Changes In Longwave Radiation Research Articles

Enhanced Asian warming increases Arctic amplification

The Arctic has been experiencing prominent warming amplification. However, despite anthropogenic emissions and oceanic variability, whether Arctic amplification has a connection with land in the lower latitudes remains unknown. Here, we newly identify enhanced Asian warming as a factor underlying Arctic amplification. The simulations demonstrate that enhanced Asian warming contributes 22% of the wintertime amplified warming over the Barents–Kara Seas (BKS). We demonstrate that Asian warming remotely affects the Arctic by affecting poleward atmospheric heat and moisture transport. The external anomalous heat and moisture further trigger local feedbacks concerning sea ice-albedo feedback and changes in longwave radiation and evaporation, thus facilitating BKS warming amplification. The capacitor effect of the Arctic Ocean further modulates the seasonality of BKS warming via turbulent heat flux exchange between the atmosphere and ocean. Moreover, anomalous Rossby wave trains are responsible for the anomalous atmospheric circulations favoring the atmospheric heat and moisture transport into BKS. Our findings illuminate a new factor from remote lower latitudes affecting Arctic climate change.

The Role of Soil Temperature Feedbacks for Summer Air Temperature Variability Under Climate Change Over East Asia

AbstractThe ongoing climate change has posed severe threat to the natural system and human society. However, how soil temperature feedbacks matter for climate change projections is not yet well explored. In this study, we assess the role of soil temperature feedbacks for summer air temperature variability over East Asia under global warming. Regional climate model simulations with and without soil temperature‐atmosphere interactions were performed to isolate the role of soil temperature feedbacks under historical (1976–2005) and future (2071–2100) warming conditions. Results indicate that soil temperature feedbacks can largely enhance interannual variability of summer daily mean and minimum surface air temperatures over East Asia, with strongest impacts over Mongolia and many areas of northern Tibetan Plateau and northern China, in the historical period. The soil temperature feedback strength below 925‐hPa mainly depends on changes in longwave radiation, surface heat flux partitioning, and temperature advections, while it is largely determined by the diabatic heating processes in the lower troposphere. The spatial distribution of soil temperature feedbacks is projected to change notably over East Asia under future warming condition. In particular, the impacts of soil temperature feedbacks on summer temperature are shown to be stronger in the lower troposphere under future condition. These results imply the important role of soil temperature feedbacks on air temperature variability for regional climate modeling especially in the context of climate change.

Comparison of elevation-dependent warming and its drivers in the tropical and subtropical Andes

Mountain regions have been recognized to be more sensitive to climate and environmental changes, and in particular to global warming. Several studies report on elevation-dependent warming (EDW), i.e., when warming rates are different in different altitude ranges, particularly focusing on the enhancement of warming rates with elevation. The Andean chain proved to be a relevant climate change hot-spot with positive temperature trends and a widespread glacier retreat over the recent decades. To assess and to better understand elevation dependent warming in this mountain region and to identify its possible dependence on latitude, the Andean Cordillera was split into five domains, three pertaining to the tropical zone and two pertaining to the Subtropics. Further, for each area the eastern and western faces of the mountain range were separately analyzed. An ensemble of regional climate model (RCM) simulations participating in the Coordinated Regional Climate Downscaling Experiment (CORDEX), consisting of one RCM nested into eight different global climate models from the CMIP5 ensemble was considered in this study. EDW was assessed by calculating the temperature difference between the end of the century (2071–2100) and the period 1976–2005 and relating it to the elevation. Future projections refer to the RCP 8.5 high-emission scenario. Possible differences in EDW mechanisms were identified using correlation analyses between temperature changes and all the variables identified as possible EDW drivers. For the maximum temperatures, a positive EDW signal (i.e. enhancement of warming rates with elevation) was identified in each side of both the tropical and subtropical Andes and in all seasons. For the minimum temperatures, on the contrary, while a positive EDW was identified in the Subtropics (particularly evident in the western side of the chain), the Tropics are characterized by a negative EDW throughout the year. Therefore, the tropical boundary marks a transition between discordant EDW behaviours in the minimum temperature. In the Tropics and particularly in the inner Tropics, different EDW drivers were identified for the minimum temperature, whose changes are mostly associated with changes in downward longwave radiation, and for the maximum temperature, whose changes are mainly driven by changes in downward shortwave radiation. This might explain the opposite EDW signal found in the tropical Andes during daytime and nighttime. Changes in albedo are an ubiquitous driver for positive EDW in the Subtropics, for both the minimum and the maximum temperature. Changes in longwave radiation and humidity are also EDW drivers in the Subtropics but with different relevance throughout the seasons and during daytime and nighttime. Also, the western and eastern sides of the Cordillera might be influenced by different EDW drivers.

Stomatal response to humidity and CO2 implicated in recent decline in US evaporation.

Evapotranspiration, defined as the total flux of water from the land surface to the atmosphere, is a major component of the hydrologic cycle and surface energy balance. Although evapotranspiration is expected to intensify with increasing temperatures, long-term, regional trends in evapotranspiration remain uncertain due to spatially and temporally limited direct measurements. In this study, we utilize an emergent relation between the land surface and atmospheric boundary layer to infer daily evapotranspiration from historical meteorological data collected at 236 weather stations across the United States. Our results suggest a statistically significant (α=0.05) decrease in evapotranspiration of approximately 6% from 1961 to 2014, with a significant (α=0.05) sharp decline of 13% from 1998 to 2014. We attribute the decrease in evapotranspiration mostly to declines in surface conductance, but also to offsetting changes in longwave radiation, wind speed, and incoming solar radiation. Using an established stomatal conductance model, we explain the changes in inferred surface conductance as a response to increases in carbon dioxide and, more recently, to an abrupt decrease in atmospheric humidity.

Cosmic ray modulation of infra-red radiation in the atmosphere

Cosmic rays produce molecular cluster ions as they pass through the lower atmosphere. Neutral molecular clusters such as dimers and complexes are expected to make a small contribution to the radiative balance, but atmospheric absorption by charged clusters has not hitherto been observed. In an atmospheric experiment, a narrowband thermopile filter radiometer centred on 9.15 μm, an absorption band previously associated with infra-red absorption of molecular cluster ions, was used to monitor changes following events identified by a cosmic ray telescope sensitive to high-energy (>400 MeV) particles, principally muons. The average change in longwave radiation in this absorption band due to molecular cluster ions is 7 mWm−2. The integrated atmospheric energy density for each event is 2 Jm−2, representing an amplification factor of 1012 compared to the estimated energy density of a typical air shower. This absorption is expected to occur continuously and globally, but calculations suggest that it has only a small effect on climate.

Evaporation Change and Global Warming: The Role of Net Radiation and Relative Humidity

The change in evaporation over the oceans in climate models is analyzed from the perspective of air‐sea turbulent fluxes of water and energy. The results challenge the view that the change in evaporation is predominantly constrained by the change in the net radiation at the surface. For fixed net radiation change, it is found that (1) robust increases in near‐surface relative humidity and (2) robust decreases in turbulent exchange coefficient lead to a substantial reduction in evaporation below the rate of increase implied by the net radiation alone. This reduction of evaporation is associated with corresponding changes in the sensible heat flux. In addition, a net imbalance in the surface energy budget under transient greenhouse gas forcing provides a further reduction in the evaporation change in climate models. Further results also suggest that it might be more physical to view the evaporation change as a function of relative humidity change rather than net radiation. In this view, the relative humidity controls the net surface shortwave radiation through changes in low‐level cloudiness and the temperature controls the net surface radiation through the changes in longwave radiation. In addition, the results demonstrate the dominant role of both the air‐sea temperature difference and relative humidity over, for example, wind speed in reducing the evaporation change in climate models below the Clausius‐Clapeyron rate.

Strong-wind events and their impact on the near-surface climate at Kohnen Station on the Antarctic Plateau

AbstractStrong-wind events occur 10–20 times per year at Kohnen Station, East Antarctica (75°00′S, 0°04′E, 2892 m above sea level), and are often caused by warm-core cyclones in the north-eastern Weddell Sea. An uncommon event occurred in January 2002, when blocking both in the south Atlantic Ocean and in the south Tasman Sea caused a split-up of the circumpolar vortex, and large amounts of heat and moisture were transported onto the Antarctic Plateau. During strong-wind events over the plateau the near-surface temperature can increase by tens of degrees, which is partly caused by the advection of heat, but for an important part by the destruction of the stable temperature-deficit layer by enhanced vertical mixing. The temperature rise is larger during the winter/night than during the summer/day, due to a better-developed temperature deficit. Snowdrift during the January 2002 event linearly increased surface roughness for momentum with friction velocity, for values over about 0.18 m s-1. The cloud cover during the event reduced down-welling solar radiation by 32%, and increased the albedo from about 0.86 to 0.92. Changes in longwave radiation largely cancelled the daytime changes in shortwave radiation, thus net radiation was most affected at night.

Simulating the thermal dynamics of Lake Kinneret

In this study, we use the one-dimensional hydrodynamic model DYRESM to simulate the Lake Kinneret thermal structure over a period of 45 months. We focus on the application of a new version of the one-dimensional hydrodynamic model to Lake Kinneret and simulation of the physical processes. DYRESM is calibration-free process-based model that simulates the vertical distribution of temperature and salinity in lakes and reservoirs. The current model is a complete rewrite of the original DYRESM code. Each algorithm was simplified to the essential minimum physical description yielding a more robust and faster model. The new model underwent validation with a high quality dataset from Lake Kinneret. The model simulation reproduced the temperature of the surface layer of the water column to within less than 1 °C. Results of a sensitivity analysis indicated that the surface and bottom water temperatures were most sensitive to changes in long-wave radiation. We conclude from the results that the simplified version of DYRESM accurately simulated the physical processes in the lake although planned inclusion of a benthic boundary-layer routine will improve the model’s ability to simulate the bottom temperatures.