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Hongqing Yang
and
Ke Fan

Abstract

The subseasonal variability of winter air temperature in China during 2021/22 underwent significant changes, showing warm, warm, and cold anomalies during 2–23 December 2021 (P1), 1–27 January 2022 (P2), and 28 January–24 February 2022 (P3). The strong (weak) zonal circulation over East Asia led to positive (negative) surface air temperature anomalies (SATA) during P1 and P2 (P3). The position of the Siberian high affected the distribution of the warmest center of SATA over northeastern and northwestern China in P1 and P2, respectively. Further investigations indicated that intraseasonal components (10–90 days) primarily drove the warm-to-cold transition in China during P2 and P3, contributing to 79.5% of the variance in SATA in winter 2021/22. Strong (weak) East Asian intraseasonal zonal circulations corresponded to positive (negative) meridional wind anomalies over China–Lake Baikal, affecting the guidance of cold air into China during P2 (P3). East Asian circulation alternations from P2 to P3 were associated with a shift in intraseasonal geopotential height anomalies over the North Atlantic region from positive to negative in the mid-to-high troposphere through the propagation of north and south branch wave trains. The reversal of the North Atlantic geopotential height anomalies between P2 and P3 was modulated by intraseasonal higher-latitude SST anomalies over the North Atlantic and the location of intraseasonal stratospheric polar vortex. Furthermore, the intensified south branch wave train from the Indian Peninsula to China in the mid-to-high troposphere was associated with active convection over the tropical western Indian Ocean during P3. These processes could be verified by using the Linear Baroclinic Model.

Restricted access
Laurence Coursol
,
Sylvain Heilliette
, and
Pierre Gauthier

Abstract

With hyperspectral instruments measuring radiation emitted by Earth and its atmosphere in the thermal infrared range in multiple channels, several studies were made to select a subset of channels in order to reduce the number of channels to be used in a data assimilation system. An optimal selection of channels based on the information content depends on several factors related to observation and background error statistics and the assimilation system itself. An optimal channel selection for the Cross-track Infrared Sounder (CrIS) was obtained and then compared to selections made for different NWP systems. For instance, the channel selection of Carminati has 224 channels also present in our optimal selection, which includes 455 channels. However, in terms of analysis error variance, the difference between the two selections is small. Integrated over the whole profile, the relative difference is equal to 15.3% and 4.5% for temperature and humidity, respectively. Also, different observation error covariance matrices were considered to evaluate the impact of this matrix on channel selection. Even though the channels selected optimally were different in terms of which channels were selected for the various R matrices, the results in terms of analysis error are similar.

Significance Statement

Satellites measure radiation from Earth and its atmosphere in the thermal infrared. Those radiance data contain thousands of measurements, called channels, and thus, a selection needs to be done retaining most of the information content since the large number of individual pieces of information is not usable for numerical weather prediction systems. The goal of this paper is to find an optimal selection for the instrument CrIS and to compare this selection with selections made for different numerical weather prediction systems. It was found that even though the channels selected optimally were different in terms of which channels were selected compared to other selections, the results in terms of precision of the analysis are similar and the results in terms of analysis error are similar due to the nature of hyperspectral instruments, which have multiple Jacobians overlapping.

Open access
Martin Radenz
,
Ronny Engelmann
,
Silvia Henning
,
Holger Schmithüsen
,
Holger Baars
,
Markus M. Frey
,
Rolf Weller
,
Johannes Bühl
,
Cristofer Jimenez
,
Johanna Roschke
,
Lukas Ole Muser
,
Nellie Wullenweber
,
Sebastian Zeppenfeld
,
Hannes Griesche
,
Ulla Wandinger
, and
Patric Seifert

Abstract

Novel observations of aerosol and clouds by means of ground-based remote sensing have been performed in Antarctica over the Ekström ice shelf on the coast of Dronning Maud Land at Neumayer Station III (70.67°S, 8.27°W) from January to December 2023. The deployment of OCEANET-Atmosphere remote-sensing observatory in the framework of the Continuous Observations of Aerosol-cLoud interAction (COALA) campaign brought ACTRIS aerosol and cloud profiling capabilities next to meteorological and air chemistry in-situ observations at the Antarctic station. We present an overview of the site, the instrumental setup and data analysis strategy and introduce 3 scientific highlights from austral fall and winter, namely: 1. Observations of a persistent mixed-phase cloud embedded in a plume of marine aerosol. Remote-sensing-based retrievals of cloud-relevant aerosol properties and cloud microphysical parameters confirm that the free-tropospheric mixed phase cloud layer formed in an aerosol-limited environment. 2. Two extraordinary warm air intrusions. One with intense snowfall produced the equivalent of 10% of the yearly snow accumulation, a second one with record-breaking maximum temperatures and heavy icing due to supercooled drizzle. 3. Omnipresent aerosol layers in the stratosphere. Our profiling capabilities could show that 50% of the 500-nm aerosol optical depth of 0.06 was caused by stratospheric aerosol, while the troposphere was usually pristine. As demonstrated by these highlights, the one-year COALA observations will serve as a reference dataset for the vertical structure of aerosol and clouds above the region, enabling future observational and modeling studies to advance understanding of atmospheric processes in Antarctica.

Open access
Hongyu Luo
,
Haipeng Yu
,
Zeyong Hu
,
Jie Zhou
,
Bofei Zhang
,
Yaoxian Yang
,
Shanling Cheng
,
Yongqi Gong
, and
Yu Ren

Abstract

The summer atmospheric heat source (AHS) over the Tibetan Plateau (TP) induces meridional circulations in TP and its surrounding areas. Previous studies mainly focused on the monsoon circulation on the south side of TP, while the formation and maintenance mechanisms of meridional circulation on its north side remain unclear. This study compared three calculation methods of the AHS, analyzed the spatial–temporal variability of the summer AHS over the TP, and discussed its influence on the interannual variability of meridional circulation on the north side of the TP based on the two-dimensional decomposition method of atmospheric circulation and sensitivity experiments. The results indicate that in the positive AHS anomalies years, the diabatic heating of condensation latent release in southeastern TP could motivate anomalous ascending motion. Simultaneously, the increased meridional temperature gradient between the mid- and high latitudes of East Asia leads to an enhanced southward westerly jet. In this context, the region on the north side of TP, located on the north side of the westerly jet entrance, is affected by negative anomalous relative vorticity advection, prevailing anomalous descending motion, which makes the descending branch of meridional circulation significantly presented. Unlike previous studies that considered the descending branch of meridional circulation as the compensation for upward flow, the results of the linear baroclinic model (LBM) verify that the descending branch is mainly influenced by the vorticity advection related to regional scale variability of the westerly jet. This study reveals the physical mechanism of meridional circulation on the north side of TP, which offers valuable implications for seasonal forecasting in TP and Northwest China.

Restricted access
Ali Fallah
,
Mathew A. Barlow
,
Laurie Agel
,
Junghoon Kim
,
Justin Mankin
,
David M. Mocko
, and
Christopher B. Skinner

Abstract

Predicting and managing the impacts of flash droughts is difficult owing to their rapid onset and intensification. Flash drought monitoring often relies on assessing changes in root-zone soil moisture. However, the lack of widespread soil moisture measurements means that flash drought assessments often use process-based model data like that from the North American Land Data Assimilation System (NLDAS). Such reliance opens flash drought assessment to model biases, particularly from vegetation processes. Here we examine the influence of vegetation on NLDAS-simulated flash drought characteristics by comparing two experiments covering 1981-2017: open loop, (OL) which uses NLDAS surface meteorological forcing to drive a land-surface model using prognostic vegetation, and data assimilation (DA), which instead assimilates near-real-time satellite-derived leaf area index (LAI) into the land-surface model. The OL simulation consistently underestimates LAI across the U.S., causing relatively high soil moisture values. Both experiments produce similar geographic patterns of flash droughts, but OL produces shorter duration events and regional trends in flash drought occurrence that are sometimes opposite to those in DA. Across the Midwest and Southern U.S., flash droughts are four weeks (about 70%) longer on average in DA than OL. Moreover, across much of the Great Plains, flash drought occurrence has trended upward according to the DA experiment, opposite to the trend in OL. This sensitivity of flash drought to the representation of vegetation suggests that representing plants with greater fidelity could aid in monitoring flash droughts and improve the prediction of flash drought transitions to more persistent and damaging long-term droughts.

Restricted access
Takuro Matsuta
and
Humio Mitsudera

Abstract

Recent studies have shown that the sensitivity of the circumpolar transport of channels to the westerlies is controlled by wind-driven gyre circulations. Although the form stress associated with the gyres has been shown to be controlled by eddies, bottom friction, and topographic width, the role of inertial effects has not been fully understood. In this study, we conduct a series of sensitivity analyses using the barotropic model with and without the advection term (hereinafter, the model without the advection term is denoted as linear model). Experiments showed that the sensitivity of the circumpolar transport decreased under the westerly winds compared to the linear model, while it increased under the easterly winds. We show that the inertial effect of western boundary currents generates anomalous anticyclonic circulations over the topography, producing the westward topographic form stress anomalies regardless of the wind directions. In addition, we discuss the sensitivity of the inertial effect mechanism to topographic height, width, and geometries. The inertial effect mechanism is robust as long as the gyre circulations dominate while its relative importance changes. We also found that the dynamics of the barotropic channel strongly depend on the geometries of geostrophic contours f/h. Therefore, we conclude that the dynamics of barotropic channel models might be interpreted with caution to understand the dynamics of the Southern Ocean.

Significance Statement

Previous studies have studied baroclinic and barotropic channel models as a benchmark of the Southern Ocean, but the nonlinear dynamics of channels have not been fully understood. In this paper, we show that the inertial effect by the mean flow works to decrease the sensitivity to the westerly winds in the barotropic channel models using numerical experiments. We also show that the inertial effect is robust as long as gyre circulations exist, while its relative importance differs.

Restricted access
Yunshuai Zhang
,
Cunbo Han
,
Yaoming Ma
,
Shizuo Fu
,
Hongchao Zuo
, and
Qian Huang

Abstract

Applying 1D surface heterogeneity and observed atmospheric vertical profiles as initial conditions, two sets of large-eddy simulation experiments provided insight into the influence of lake size and soil moisture (SM) on the development of lake breezes and moist convection over land beside the lake. When the lake diameter increased from 20 km to 50 and 70 km, the maximum precipitation increased by 71.4% and 1.29 times, respectively. There are two reasons for larger precipitation over land in large-lake simulations: 1) Stronger and broader updrafts were found near the lake-breeze front (LBF); 2) the air at 2–4 km was moister, probably because more water vapor below 2 km was advected by the lake breezes and transported upward through turbulent exchange. Moreover, when the lake diameter increased from 20 km to more than 50 km, the deep moist convection (DMC) occurred 20 min earlier. This may be related to broader shallow convective cloud and larger vertical velocity of cloud-initiating parcels in large-lake simulations. Shallow moist convection transitioned to DMC earlier with broader clouds under moderate and high soil moisture conditions. Notably, stronger and broader updrafts near the LBFs, along with the advection of moisture induced by the lake breezes, caused the shallow moist convection to reach its peak 1 h earlier in the driest soil moisture case. However, smaller evapotranspiration could not provide sufficient moisture for the development of DMC. Our simulation results show that lake-breeze circulations are essential for the development of moist convections in the lake region.

Restricted access
Christopher M. Hartman
,
Falko Judt
, and
Xingchao Chen

Abstract

Tropical cyclone formation is known to require abundant water vapor in the lower to middle troposphere within the incipient disturbance. In this study, we assess the impacts of local water vapor analysis uncertainty on the predictability of the formation of Hurricane Irma (2017). To this end, we reduce the magnitude of the incipient disturbance’s water vapor perturbations obtained from an ensemble-based data assimilation system that constrained moisture by assimilating all-sky infrared and microwave radiances. Five-day ensemble forecasts are initialized two days before genesis using each set of modified analysis perturbations. Growth of convective differences and intensity uncertainty are evaluated for each ensemble forecast. We observe that when initializing an ensemble forecast with only moisture uncertainty within the incipient disturbance, the resulting intensity uncertainty at every lead time exceeds half that of an ensemble containing initial perturbations to all variables throughout the domain. Although ensembles with different initial moisture uncertainty amplitudes reveal a similar pathway to genesis, uncertainty in genesis timing varies substantially across ensembles since moister members exhibit earlier spinup of the low-level vortex. These differences in genesis timing are traced back to the first 6–12 h of integration, when differences in the position and intensity of mesoscale convective systems across ensemble members develop more quickly with greater initial moisture uncertainty. In addition, the rapid growth of intensity uncertainty may be greatly modulated by the diurnal cycle. Ultimately, this study underscores the importance of targeting the incipient disturbance with high spatiotemporal water vapor observations for ingestion into data assimilation systems.

Significance Statement

Hurricanes form from clusters of thunderstorms that organize into a coherent system. One of the key ingredients for the formation process is an abundance of moisture. In this study, we test the sensitivity of hurricane formation to the initial moisture content in the vicinity of the cluster of thunderstorms that would become Hurricane Irma (2017). To do so, we initialize sets of forecasts each having a different variability of initial moisture content within the embryonic disturbance. Our results show that the predictability of hurricane formation is highly dependent on the uncertainty of the moisture content within the initial disturbance. Consequently, more high-quality observations of the moisture within the precursor disturbances to hurricanes are expected to improve forecasts of their formation.

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Gaspard Geoffroy
,
Friederike Pollmann
, and
Jonas Nycander

Abstract

The solution from linear theory for the barotropic-to-baroclinic tidal energy conversion into vertical modes is validated with numerical simulations and analytical results. The main result is the translation of the traditional critical slope condition into a mode-wise condition on the topographic height only. Our findings are then used for estimates of the global M 2 tidal conversion into the first 10 vertical modes in the open ocean (excluding the continental shelves and slopes). We observe a rapid increase with mode number of the fraction of the world ocean where linear theory is invalid. In terms of conversion, which is highly variable in space, this corresponds to an even more rapid increase with mode number of the fraction of the converted energy that is strongly affected by nonlinear effects. Out of the 373.6 GW of the globally integrated conversion into modes 1-10, only 241.7 GW occur in locations where linear theory is valid. While it represents 95% for mode 1, this fraction rapidly drops with mode number to reach 27% for mode 10. Moreover, for the conversion into a single mode, we show that capping the linear solution at supercritical topography is inappropriate. Hence, linear theory appears unfit to directly quantify the role played by high-mode internal tides in the internal wave energy budget.

Restricted access
Matthew D. LaPlante
,
Luthiene Alves Dalanhese
,
Liping Deng
, and
Shih-Yu Simon Wang

Abstract

Annual wheat yields have steadily risen over the past century, but harvests remain highly variable and dependent on myriad weather conditions during a long growing season. In Kansas, for example, the 2014 crop year brought the lowest average yield in decades at 28 bushels per acre, while in 2016 farmers in the Wheat State, as Kansas is often called, enjoyed an historic high of 57 bushels per acre. It is broadly known that remote forces like the El Niño-Southern Oscillation contribute to meteorological outcomes across North America, including in the wheat growing regions of the U.S. Midwest, but the differential imprints of ENSO phases and flavors have not been well explored as leading indicators for harvest outcomes in highly specific agricultural regions, such as the more than 7 million acres upon which wheat is grown in Kansas. Here, we demonstrate a strong, steady, and long-term association between a simple “wheat yield index” and sea surface temperature anomalies, more than a year earlier, in the East Pacific, potentially offering insights into forthcoming harvest yields several seasons before planting commences.

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