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Hengchun Ye

Abstract

Spatial and temporal characteristics of winter snow depth variation over northern Eurasia and their connections to sea surface temperatures (SSTs) and associated atmospheric circulation anomalies, surface air temperatures, and precipitation are examined by using 60 yr (1936–95) of station data records. This study found that snow depth variation over the region east of the Caspian Sea and west of China, explaining 10.1% of total snow depth variance, has a quasi-biennial variability of about 2.5 yr. The snow depth variation over central European Russia and western-central Siberia, explaining 8.1% of the total snow depth variance, has a quasi-decadal variability of about 11.8 yr. The snow depth variation over the northern Ural Mountains, explaining 7.5% of the total snow depth variance has, variability of about 8 and 14 yr.

The quasi-biennial snow depth variation is associated with SSTs over the northern North Pacific and tropical western Atlantic extending into the Gulf of Mexico. The associated atmospheric circulation pattern of Eurasia 1 (EU-1) and the Pacific–North American (PNA) pattern determine the surface air temperature conditions and thus snow depth at the biennial timescale. The quasi-decadal snow variation is associated with a well-known SST anomaly pattern over the Atlantic, having opposite SST variations in alternating latitudinal belts, and SSTs over the tropical Pacific Ocean. The associated atmospheric North Atlantic oscillation (NAO) and the circulation anomaly over central Siberia affect both surface air temperature and precipitation and thus snow depth anomaly on this quasi-decadal timescale. The results provide observational evidence of possible causes for snow depth variability over high-latitude regions.

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Hengchun Ye

Abstract

This study reveals spatial and temporal characteristics of precipitation variability and their teleconnections to sea surface temperatures (SSTs) over the Atlantic and Pacific Oceans by analyzing 68 yr of recent precipitation records over the former Soviet Union. In addition to a general increasing trend of about 0.4 mm yr−1 or 6% decade−1 over much of the study region, three major modes of precipitation variation are identified. A quasi-biennial variation of 2–3 yr is found over the region surrounding the Caspian and Aral Seas. An El Niño timescale precipitation variation of 4–5 yr is present over southern central Siberia and is associated with eastern tropical Pacific SSTs. A quasi-decadal timescale variation of about 11–12 yr is evident over central European Russia. This quasi-decadal precipitation variation is closely linked to a major SST anomaly pattern of alternating latitudinal belts over the Atlantic and SST variations over the equatorial Pacific Ocean. These associations to SSTs are stronger at the timescales identified in this study than at an interannual timescale.

The atmospheric west Atlantic pattern and atmospheric anomalies over the eastern tropical Pacific bridge the teleconnection between precipitation over central European Russia and both oceans at quasi-decadal timescales. The atmospheric anomalies over the eastern Pacific coupled with eastern tropical SST anomalies are teleconnected to those over northern Asia and, thus, are responsible for the precipitation variation over southern central Siberia at an El Niño timescale.

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Hengchun Ye

Abstract

Potential benefits or disadvantages of increasing precipitation in high-latitude regions under a warming climate are dependent on how and in what form the precipitation occurs. Precipitation frequency and type are equally as important as quantity and intensity to understanding the seasonality of hydrological cycles and the health of the ecosystem in high-latitude regions. This study uses daily historical synoptic observation records during 1936–90 over the former USSR to reveal associations between the frequency of precipitation types (rainfall, snowfall, mixed solid and liquid, and wet days of all types) and surface air temperatures to determine potential changes in precipitation characteristics under a warming climate. Results from this particular study show that the frequency of precipitation of all types generally increases with air temperature during winter. However, both solid and liquid precipitation days predominantly decrease with air temperature during spring with a reduction in snowfall days being most significant. During autumn, snowfall days decrease while rainfall days increase resulting in overall decreases in wet days as air temperature increases. The data also reveal that, as snowfall days increase in relationship to increasing air temperatures, this increase may level out or even decrease as mean surface air temperature exceeds −8°C in winter. In spring and autumn, increasing rainfall days switch to decreasing when the mean surface air temperature goes above 6°C. The conclusion of this study is that changes in the frequency of precipitation types are highly dependent on the location’s air temperature and that threshold temperatures exist beyond which changes in an opposite direction occur.

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Hengchun Ye
,
Judah Cohen
, and
Michael Rawlins

Abstract

Daily synoptic observations were examined to determine the critical air temperatures and dewpoints that separate solid versus liquid precipitation for the fall and spring seasons at 547 stations over northern Eurasia. The authors found that critical air temperatures are highly geographically dependent, ranging from −1.0° to 2.5°C, with the majority of stations over European Russia ranging from 0.5° to 1.0°C and those over south-central Siberia ranging from 1.5° to 2.5°C. The fall season has a 0.5°–1.0°C lower value than the spring season at 42% stations. Relative humidity, elevation, the station's air pressure, and climate regime were found to have varying degrees of influences on the distribution of critical air temperature, although the relationships are very complex and cannot be formulated into a simple rule that can be applied universally. Although the critical dewpoint temperatures have a spread of −1.5° to 1.5°C, 92% of stations have critical values of 0.5°–1.0°C. The critical dewpoint is less dependent on environmental factors and seasons. A combination of three critical dewpoints and three air temperatures is developed for each station for spring and fall separately that has improved snow event predictability when the dewpoint is in the range of −0.5°–1.5°C and has improved rainfall event predictability when the dewpoint is higher than or equal to 0°C based on the statistics of all 537 stations. Results suggest that application of site-specific critical values of air temperature and dewpoint to discriminate between solid and liquid precipitation is needed to improve snow and hydrological modeling at local and regional scales.

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Laurence S. Kalkstein
,
Paul C. Dunne
, and
Hengchun Ye

Abstract

It has been suggested that previous results indicating an increase in surface temperatures over the past 40 years within the coldest air masses at four stations in the western North American Arctic may be attributed to the shorter residence lime of these air masses through the time period. If true, this contradicts the original contention that these air masses have undergone physical character changes, possibly attributed to anthropogenic sources, during the period. A reevaluation of the data at two of these stations indicates that a long-term warming is, in fact, taking place even when residence time is kept constant. Thus, it is suggested that changes in the physical character of these very cold air masses are due to factors other than residence time.

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Hengchun Ye
,
Han-Ru Cho
, and
Philip E. Gustafson

Abstract

Winter snow depth observations from 119 Russian stations during the years 1936–83 are selected. These irregularly spaced station data are then interpolated into 220 regular grids of 2° lat × 5.24° long that cover a region of 50°–70°N, 30°–140°E. The spatial variation patterns of the annual Russian winter snow accumulation during the period of 1936–83 are identified by using principal components analyses. Statistically significant trends in major snow depth variation patterns are detected. A method is constructed to estimate the spatial distributions of the total amount of snow depth change based on the significant trends of component scores during the period of 1936–83.

The study found that snow depth has increased over most of northern Russia and decreased over most of southern Russia during the study period. Exceptions are found in northern European Russia, where a slight decrease in snow depth has occurred and in southern west Siberia where the snow depth has increased. The total amount of snow depth increase more than compensates for the total amount of decrease in Russia. The most significant snow increase regions are found in the northern Ural Mountains (about 60°–70°N and 50°–70°E) and northern central Siberia (60°–70°N and 110°–130°E). The most significant snow decrease is found on the southern Ural Mountains (50°–55°N, 55°–65°E).

An increase of 4.7% per decade in the snow depth is estimated in northern Russia (north of 60°N), which is fairly consistent with the amount of snowfall increase estimated in northern Canada in previous studies. The total snow depth change in the study region for the period of 1936–83 is estimated to be equivalent to 43.23 km3 of water. The study suggests that the winter snow depth increase in polar continents might be a circumpolar phenomena.

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Hengchun Ye
,
Steve Ladochy
,
Daqing Yang
,
Tingjun Zhang
,
Xuebin Zhang
, and
Mark Ellison

Abstract

The influences of surface climate conditions and atmospheric circulation on seasonal river discharges of the Ob, Yenisei, and Lena River basins during 1936–95 have been examined and quantified. Climatic variables include seasonal basin-averaged surface air temperatures, precipitation, maximum snow accumulation depth, and starting and ending dates of the basins' continuous snow cover. Atmospheric circulation is represented by the Northern Hemisphere annular mode (NAM) index. The combinations of these climatic and atmospheric variables explain about 31% to 55% of the variance of the annual total discharges of these rivers. On average, climatic and atmospheric variables explain 35% to 69% variance of spring discharges, 34% to 47% variance of summer discharges, 21% to 50% variance of fall discharges, and 18% to 36% variance of winter discharges. This study reveals that the spring thermal condition is most significant for spring discharge and negatively affects summer discharge. Climatic conditions during the previous winter through fall influence fall discharges, while the atmospheric conditions of the previous summer and fall affect winter discharges. Also, winter snow accumulation influences summer and fall discharges of the Ob and Yenisei Rivers but affects winter and spring discharges of the Lena River, suggesting the importance of topography and permafrost conditions to river discharges over high-latitude regions.

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Sun Wong
,
Eric J. Fetzer
,
Baijun Tian
,
Bjorn Lambrigtsen
, and
Hengchun Ye

Abstract

The possibility of using remote sensing retrievals to estimate apparent water vapor sinks and heat sources is explored. The apparent water vapor sinks and heat sources are estimated from a combination of remote sensing, specific humidity, and temperature from the Atmospheric Infrared Sounder/Advanced Microwave Sounding Unit (AIRS) and wind fields from the National Aeronautics and Space Administration (NASA)’s Goddard Space Flight Center (GSFC)’s Modern Era Retrospective-Analysis for Research and Applications (MERRA). The intraseasonal oscillation (ISO) of the Indian summer monsoon is used as a test bed to evaluate the apparent water vapor sink and heat source. The ISO-related northward movement of the column-integrated apparent water vapor sink matches that of precipitation observed by the Tropical Rainfall Measuring Mission (TRMM) minus the MERRA surface evaporation, although the amplitude of the variation is underestimated by 50%. The diagnosed water vapor and heat budgets associated with convective events during various phases of the ISO agree with the moisture–convection feedback mechanism. The apparent heat source moves northward coherently with the apparent water vapor sink associated with the deep convective activity, which is consistent with the northward migration of the precipitation anomaly. The horizontal advection of water vapor and dynamical warming are strong north of the convective area, causing the northward movement of the convection by the destabilization of the atmosphere. The spatial distribution of the apparent heat source anomalies associated with different phases of the ISO is consistent with that of the diabatic heating anomalies from the trained heating (TRAIN Q1) dataset. Further diagnostics of the TRAIN Q1 heating anomalies indicate that the ISO in the apparent heat source is dominated by a variation in latent heating associated with the precipitation.

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Sun Wong
,
Eric J. Fetzer
,
Brian H. Kahn
,
Baijun Tian
,
Bjorn H. Lambrigtsen
, and
Hengchun Ye

Abstract

The authors investigate if atmospheric water vapor from remote sensing retrievals obtained from the Atmospheric Infrared Sounder/Advanced Microwave Sounding Unit (AIRS) and the water vapor budget from the NASA Goddard Space Flight Center (GSFC) Modern Era Retrospective-analysis for Research and Applications (MERRA) are physically consistent with independently synthesized precipitation data from the Tropical Rainfall Measuring Mission (TRMM) or the Global Precipitation Climatology Project (GPCP) and evaporation data from the Goddard Satellite-based Surface Turbulent Fluxes (GSSTF). The atmospheric total water vapor sink (Σ) is estimated from AIRS water vapor retrievals with MERRA winds (AIRS–MERRA Σ) as well as directly from the MERRA water vapor budget (MERRA–MERRA Σ). The global geographical distributions as well as the regional wavelet amplitude spectra of Σ are then compared with those of TRMM or GPCP precipitation minus GSSTF surface evaporation (TRMM–GSSTF and GPCP–GSSTF PE, respectively). The AIRS–MERRA and MERRA–MERRA Σs reproduce the main large-scale patterns of global PE, including the locations and variations of the ITCZ, summertime monsoons, and midlatitude storm tracks in both hemispheres. The spectra of regional temporal variations in Σ are generally consistent with those of observed PE, including the annual and semiannual cycles, and intraseasonal variations. Both AIRS–MERRA and MERRA–MERRA Σs have smaller amplitudes for the intraseasonal variations over the tropical oceans. The MERRA PE has spectra similar to that of MERRA–MERRA Σ in most of the regions except in tropical Africa. The averaged TRMM–GSSTF and GPCP–GSSTF PE over the ocean are more negative compared to the AIRS–MERRA, MERRA–MERRA Σs, and MERRA PE.

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Terence J. Pagano
,
Duane E. Waliser
,
Bin Guan
,
Hengchun Ye
,
F. Martin Ralph
, and
Jinwon Kim

Abstract

Atmospheric rivers (ARs) are long and narrow regions of strong horizontal water vapor transport. Upon landfall, ARs are typically associated with heavy precipitation and strong surface winds. A quantitative understanding of the atmospheric conditions that favor extreme surface winds during ARs has implications for anticipating and managing various impacts associated with these potentially hazardous events. Here, a global AR database (1999–2014) with relevant information from MERRA-2 reanalysis, QuikSCAT, and AIRS satellite observations is used to better understand and quantify the role of near-surface static stability in modulating surface winds during landfalling ARs. The temperature difference between the surface and 1 km MSL (ΔT; used here as a proxy for near-surface static stability), along with integrated water vapor transport (IVT), is analyzed to quantify their relationships to surface winds using bivariate linear regression. In four regions where AR landfalls are common, the MERRA-2-based results indicate that IVT accounts for 22%–38% of the variance in surface wind speed. Combining ΔT with IVT increases the explained variance to 36%–52%. Substitution of QuikSCAT surface winds and AIRS ΔT in place of the MERRA-2 data largely preserves this relationship (e.g., 44% as compared with 52% explained variance for U.S. West Coast). Use of an alternate static stability measure—the bulk Richardson number—yields a similar explained variance (47%). Last, AR cases within the top and bottom 25% of near-surface static stability indicate that extreme surface winds (gale or higher) are more likely to occur in unstable conditions (5.3% and 14.7% during weak and strong IVT, respectively) than in stable conditions (0.58% and 6.15%).

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