years to better understand the response of key climate quantities such as energy fluxes, temperature and precipitation after a sudden increase in either carbon dioxide (CO2), black carbon (BC) or sulfate (SO4) aerosols. The results are based on idealized simulations from six global climate models. We find that the effect of changing ocean temperatures kicks in after a couple of months. Rapid precipitation reductions start occurring instantly and are established after just a few days. For BC, they constitute most of the equilibrium response. For CO2 and SO4, the magnitude of the precipitation response gradually increases as surface warming/cooling evolves, and for CO2 the sign of the response changes from negative to positive after two years. Rapid cloud adjustments are typically established within the first 24 hours and while the magnitude of cloud feedbacks for CO2 and SO4 increases over time, the geographical pattern of the equilibrium cloud change is present after one year. While there are model differences, our work underscores the overall similarity of the major time-varying processes and responses simulated by current global models, and hence the robustness of key features of simulated responses to historical and future anthropogenic forcing.

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Guo Deng
,
Jun Du
,
Yushu Zhou
,
Ling Yan
,
Jing Chen
,
Fajing Chen
,
Hongqi Li
, and
Jingzhou Wang

Abstract

Using a 3-km regional ensemble prediction system (EPS), this study tested a three-dimensional (3D) rescaling mask for initial condition (IC) perturbation. Whether the 3D mask-based EPS improves ensemble forecasts over current two-dimensional (2D) mask-based EPS has been evaluated in three aspects: ensemble mean, spread, and probability. The forecasts of wind, temperature, geopotential height, sea level pressure, and precipitation were examined for a summer month (1–28 July 2018) and a winter month (1–27 February 2019) over a region in North China. The EPS was run twice per day (initiated at 0000 and 1200 UTC) to 36 h in forecast length, providing 56 warm-season forecast cases and 54 cold-season cases for verification. The warm and cold seasons are verified separately for comparison. The study found the following: 1) The vertical profile of IC perturbation becomes closer to that of analysis uncertainty with the 3D rescaling mask. 2) Ensemble performance is significantly improved in all three aspects. The biggest improvement is in the ensemble spread, followed by the probabilistic forecast, and the least improvement is in the ensemble mean forecast. Larger improvements are seen in the warm season than in the cold season. 3) More improvement is in the shorter time range (<24 h) than in the longer range. 4) Surface and lower-level variables are improved more than upper-level ones. 5) The underlying mechanism for the improvement has been investigated. Convective instability is found to be responsible for the spread increment and, thus, overall ensemble forecast improvement. Therefore, using a 3D rescaling mask is recommended for an EPS to increase its utility especially for shorter time range and surface weather elements.

Significant Statement

A weather prediction model is a complex system that consists of nonlinear differential equations. Small errors in either its inputs or model itself will grow with time during model integration, which will contaminate a forecast. To quantify such contamination (“uncertainty”) of a forecast, the ensemble forecasting technique is used. An ensemble of forecasts is a multiple of model runs at the same time but with slightly “perturbed” inputs or model versions. These small perturbations are supposed to represent true “uncertainty” in inputs or model representation. This study proposed a technique that makes a perturbation’s vertical structure more resemble real uncertainty (intrinsic error) in input data and confirmed that it can significantly improve ensemble forecast quality especially for a shorter time range and lower-level weather elements. It is found that convective instability is responsible for the improvement.

Open access
Yuan Lian
,
Mark I. Richardson
,
Claire E. Newman
,
Chris Lee
,
Anthony Toigo
,
Scott Guzewich
, and
Roger V. Yelle

Abstract

Atmospheric oscillations with daily periodicity are observed in in situ near-surface pressure, temperature, and winds observations and also in remotely sensed temperature and pressure observations of the Martian atmosphere. Such oscillations are interpreted as thermal tides driven by the diurnal cycle of solar radiation and occur at various frequencies, with the most prominent being the diurnal, semidiurnal, terdiurnal, and quadiurnal tides. Mars global circulation models reproduce these tides with varying levels of success. Until recently, both the MarsWRF and newly developed MarsMPAS models were able to produce realistic diurnal and semidiurnal tide amplitudes but predicted higher-order mode amplitudes that were significantly weaker than observed. We use linear wave analysis to show that the divergence damping applied within both MarsWRF and MarsMPAS is responsible for suppressing the amplitude of thermal tides with frequency greater than 2 per sol, despite being designed to suppress only acoustic wave modes. Decreasing the strength of the divergence damping in MarsWRF and MarsMPAS allows for excellent prediction of the higher-order tidal modes. This finding demonstrates that care must be taken when applying numerical dampers and filters that may eliminate some desired dynamical features in planetary atmospheres.

Open access
Jared W. Marquis
,
Erica K. Dolinar
,
Anne Garnier
,
James R. Campbell
,
Benjamin C. Ruston
,
Ping Yang
, and
Jianglong Zhang

Abstract

The assimilation of hyperspectral infrared sounders (HIS) observations aboard earth-observing satellites has become vital to numerical weather prediction, yet this assimilation is predicated on the assumption of clear-sky observations. Using co-located assimilated observations from the Atmospheric Infrared Sounder (AIRS) and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), it is found that near 7.7% of HIS observations assimilated by the Naval Research Laboratory Variational Data Assimilation System – Accelerated Representer (NAVDAS-AR) are contaminated by cirrus clouds. These contaminating clouds primarily exhibit visible cloud optical depths at 532nm (COD532nm) below 0.10 and cloud top temperatures between 240 K and 185 K as expected for cirrus clouds. These contamination statistics are consistent with simulations from the Radiative Transfer for TOVS (RTTOV) radiative transfer model showing a cirrus cloud with a COD532nm of 0.10 imparts brightness temperature differences below typical innovation thresholds used by NAVDAS-AR. Using a one-dimensional variational (1DVar) assimilation system coupled with RTTOV for forward and gradient radiative transfer, the analysis temperature and moisture impact of assimilating cirrus contaminated HIS observations is estimated. Large differences of 2.5 K in temperature and 11 K in dew point are possible for a cloud with COD532nm of 0.10 and cloud top temperature of 210 K. When normalized by the contamination statistics, global differences of near 0.11 K in temperature and 0.34 K in dew point are possible, with temperature and dew point tropospheric root-mean-squared-error (RMSD) as large as 0.06 and 0.11 K, respectively. While in isolation these global estimates are not particularly concerning, differences are likely much larger in regions with high cirrus frequency.

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Lu Su
,
Qian Cao
,
Shraddhanand Shukla
,
Ming Pan
, and
Dennis P. Lettenmaier

Abstract

Predictions of drought onset and termination at subseasonal (from two weeks to one month) lead times could provide a foundation for more effective and proactive drought management. We used reforecasts archived in NOAA’s Subseasonal Experiment (SubX) to force the Noah Multi-parameterization (Noah-MP), which produced forecasts of soil moisture from which we identified drought levels D0-D4. We evaluated forecast skill of major and more modest droughts, with leads from one to four weeks, and with particular attention to drought termination and onset. We find usable drought termination and onset forecast skill at leads one and two weeks for major D0 -D2 droughts; and limited skill at week three for major D0-D1 droughts, with essentially no skill at week four regardless of drought severity. Furthermore, for both major and more modest droughts, we find limited skill or no skill for D3 -D4 droughts. We find that skill is generally higher for drought termination than for onset for all drought events. We also find that drought prediction skill generally decreases from north to south for all drought events.

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Monika Kopacz
,
Victoria Breeze
,
Shobha Kondragunta
,
Gregory Frost
,
Susan Anenberg
,
Lori Bruhwiler
,
Sean Davis
,
Arlindo da Silva
,
Joost de Gouw
,
Riley Duren
,
Lawrence Flynn
,
Audrey Gaudel
,
Michael Geigert
,
Gretchen Goldman
,
Joanna Joiner
,
Brian McDonald
,
Lesley Ott
,
Vincent-Henri Peuch
,
Sally E. Pusede
,
Ivanka Stajner
,
Colin Seftor
,
Colm Sweeney
,
Lukas C. Valin
,
Jun Wang
,
James Whetstone
, and
Satya Kalluri
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