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Author Topic: Rain-On-Snow Events (ROS)  (Read 410 times)

gerontocrat

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Rain-On-Snow Events (ROS)
« on: September 04, 2020, 07:38:12 PM »
Don't know where to put this, and it is significant in tracking the threats to Arctic Sea Ice.
If anybody can find a good home for it.....?

https://tc.copernicus.org/preprints/tc-2020-214/   Open Access
Trends and spatial variation in rain-on-snow events over the Arctic Ocean during the early melt season

Received: 28 Jul 2020 – Accepted for review: 25 Aug 2020 – Discussion started: 28 Aug 2020

Abstract.
Quote
Rain-on-snow (ROS) events can accelerate the surface ablation of sea ice, thus greatly influencing the ice-albedo feedback. However, the variability of ROS events over the Arctic Ocean is poorly understood due to limited historical station data in this region. In this study early melt season ROS events were investigated based on four widely-used reanalysis products (ERA-Interim, JRA-55, MERRA2 and ERA5) in conjunction with available observations at Arctic coastal stations. The performance of the reanalysis products in representing the timing of ROS events and the phase change of precipitation was assessed.

Our results show that ERA-Interim better represents the onset date of ROS events in spring and ERA5 better represents the phase change of precipitation associated with ROS events. All reanalyses indicate that ROS event timing has shifted to earlier dates in recent decades (with maximum trends up to −4 to −6 days/decade in some regions in ERA-Interim), and that sea ice melt onset in the Pacific sector and most of the Eurasian marginal seas is correlated with this shift.

There has been a clear transition from solid to liquid precipitation, leading to more ROS events in spring, although large discrepancies were found between different reanalysis products. In ERA5, the shift from solid to liquid precipitation phase during the early melt season has directly contributed to a reduction in spring snow depth on sea ice by more than −0.5 cm/decade averaged over the Arctic Ocean since 1980, with the largest contribution (about −2.0 cm/decade) in the Kara-Barents Seas and Canadian Arctic Archipelago.
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kassy

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Re: Rain-On-Snow Events (ROS)
« Reply #1 on: September 05, 2020, 07:27:44 PM »
It is rather abstract. Basically it says ROS is bad but it only evaluates it in model output.
It will drift down.  ;)
Þetta minnismerki er til vitnis um að við vitum hvað er að gerast og hvað þarf að gera. Aðeins þú veist hvort við gerðum eitthvað.

gerontocrat

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Re: Rain-On-Snow Events (ROS)
« Reply #2 on: September 07, 2020, 12:42:07 PM »
One of the many speculations that belong to me is that rain on snow may be going to be a major factor in the demise of the Arctic sea ice.

I am really grateful to SimonF92 and stjuuv for pointing towards this....

https://www.nature.com/articles/s41598-018-34450-3
The impact of Arctic warming on increased rainfall
Quote
Abstract
The Arctic region is warming two to three times faster than the global mean, intensifying the hydrological cycle in the high north. Both enhanced regional evaporation and poleward moisture transport contribute to a 50–60% increase in Arctic precipitation over the 21st century.

The additional precipitation is diagnosed to fall primarily as rain, but the physical and dynamical constraints governing the transition to a rain-dominated Arctic are unknown. Here we use actual precipitation, snowfall, rainfall output of 37 global climate models in standardised 21st-century simulations to demonstrate that, on average, the main contributor to additional Arctic (70–90°N) rainfall is local warming (~70%), whereas non-local (thermo)dynamical processes associated with precipitation changes contribute only 30%.

Surprisingly, the effect of local warming peaks in the frigid high Arctic, where modest summer temperature changes exert a much larger effect on rainfall changes than strong wintertime warming.

This counterintuitive seasonality exhibits steep geographical gradients, however, governed by non-linear changes in the temperature-dependent snowfall fraction, thereby obscuring regional-scale attribution of enhanced Arctic rainfall to climate warming. Detailed knowledge of the underlying causes behind Arctic snow/rainfall changes will contribute to more accurate assessments of the (possibly irreversible) impacts on hydrology/run-off, permafrost thawing, ecosystems, sea ice retreat, and glacier melt.

So in the Arctic, especially the high Arctic, we are bound to see a lot more rain, and a bit more snow. For much of the year that rain will fall on snow.Snow protects ice, rain destroys ice. Wet air melts ice faster than dry air.


Quote
Projected climate model mean (37 models), Arctic mean (70–90°N) and annual mean changes in total precipation, snowfall and rainfall. The contributions of temperature and precipitation changes to Arctic rainfall are indicated by ΔRΔT and ΔRΔP, respectively (see Supplementary Information). Error bars represent the multi-model standard deviation and indicate model uncertainty.

While all 37 climate models agree that Arctic rainfall will increase over the next century, the magnitude of the simulated changes is much less robust among models (Fig. 2). The increase in Arctic rainfall varies by a factor of 4 between the most extreme models, but in all models local temperature changes govern increases in annually averaged Arctic rainfall.

Quote

Projected Arctic mean (70–90°N) and annual mean changes in rainfall, including the contributions of changes in local temperature and precipitation, for each climate model. Black squares indicate the percentage by which local warming determines rainfall changes.

Apparently the increases in total precipitation and (local warming induced) absolute increases in rainfall are such that the fractional contribution of local warming induced rainfall is largely model independent and therefore a robust feature of Arctic climate change.

Spatial and Seasonal Variations
The effect of local warming on rainfall changes exhibits a complex geographical distribution, with peak values over the Arctic Ocean and North Atlantic, comparatively low values over the subarctic continents and again higher values in the mild southern regions (Fig. 3a). The fact that the local warming effect peaks in the high Arctic Ocean is a unexpected result, as one would instinctively not assume this factor to dominate in the coldest regions of the Arctic where liquid precipitation is relatively scarce to begin with. However, it is the region where projected temperature changes are largest (a phenomenon coined Arctic amplification)2,4, potentially amplifying the temperature effect on rainfall changes. Interestingly, the maximum values over the central Arctic Ocean can be attributed largely to summer conditions (Fig. 3b), during which changes in snowfall fraction are maximum in the central Arctic where current summer temperatures are around the freezing point.

Projected Arctic rainfall changes attributed to changes in local temperature, for annual mean (top), summer (JJA, June-July-August; bottom left) and winter (DJF, December-January-February; bottom right). 100% percent means that the increase in rainfall can be totally attributed to local warming.
"Para a Causa do Povo a Luta Continua!"
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"Damn, I wanted to see what happened next" (Epitaph)

aslan

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Re: Rain-On-Snow Events (ROS)
« Reply #3 on: September 11, 2020, 10:30:21 AM »
Rainfall is already on the increase by the way (and this summer can testify, with record amount of rain here and there). One point that is not discussed to my knowledge is the study of warm rain in the Arctic. This study only cover the question of snow melting before reaching surface, which obviously is the biggest factor. But given the ongoing trend toward stronger shallow convection, and soundings like this one during the rain event of the 23rd and 24th of August, the question of warm rain microphysic could be asked.