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This study employs a long time series (1997-2017) of reforecasts based on a version of the ECMWF Integrated Forecast System to evaluate the dependence of medium-range (i.e., 3-15 days) precipitation forecast skill over California on the state of the large-scale atmospheric flow. As a basis for this evaluation, four recurrent large-scale flow regimes over the North Pacific and western North America associated with precipitation in a domain encompassing northern and central California were objectively identified in ECMWF ERA5 reanalysis data for November-March 1981-2017. Two of the regimes are characterized by zonal upper-level flow across the North Pacific, and the other two are characterized by wavy, blocked flow. Forecast verification statistics conditioned on regime occurrence indicate considerably lower medium-range precipitation skill over California in blocking regimes than in zonal regimes. Moreover, forecasts of blocking regimes tend to exhibit larger errors and uncertainty in the synoptic-scale flow over the eastern North Pacific and western North America compared with forecasts of zonal regimes. Composite analyses for blocking forecasts reveal a tendency for errors to develop in conjunction with the amplification of a ridge over the western and central North Pacific. The errors in the ridge tend to be communicated through the large-scale Rossby wave pattern, resulting in misforecasting of downstream trough amplification and, thereby, moisture flux and precipitation over California. The composites additionally indicate that error growth in the blocking ridge can be linked to misrepresentation of baroclinic development as well as upper-level divergent outflow associated with latent heat release. [ABSTRACT FROM AUTHOR]

Water bodies such as lakes and reservoirs affect the regional climate by acting as heat sinks and sources through the evaporation of substantial quantities of water over several months of the year. Unfortunately, energy exchange observations between deep reservoirs and the atmosphere remain rare in northeastern North America, which has one of the highest densities of water bodies in the world. This study characterizes the dynamics of turbulent heat fluxes by analysing in‐situ observations of a compact and dimictic reservoir (50.69° N, 63.24° W) located in a subarctic environment. The reservoir is characterized by a mean depth of 44 m and a surface area of 85 km2. Two eddy covariance (EC) systems, one on a raft and one onshore, were deployed from 27 June 2018 to 12 June 2022. The thermal regime of the reservoir was monitored using two vertical chains of thermistors. Results indicate a mean annual evaporation rate of 590 ± 66 mm, which is equivalent to ≈51% of the annual precipitation, with 84% of the evaporation occurring at a high rate from August to freeze‐up in late December through episodic pulses. It was difficult to close the energy balance because of the magnitude and the large time lag of the heat storage term. To circumvent this problem, we opted to perform calculations for a year that started from the first of March, as the heat storage in the water column was at its lowest at that point and could thus be ignored. From June to December, monthly Bowen ratios increased from near‐zero negative values to about 1.5. After September, due to smaller vapour pressure deficits, latent heat fluxes steadily decreased until the reservoir had a complete ice cover. Opposite diurnal cycles of sensible and latent heat fluxes were revealed during the open water period, with sensible heat fluxes peaking at night and latent heat fluxes peaking in the afternoon. [ABSTRACT FROM AUTHOR]

Climate anomalies over North America and Europe are closely related to the North Atlantic Oscillation (NAO). The persistence, decay or switch of the NAO phase across the boreal winter determines whether the NAO‐related climate anomalies are sustained, weakened or reversed during the boreal winter. This study compares the spatiotemporal evolution of atmospheric wind and sea surface temperature (SST) anomalies over the North Atlantic corresponding to persistence and switch of the NAO phase across the boreal winter. It is revealed that the persistence of the NAO from early to late winter is associated with a strong coupling between the NAO and the North Atlantic tripole SST anomalies. The formation of the tripole SST anomalies is attributed to the wind‐related surface latent heat flux changes. The tripole SST anomalies feedback on the NAO through eddy‐mean flow interaction. In contrast, during the winters with the switch of the NAO phase, the North Atlantic SST anomalies follow the wind changes, but do not have obvious feedback on the NAO. Our analysis identifies a prominent change of the winter NAO evolution in the early 1990s. The NAO switches its phase from early to late winter during 1970s through the early 1990s, but tends to maintain its phase during the boreal winter after the early 1990s. This interdecadal change appears to be related to the mean SST change. The SST increase in both the tropical and mid‐latitude North Atlantic Ocean in the early 1990s enhances the feedback of the NAO‐induced tripole SST anomalies on the NAO. Plain Language Summary: The North Atlantic Oscillation (NAO) is an important climate mode in the North Atlantic region. It has a large influence on climate in the surrounding regions. Different phases of the NAO are accompanied by distinct climate anomalies over North America and Europe. The persistence, decay or phase transition of the NAO across the boreal winter signifies the sustenance, weakening or reversal of the associated climate anomalies. This study reveals that the temporal evolution of the NAO from early to late winter is associated with the role of the North Atlantic air‐sea coupling. When the NAO is coupled closely with the North Atlantic tripole sea surface temperature anomalies, the air‐sea coupling processes maintain the phase of the NAO. When the air‐sea coupling is weak over the North Atlantic Ocean, the internal atmospheric processes control the temporal evolution of wind anomalies over the North Atlantic Ocean. In that case, the NAO cannot maintain its phase through the boreal winter and it either decays or switches its phase from early to late winter. The NAO tends to switch and maintain its phase during the boreal winter during 1970s‐early 1990s and after the early 1990s, respectively. Key Points: The North Atlantic Oscillation (NAO) displays two distinct evolutions across the boreal winter with the same phase persisting and a switch of the phaseThe NAO phase persistence is associated with a strong coupling between the NAO and the North Atlantic tripole sea surface temperature anomaliesThe winter NAO evolution experienced an interdecadal change from the phase switch to the phase persisting dominant in the early 1990s [ABSTRACT FROM AUTHOR]

The increasing frequency of very high summertime temperatures has motivated growing interest in the processes determining the probability distribution of surface temperature over land. Here, we show that on monthly time scales, temperature anomalies can be modeled as linear responses to fluctuations in shortwave radiation and precipitation. Our model contains only three adjustable parameters, and, surprisingly, these can be taken as constant across the globe, notwithstanding large spatial variability in topography, vegetation, and hydrological processes. Using observations of shortwave radiation and precipitation from 2000 to 2017, the model accurately reproduces the observed pattern of temperature variance throughout the Northern Hemisphere midlatitudes. In addition, the variance in latent heat flux estimated by the model agrees well with the few long-term records that are available in the central United States. As an application of the model, we investigate the changes in the variance of monthly averaged surface temperature that might be expected due to anthropogenic climate change. We find that a climatic warming of 4°C causes a 10% increase in temperature variance in parts of North America. [ABSTRACT FROM AUTHOR]

Meridional sensible and latent heat transport is evaluated for regions with landfalling atmospheric rivers using both MERRA‐2 reanalysis and fully coupled CESM1.3 high‐resolution climate projections. Western North America, the United Kingdom, and the Iberian Peninsula are chosen to represent the regions significantly impacted by atmospheric rivers (ARs). CESM1.3 historical simulations can accurately represent both sensible and latent regional meridional heat transports compared to MERRA‐2 both for the total period analyzed (1980–2016) and for days with atmospheric rivers only. Uncertainty in these calculations due to AR identification is assessed by applying available Tier 1 AR‐catalogs from Atmospheric Tracking Method Intercomparison Project (ARTMIP) to the MERRA‐2 analysis. CESM1.3 climate projections suggest that under global warming, latent heat transport increases across all regions in the mid‐latitudes where sensible heat decreases (increases) for western North America (Europe). Generally, changes to the meridional heat transport are forced by the upper‐level meridional wind component. Plain Language Summary: Atmospheric rivers (ARs) are long, filamentary structures in the atmosphere that transport significant amounts of water and energy from lower latitudes to higher latitudes. They can be considered a subset of an extratropical storm and are commonly found in the mid‐latitudes. To date, the majority of research has focused on water transport simply because ARs are an important part of Earth's hydrological cycle and can act as either drought‐busters or mechanisms for catastrophic floods, particularly in regions such as western North America and western Europe. Here, rather than focusing on water transport, we analyze two key contributors to total energy transport in the atmosphere: (1) heat produced by the phase changes of water (latent heat) and (2) heat produced by a change in temperature (sensible heat). With global warming, for days with landfalling atmospheric rivers, we find that sensible heat transport decreases for western North America but increases for western Europe. Latent heat transport, however, increases across all regions. Key Points: Heat transport during landfalling atmospheric rivers is explicitly computed for western North America and EuropeUnder global warming, latent heat transport increases across all regions in the mid‐latitudes where sensible heat decreases (increases) for western North America (Europe)Upper‐level meridional wind component dominates changes in heat transport [ABSTRACT FROM AUTHOR]

A carbon and nitrogen coupled dynamic vegetation ecosystem model (CLASS-CTEM N+ ) was used to assess the effects of nitrogen controls on simulated carbon, water and energy exchanges in a range of vegetation ecosystems. Standardized meteorological forcing data and eddy covariance flux measurements of carbon, water and energy from 32 FLUXNET sites covering eight major plant functional types (PFTs) across North America were used in the analysis. Two versions of the model, a carbon and nitrogen (C N) coupled version and carbon (C) only version, were employed. Simulated diurnal, daily, seasonal and annual values of gross ecosystem productivity (GEP), ecosystem respiration (Re), net ecosystem productivity (NEP), net radiation (Rn), sensible heat flux (H), latent heat flux (LE) and vegetation biomass and soil carbon stocks were compared with available measured values to evaluate the model's performance in each PFT. The C N version of the model simulated annual mean NEP for all sites was 211 g C m −2 yr −1 , compared to 174 g C m −2 yr −1 from observation and 253 g C m −2 yr −1 by the C version, respectively. Overall, the inclusion of a nitrogen cycle with the carbon cycle in the model resulted in better accuracy scores (e.g., reduced RMSE by 0.31, 0.10 and 0.09 g C m −2 yr −1 in the C N version relative to the C version, for GEP, Re and NEP, respectively) when compared with observations. Simulated Rn, H and LE were usually improved with the inclusion of a nitrogen cycle, but the changes were not always statistically significant at the site level as they were with carbon exchanges. Results indicated the simulated N limitation effects varied among PFTs, but were strongest for boreal forests during the early-growing season. Evaluation of N deposition impacts on GEP, NEP and biomass pools showed considerable variability between and within forest types due to non-linearity of N effects and spatial heterogeneity of C and N cycle interactions. Inclusion of the N cycle in the model will help in its application at regional and global scales to evaluate N availability impacts on the C cycle in terrestrial ecosystems and to determine N cycle feedbacks on Earth's climate. [ABSTRACT FROM AUTHOR]

An extended version of the Q-vector form for the ω-equation that includes diabatic (in particular latent) heating in the Q-vector itself is derived and tested for use in analyzing the life-cycle of a midlatitude cyclone that developed over the central United States during 24–26 December 2009. While the inclusion of diabatic heating in the Q-vector ω-equation is not unique to this work, the inclusion of diabatic heating in the Q-vector itself is a unique formulation. Here it is shown that the diabatic Q-vector gives a better representation of the forcing contributing to the life-cycle of the Christmas Storm of 2009 using analyses derived from the 80-km NAM. [ABSTRACT FROM AUTHOR]

• Turbulent heat flow is reduced to an advective-conductive flow on a daily timescale. • This analogy allows directly linking the daily mean sensible and ground heat fluxes. • The new relationship allows an analytical closure of the surface energy balance equation. Characterizing physical relationships between the sensible, latent and ground heat flux at the Earth surface is of crucial importance in studying the global energy, water, and carbon cycles. Here we demonstrate an analogy between the daily mean turbulent heat transfer in the atmospheric surface layer and conductive-advective heat transfer in soil. This finding, in conjunction with temperature continuity at the land–atmosphere interface, uncovers a physical linkage between sensible and ground heat flux. The proposed relationship and its ramification for estimation of latent heat flux are validated based on daily averaged data from six AmeriFlux eddy covariance stations in North America over a wide range of land covers and climate regimes. [ABSTRACT FROM AUTHOR]

Understanding the differences between regional simulations of land-atmosphere interactions and near-surface conditions is crucial for a more reliable representation of past and future climate. Here, we explore the effect of changes in the model's horizontal resolution on the simulated energy balance at the surface and near-surface conditions using the Weather Research and Forecasting (WRF) model. To this aim, an ensemble of twelve simulations using three different horizontal resolutions (25 km, 50 km and 100 km) and four different Land Surface Model (LSM) configurations over North America from 1980 to 2013 is developed. Our results show that finer resolutions lead to higher surface net shortwave radiation and maximum temperatures at mid- and high latitudes. At low latitudes over coastal areas, an increase in resolution leads to lower values of sensible heat flux and higher values of latent heat flux, as well as lower values of surface temperatures and higher values of precipitation and soil moisture in summer. The use of finer resolutions leads then to an increase in summer values of latent heat flux, convective and non-convective precipitation and soil moisture at low latitudes. The effect of the LSM choice is larger than the effect of horizontal resolution on the near-surface temperature conditions. By contrast, the effect of the LSM choice on the simulation of precipitation is weaker than the effect of horizontal resolution, showing larger differences among LSM simulations in summer and over regions with high latent heat flux. Comparison between observations and the simulation of daily maximum and minimum temperatures and accumulated precipitation indicates that the CLM4 LSM yields the lowest biases in maximum and minimum mean temperatures, but the highest biases in extreme temperatures. Increasing horizontal resolution leads to larger biases in accumulated precipitation over all regions particularly in summer. The reasons behind relate the partition between convective and non-convective precipitation, specially noticeable over western US. [ABSTRACT FROM AUTHOR]

In this study latent heat flux (λE) measurements made at 65 boreal and arctic eddy-covariance (EC) sites were analyses by using the Penman-Monteith equation. Sites were stratified into nine different ecosystem types: harvested and burnt forest areas, pine forests, spruce or fir forests, Douglas-fir forests, broadleaf deciduous forests, larch forests, wetlands, tundra and natural grasslands. The Penman-Monteith equation was calibrated with variable surface resistances against half-hourly eddy-covariance data and clear differences between ecosystem types were observed. Based on the modeled behavior of surface and aerodynamic resistances, surface resistance tightly control λE in most mature forests, while it had less importance in ecosystems having shorter vegetation like young or recently harvested forests, grasslands, wetlands and tundra. The parameters of the Penman-Monteith equation were clearly different for winter and summer conditions, indicating that phenological effects on surface resistance are important. We also compared the simulated λE of different ecosystem types under meteorological conditions at one site. Values of λE varied between 15% and 38% of the net radiation in the simulations with mean ecosystem parameters. In general, the simulations suggest that λE is higher from forested ecosystems than from grasslands, wetlands or tundra-type ecosystems. Forests showed usually a tighter stomatal control of λE as indicated by a pronounced sensitivity of surface resistance to atmospheric vapor pressure deficit. Nevertheless, the surface resistance of forests was lower than for open vegetation types including wetlands. Tundra and wetlands had higher surface resistances, which were less sensitive to vapor pressure deficits. The results indicate that the variation in surface resistance within and between different vegetation types might play a significant role in energy exchange between terrestrial ecosystems and atmosphere. These results suggest the need to take into account vegetation type and phenology in energy exchange modeling., (© 2014 John Wiley & Sons Ltd.)