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Alberto Meucci
,
Ian R. Young
,
Mark Hemer
,
Claire Trenham
, and
Ian G. Watterson

Abstract

We present four 140-year wind-wave climate simulations (1961-2100) forced with surface wind speed and sea ice concentration from two CMIP6 GCMs under two different climate scenarios: SSP1-2.6 and SSP5-8.5. A global three-grid system is implemented in WAVEWATCH III® to simulate the wave-ice interactions in the Arctic and Antarctic regions. The models perform well in comparison with global satellite altimeter and in-situ buoys climatology. The comparison with traditional trend analyses demonstrates the present GCM-forced wave models’ ability to reproduce the main historical climate signals. The long-term datasets allow a comprehensive description of the 20th and 21st century wave climate and yield statistically robust trends. Analysis of the latest IPCC ocean climatic regions highlights four regions where changes in wave climate are projected to be most significant: the Arctic, the North Pacific, the North Atlantic, and the Southern Ocean. The main driver of offshore wave climate change is the wind, except for the Arctic where the significant sea ice retreat causes a sharp increase in the projected wave heights. Distinct changes in the wave period and the wave direction are found in the Southern Hemisphere, where the poleward shift of the Southern Ocean westerlies causes an increase in the wave period of up to 5% and a counter-clockwise change in wave direction of up to 5°. The new CMIP6 forced wave models improve in performance compared to previous CMIP5 forced wave models, and will ultimately contribute to a new CMIP6 wind-wave climate model ensemble, crucial for coastal adaptation strategies and the design of future marine offshore structures and operations.

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Benjamin Bass
,
Stefan Rahimi
,
Naomi Goldenson
,
Alex Hall
,
Jesse Norris
, and
Zachary J. Lebo

Abstract

In this study, we calibrate a regional climate model’s (RCMs) underlying land surface model (LSM). In addition to providing a realistic representation of runoff across the hydroclimatically diverse western United States, this is done to take advantage of the RCMs ability to physically resolve meteorological forcing data in ungauged regions, and to prepare the calibrated hydrologic model for tight-coupling, or the ability to represent land surface-atmosphere interactions, with the RCM. Specifically, we use a 9km resolution meteorological forcing dataset across the western United States (US), from ECMWF Reanalysis 5th Generation (ERA5) downscaled by the Weather Research Forecasting (WRF) regional climate model, as an offline forcing for Noah-Multiparameterization (Noah-MP). We detail the steps involved in producing an LSM capable of accurately representing runoff, including physical parameterization selection, parameter calibration, and regionalization to ungauged basins. Based on our model evaluation from 1954 to 2021 for 586 basins with daily natural streamflow, the streamflow bias is reduced from 24.2% to 4.4%, and the median daily Nash-Sutcliffe Efficiency (NSE) is improved from 0.12 to 0.36. When validating against basins with monthly natural streamflow data, we obtain a similar reduction in bias and a median monthly NSE improvement from 0.18 to 0.56. In this study, we also discover the optimal setup when using a donor-basin method to regionalize parameters to ungauged basins, which can vary by 0.06 NSE for unique designs of this regionalization method.

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Nisam Mang Luxom
and
Rishi Kumar Sharma

Abstract

Large expanses of snow leopard habitat overlap with extensively used areas for livestock grazing. A fundamental question for conservationists is to determine whether livestock production can be reconciled with the conservation of rare and threatened large carnivores. Therefore, numerous studies focus on the relationship between carnivore densities and space use and environmental, anthropogenic, and topographic variables. Using snow leopard sign surveys in areas with high and low grazing disturbance, Hong et al. posit that livestock grazing directly impacts fine-scale habitat selection by snow leopards. The authors recommend controlling livestock grazing to help restore habitat complexity and alpine environment diversity. However, the approach by which Hong et al. have reached this conclusion is inadequate and is based on a methodology that fails to address the research question appropriately. We argue that 1) identification of a biologically relevant scale of study is the first essential step toward inferring carnivore–habitat relationships, 2) the authors draw inconsistent conclusions from their data on sign densities in high and low grazing disturbance areas, 3) ideally, the snow leopard–livestock relationship needs to be examined across a gradient of livestock grazing intensities and at multiple spatial scales, and 4) it is inappropriate to draw conclusions for landscape/regional scales from a study conducted at a finer and undefined scale. We suggest that future studies should clearly define the scale of the study, identify appropriate habitat variables of interest, and use meaningful measurement instruments to serve as proxies for variables of interest.

Free access
Zhiling Liao
,
Shaowu Li
,
Ye Liu
, and
Qingping Zou
Restricted access
Deepak Waman
,
Sachin Patade
,
Arti Jadav
,
Akash Deshmukh
,
Ashok Kumar Gupta
,
Vaughan T. J. Phillips
,
Aaron Bansemer
, and
Paul J. DeMott

Abstract

Various mechanisms of secondary ice production (SIP) cause multiplication of numbers of ice particle, after the onset of primary ice. A measure of SIP is the ice enhancement ratio (“IE ratio”) defined here as the ratio between number concentrations of total ice (excluding homogeneously nucleated ice) and active ice-nucleating particles (INPs). A convective line observed on 11 May 2011 over the Southern Great Plains in the Mesoscale Continental Convective Cloud Experiment (MC3E) campaign was simulated with the “Aerosol–Cloud” (AC) model. AC is validated against coincident MC3E observations by aircraft, ground-based instruments, and satellite. Four SIP mechanisms are represented in AC: the Hallett–Mossop (HM) process of rime splintering, and fragmentation during ice–ice collisions, raindrop freezing, and sublimation. The vertical profile of the IE ratio, averaged over the entire simulation, is almost uniform (102 to 103) because fragmentation in ice–ice collisions dominates at long time scales, driving the ice concentration toward a theoretical maximum. The IE ratio increases with both the updraft (HM process, fragmentation during raindrop freezing, and ice–ice collisions) and downdraft speed (fragmentation during ice–ice collisions and sublimation). As reported historically in aircraft sampling, IE ratios were predicted to peak near 103 for cloud-top temperatures close to the −12°C level, mostly due to the HM process in typically young clouds with their age less than 15 min. At higher altitudes with temperatures of −20° to −30°C, the predicted IE ratios were smaller, ranging from 10 to 102, and mainly resulted from fragmentation in ice–ice collisions.

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Anirban Sinha
,
Jörn Callies
, and
Dimitris Menemenlis

Abstract

Submesoscale baroclinic instabilities have been shown to restratify the surface mixed layer and to seasonally energize submesoscale turbulence in the upper ocean. But do these instabilities also affect the large-scale circulation and stratification of the upper thermocline? This question is addressed for the North Atlantic subtropical mode water region with a series of numerical simulations at varying horizontal grid spacings (16, 8, 4, and 2 km). These simulations are realistically forced and integrated long enough for the thermocline to adjust to the presence or absence of submesoscales. Linear stability analysis indicates that a 2 km grid spacing is sufficient to resolve the most unstable mode of the wintertime mixed-layer instability. As the resolution is increased, spectral slopes of horizontal kinetic energy flatten and vertical velocities increase in magnitude, consistent with previous regional and short-time simulations. The equilibrium stratification of the thermocline changes drastically as the grid spacing is refined from 16 to 8 km and mesoscale eddies are fully resolved. The thermocline stratification remains largely unchanged, however, between the 8, 4, and 2 km runs. This robustness is argued to arise from a mesoscale constraint on the buoyancy variance budget. Once mesoscale processes are resolved, the rate of mesoscale variance production is largely fixed. This constrains the variance destruction by submesoscale vertical buoyancy fluxes, which thus remain invariant across resolutions. The bulk impact of mixed-layer instabilities on upper-ocean stratification in the subtropical mode water region through an enhanced vertical buoyancy flux is therefore captured at 8 km grid spacing, even though the instabilities are severely under-resolved.

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Andrew Heymsfield
,
Aaron Bansemer
,
Gerald Heymsfield
,
David Noone
,
Mircea Grecu
, and
Darin Toohey

Abstract

Coincident radar data with Doppler radar measurements at X, Ku, Ka, and W bands on the NASA ER-2 aircraft overflying the NASA P3 aircraft acquiring in-situ microphysical measurements are used to characterize the relationship between radar measurements and ice microphysical properties. The data were obtained from the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS. Direct measurements of the condensed water content as well as coincident Doppler radar measurements were acquired, facilitating improved estimates of ice particle mass, a variable that is an underlying factor for calculating and therefore retrieving the radar reflectivity (Ze), median mass diameter (Dm), particle terminal velocity, and snowfall rate (S). The relationship between the measured ice water content (IWC) and that calculated from the particle size distributions (PSD) using relationships developed in earlier studies, and between the calculated and measured radar reflectivity at the four radar wavelengths, are quantified. Relationships are derived between the measured IWC and properties of the PSD, Dm, Ze at the four radar wavelengths and the dual-wavelength ratio. Because IWC and Ze are measured directly, the coefficients in the mass-dimensional relationship that best match both the IWC and Ze are derived. The relationships developed here, and the mass-dimensional relationship that uses both the measured IWC and Ze to find a best match for both variables, can be used in studies that characterize the properties of wintertime snow clouds.

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Yang Hong
,
Thomas Connor
,
Huan Luo
,
Xiaoxing Bian
,
Zhaogang Duan
,
Zhuo Tang
, and
Jindong Zhang

Abstract

We thank Luxom and Sharma for their attention to and comments on our study. In recent years, livestock have been expanding into snow leopard habitat, and we conducted this study to examine the effects of that encroachment on snow leopard habitat within Wolong Nature Reserve. Specific responses to Luxom and Sharma’s comments include the following: 1) Many habitat factors influence carnivore–habitat relationships at varying spatial scales, and it is difficult for any single study to address the full suite of factors acting across all scales of selection. Given this fact and the limited spatial scale of our snow leopard sign survey, we mainly focused on snow leopard space use and microhabitat selection. 2) Our results are not necessarily conflicting, but more research is required to further explain how high sign densities, concentrated space use, and weak habitat selection behaviors might relate to each other. 3) We agree that examining a gradient of grazing intensities would be preferable, but because of the difficulty in collecting sufficient field data and the nature of livestock grazing patterns in our study area, we think that dividing our survey area into high- and low-grazing-disturbance areas was appropriate. 4) The original intent of this study was to examine habitat factors and response to livestock within our study area in Wolong Nature Reserve, and we did not intend for our specific results to be used for management recommendations beyond Wolong but instead encourage similar studies to be conducted in other areas.

Free access
Abhishek Kumar Jha
,
Subrata Kumar Das
,
U. V. Murali Krishna
, and
Sachin M. Deshpande

Abstract

This study investigates the diurnal cycle, propagation, and progression of convective storms (CSs) on the eastern edge of India’s monsoon trough (MT) using 9 years of S-band radar measurements with satellite and reanalysis datasets. CSs initiate over ocean during midnight–early morning hours and propagate onshore in succeeding hours. CSs exhibit two semidiurnal peaks, one during afternoon hours over inland areas and another during midnight–early morning hours in oceanic/coastal locations. The deep and intense afternoon peak over inland regions is attributed to land surface heating and associated destabilization. The weak and shallower but organized midnight–morning peak and propagation of CSs toward the coast are attributed to the nocturnal land breeze and its interaction with prevailing onshore flow. The observed lead–lag of a few hours in the diurnal cycle of different cumulus modes correspond to the transition of congestus into deep and then, often, into overshooting modes. Moisture budget analysis showed atmospheric regulation of this transition through thermodynamic (congestus moistening) and dynamic processes (vertical advection). Theoretical time scales were invoked to estimate the relative role of vertical advective versus congestus moistening for promoting the afternoon transition from congestus to deeper modes. Comparing the time scales for congestus moistening (18–46 h) and dynamics (3 h) with the actual transition time scales (2–4 h) reveal that congestus moistening is too slow to explain the observed lead–lag in CS modes. Though both thermodynamic and dynamic processes moisten the midlevel prior to deep/overshooting convection, vertical advection is the dominant dynamic process for the observed congestus–deep–overshooting transition.

Significance Statement

Tropical rainfall is usually linked with convection in the morning and afternoon hours. We look at the basic physical processes that lead to those convective activities peaks. The afternoon peak is linked to maximum heating, resulting in an unstable environment, whereas the morning peak is linked to the interaction of large-scale monsoon flow with a land breeze. Furthermore, daily solar heating visually shows a shallow-to-deep progression of convection. The moist midlevel environment was shown to precede such convective development in a day. The large-scale monsoon flow is a dominant cause of this moistening. The monsoon dynamic flow takes roughly 2–3 h to sufficiently moist shallow storms into deep storms, whereas the local thermodynamic moistening process takes about 18–46 h.

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Manuel Tanguy
,
Gaëlle de Coëtlogon
, and
Laurence Eymard

Abstract

ERA5 reanalyses and observations of convective clouds and precipitation are used over the northern Gulf of Guinea between 7°W and 3°E to study the influence of ocean surface temperature and the land–sea temperature gradient on Guinea Coast rainfall (GCR) in boreal spring and summer. Seasonal composites are calculated around two dates indexing the onset (T ref) and demise (T end) of the GCR. The T ref date corresponds to the emergence of the equatorial upwelling in boreal spring, which “pushes” the zonal precipitation belt northward against the Guinea coast. The T end date characterizes the emergence of the coastal upwelling in July, which is known to coincide with the beginning of the “Little Dry Season” that lasts until September. Along the Guinea Coast, the diurnal cycle of the air–sea temperature gradient controls precipitation through the land–sea breeze, which explains why precipitation reaches its maximum around noon over the ocean, and in the late afternoon over the continent. The emergence of the Guinea Coast upwelling in July induces a weakening of southerlies on a seasonal scale, and a weaker land breeze on a diurnal scale. It induces a decrease in the convergence of humidity transport across the coast and in coastal oceanic precipitation. Therefore, the GCR is seasonally controlled by the latitude of the maximum tropospheric water vapor content and the annual cycle of the West African monsoon, but the ocean surface temperature is responsible for the abruptness of its onset via the intensification of the equatorial upwelling around the end of May, and possibly of its demise as well via the emergence of the coastal upwelling by early July.

Open access