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Bruce T. Anderson
and
Hideki Kanamaru

Abstract

Diurnal variations in the climatological large-scale summertime hydrologic cycle over the southwestern United States are examined using surface and upper-air observations along with regional model output. Rainfall rates are greatest during the daytime, but the hydrologic balance that supports this rainfall changes as the day progresses. During the late morning and early afternoon, the area-averaged rainfall is balanced predominantly by evapotranspiration augmented by low-level moisture convergence; moisture from these two sources is redistributed via eddy diffusion, resulting in an overall moistening of the atmosphere and a divergence of moisture aloft. During the late afternoon, vertical redistribution via eddy diffusion weakens considerably, although precipitation continues at approximately the same rate because of drying aloft, which also supports continued large-scale divergence of moisture at these levels. This large-scale divergence aloft persists at all times of day, suggesting that for the domain as a whole, precipitation is dependent upon low-level moisture sources. At finer scales, these balances are modified principally by the presence of moisture convergence/divergence centered on the elevated regions of the domain, suggesting that local balances may be more complex than the area-average balances described here.

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Bruce T. Anderson
,
Dan Gianotti
, and
Guido Salvucci

Abstract

The release of seasonal (and longer) predictions of various climatological quantities is now routine. While undoubtedly devastating to lives and livelihoods, it is unclear whether seasonal extremes in precipitation—for example, extreme dry spells leading to droughts or heavy precipitation events leading to flooding—represent a feasible target for these predictions, that is, whether they are potentially predictable or are instead inherently unpredictable more than a few days to weeks in advance. This paper assesses the potential for predicting seasonal extremes in observed precipitation as a function of region and time of year by decomposing the station-based variance into that attributable to short-memory behavior of typical meteorological events—as generated from station-specific, seasonally varying, daily time-scale stationary stochastic weather models (SSWMs)—and that attributable to longer-time-scale, potentially predictable changes in precipitation-producing processes. Findings suggest the potential for making skillful predictions of seasonal precipitation extremes over the United States is enhanced (reduced) during the cool (warm) season, particularly for heavy precipitation event accumulations. Further, this potential is accentuated along the West Coast, around the Great Lakes, and over the central plains and Ohio River valley but is diminished over the Northeast and northern Great Plains. However, findings also suggest the potential for producing seasonal (and longer) predictions of seasonal precipitation extremes is spatially and seasonally dependent. As such, this paper includes supplemental material for the potentially predictable variance of seasonal extreme dry spell lengths, heavy event accumulations, and total accumulations at 774 stations across all 365 days so readers can evaluate the potential predictability for the location, timing, and metric of most relevance to them.

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Jingyun Wang
,
Bruce T. Anderson
, and
Guido D. Salvucci

Abstract

The intraseasonal variability of summertime precipitation over the southwestern United States is examined using stochastic daily occurrence models combined with empirical daily rainfall distributions to document 1) the seasonal evolution of the frequency and intensity of rainfall events across the summertime monsoon season and 2) the climatological evolution of wet spells, dry spells, and storm events. Study results indicate that the evolution of the North American monsoon system (NAMS) is most apparent in the occurrence of daily rainfall events, which exhibit clear time dependence across the summer season over the southwestern United States and can be principally portrayed by stochastic models. In contrast, the seasonal evolution of NAMS is largely absent in the averaged daily rainfall amount time series. There is also a significant seasonal evolution in the length of dry spells. In the central area of the domain (approximately 39 out of 78 stations) dry-spell lengths tend to increase over the course of the summer season, while on the western fringe (8 out of 78 stations) dry-spell lengths tend to decrease. In contrast, wet spells tend to exhibit constant lengths over the course of the season (44 out of 78 stations). The seasonal trend for storms indicates that the number and duration of storms tend to decrease in September; however, the storm depths tend to be more intense, particularly over the western portion of the domain. Overall, 90% of the area-averaged variance for dry-spell lengths can be explained by the random daily evolution of the stochastic model alone. For wet-spell lengths, the area-averaged variance explained by the stochastic models is 98% and for storm amounts it is 92%. These results suggest that the characteristics of most intraseasonal events over this region (i.e., spell lengths and storm amounts) can be captured by the random evolution of daily rainfall models, even with constant year-to-year statistical parameters, indicating that systematic variations in the background climatic conditions from one year to the next may contribute little to the characteristics of these events.

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Bruce T. Anderson
,
Jingyun Wang
,
Suchi Gopal
, and
Guido Salvucci

Abstract

The regional variability in the summertime precipitation over the southwestern United States is studied using stochastic chain-dependent models generated from 70 yr of station-based daily precipitation observations. To begin, the spatiotemporal structure of the summertime seasonal mean precipitation over the southwestern United States is analyzed using two independent spatial cluster techniques. Four optimal clusters are identified, and their structures are robust across the techniques used. Next, regional chain-dependent models—comprising a previously dependent occurrence chain, an empirical rainfall coverage distribution, and an empirical rainfall amount distribution—are constructed over each subregime and are integrated to simulate the regional daily precipitation evolution across the summer season. Results indicate that generally less than 50% of the observed interannual variance of seasonal precipitation in a given region lies outside the regional chain-dependent models’ stochastic envelope of variability; this observed variance, which is not captured by the stochastic model, is sometimes referred to as the “potentially predictable” variance. In addition, only a small fraction of observed years (between 10% and 20% over a given subregime) contain seasonal mean precipitation anomalies that contribute to this potentially predictable variance. Further results indicate that year-to-year variations in daily rainfall coverage are the largest contributors to potentially predictable seasonal mean rainfall anomalies in most regions, whereas variations in daily rainfall frequency contribute the least. A brief analysis for one region highlights how the identification of years with potentially predictable precipitation characteristics can be used to better understand large-scale circulation patterns that modulate the underlying daily rainfall processes responsible for year-to-year variations in regional rainfall.

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Jingyun Wang
,
Bruce T. Anderson
, and
Guido D. Salvucci

Abstract

The interannual variability of summertime daily precipitation at 78 stations in the southwestern United States is studied using chain-dependent models and nonparametric empirical distributions of daily rainfall amounts. Modeling results suggest that a second-order chain-dependent model can optimally portray the temporal structure of the summertime daily precipitation process over the southwestern United States. The unconditioned second-order chain-dependent model, in turn, can explain approximately 75% of the interannual variance in the seasonal total wet days over the region and 83% of the interannual variance in the seasonal total precipitation. In addition, only a small fraction (generally smaller than 20%) of the observed years at any given station show statistically significant changes in the occurrence and intensity characteristics, related to either the number of seasonal total wet days or the distributions of daily rainfall amounts. Investigations of the year-to-year variations in the occurrence and intensity characteristics indicate that both variations are random (on interannual time scales), and they display similar significance in explaining the remaining 17% of interannual variance of seasonal total precipitation over the region. However, numerical tests suggest that the interannual variations of the two are not independent for the summertime monsoon precipitation, and that complex covariability that cannot be described with simple stochastic statistical models may exist between them.

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Bruce T. Anderson
,
Hideki Kanamaru
, and
John O. Roads

Abstract

This paper examines year-to-year variations in the large-scale summertime hydrologic cycle over the southwestern United States using a suite of regional model simulations and surface- and upper-air-based observations. In agreement with previous results, it is found that observed interannual precipitation variations in this region can be subdivided into two spatiotemporal regimes—one associated with rainfall variability over the southwestern portion of the domain centered on Arizona and the other associated with variations over the southeastern portion centered on western Texas and eastern New Mexico. Because of the limited duration of the model simulation data, it is possible to only investigate one positive rainfall season over the Arizona region and one negative rainfall season over the New Mexico region. From these investigations it appears that for the positive rainfall anomalies over Arizona excess seasonal precipitation is balanced by both enhanced evaporation and vertically integrated large-scale moisture flux convergence. Vertical profiles of these terms indicate that the anomalous large-scale moisture flux convergence is actually related to a decrease in the mean large-scale moisture flux divergence aloft; below 800 mb there is a decrease in the mean moisture flux convergence typically found at these levels, which in turn produces anomalous moisture divergence from the region. For the negative rainfall anomalies over New Mexico similar results, but of opposite sign, are found; one exception is that at the lowest levels there is an additional (negative) contribution to the vertically integrated moisture flux convergence anomaly related to a weakening of the mean low-level moisture flux convergence during the low-rainfall year. Further studies using two different model simulations with the same large-scale dynamic forcing but differing initial soil moisture values indicate that similar balances are also found for rainfall anomalies related to surface soil moisture changes within the domain, suggesting that the changes in large-scale moisture flux convergence described above can be attributed to both year-to-year variations in the regional land–atmosphere interactions as well as variations in the large-scale circulation patterns.

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Bruce T. Anderson
,
Hideki Kanamaru
, and
John O. Roads

Abstract

In this paper the authors examine the large-scale summertime hydrologic cycle associated with the northwestern branch of the North American monsoon, centered on the southwestern United States, using a suite of surface-and upper-air-based observations, reanalysis products, and regional model simulations. In general, it is found that on an area-averaged basis, seasonal precipitation is balanced predominantly by evaporation; in addition, this evaporation also supports a net, vertically integrated moisture flux divergence from the region of the same magnitude as the precipitation itself. This vertically integrated large-scale moisture flux divergence is the result of an offsetting balance between convergence of low-level moisture and divergence of moisture aloft (<750 mb). Over the western portion of the domain, most of this low-level moisture convergence is related to advection from the Gulf of California and eastern Pacific; over the eastern portion of the domain, low-level moisture convergence is related to advection from the Gulf of Mexico. The low-level moisture, supplied both by evaporation and advection, is carried aloft primarily by convection (as opposed to large-scale vertical velocities), which then feeds both the precipitation and large-scale divergence fields. The large-scale divergence augments the anticyclonic circulation of moisture aloft, resulting in enhanced exiting fluxes over the Great Plains. A new metric for measuring recycling of moisture in convective semiarid areas is introduced; this metric is designed to better capture the importance of evaporative processes for supporting regional precipitation in these types of environments. Using this metric, it is shown that about 70%–90% of the area-averaged precipitation is the result of evaporative processes, while the remaining 10%–30% is related to low-level convergence of moisture.

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Mutlu Ozdogan
,
Guido D. Salvucci
, and
Bruce T. Anderson

Abstract

The complementary relationship between actual and potential evaporation over southeastern Turkey was examined using a mesoscale climate model and field data. Model simulations of both actual and potential evaporation produce realistic temporal patterns in comparison to those estimated from field data; as evaporation from the surface increases with increasing irrigation, potential evaporation decreases. This is in accordance with the Bouchet–Morton complementary relationship and suggests that actual evapotranspiration can be readily computed from routine meteorological observations. The driving mechanisms behind irrigation-related changes in actual and potential evaporation include reduced wind velocities, increased atmospheric stability, and depressed humidity deficits. The relative role of each in preserving the complementary relation is assessed by fitting a potential evaporation model to pan evaporation data. The importance of reduced wind velocity in maintaining complementarity was unexpected, and thus examined further using a set of perturbation simulation experiments with changing roughness parameters (reflecting growing cotton crops), changing moisture conditions (reflecting irrigation), and both. Three potential causes of wind velocity reduction associated with irrigation may be increased surface roughness, decreased thermal convection that influences momentum transfer, and the development of anomalous high pressure that counteracts the background wind field. All three are evident in the mesoscale model results, but the primary cause is the pressure-induced local wind system. The apparent necessity of capturing mesoscale dynamical feedbacks in maintaining complementarity between potential and actual evaporation suggests that a theory more complicated than current descriptions (which are based on feedbacks between actual evaporation and temperature and/or humidity gradients) is required to explain the complementary relationship.

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Bruce T. Anderson
,
Guido Salvucci
,
Alex C. Ruane
,
John O. Roads
, and
Masao Kanamitsu

Abstract

The objective of this paper is to introduce a diagnostic metric—termed the local-convergence ratio—that can be used to quantify the contribution of evaporation (and transpiration) to the atmospheric hydrologic cycle, and precipitation in particular, over a given region. Previous research into regional moisture (or precipitation) recycling has produced numerous methods for estimating the contributions of “local” (i.e., evaporated) moisture to climatological precipitation and its variations. In general, these metrics quantify the evaporative contribution to the mass of precipitable water within an atmospheric column by comparing the vertically integrated atmospheric fluxes of moisture across a region with the fluxes via evaporation. Here a new metric is proposed, based on the atmospheric moisture tendency equation, which quantifies the evaporative contribution to the rate of precipitation by comparing evaporative convergence into the column with large-scale moisture-flux convergence. Using self-consistent, model-derived estimates of the moisture-flux fields and the atmospheric moisture tendency terms, the authors compare estimates of the flux-based moisture-recycling ratio with the newly introduced local-convergence ratio. Differences between the two ratios indicate that they can be considered complementary, but independent, descriptors of the atmospheric hydroclimatology for a given region.

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Bruce T. Anderson
,
Alex C. Ruane
,
John O. Roads
, and
Masao Kanamitsu

Abstract

In this paper, a diagnostic metric—termed the local-convergence ratio—is used to analyze the contribution of evaporation and atmospheric moisture-flux convergence to model-based estimates of climatological precipitation over the North American continent. Generally, the fractional evaporative contribution is largest during spring and summer when evaporation is largest and decreases as evaporation decreases. However, there appears to be at least three regions with distinct spatiotemporal seasonal evolutions of this ratio. Over both the northern and western portions of the continent, the fractional evaporative contribution peaks in spring and early summer and decreases during fall and into winter. Over the northern portion, this fall decrease is related to an increase in atmospheric moisture-flux convergence associated with enhanced meridional moisture fluxes into the region; over the western coastal regions, the fall decrease in evaporative contribution is associated with a decrease in evaporation and an increase in total moisture-flux convergence, most likely associated with increased storm activity. In contrast, over the central portions of the continent, the fractional evaporative contribution to precipitation remains relatively low in spring—when enhanced low-level jet activity increases the low-level atmospheric moisture flux convergence into the region—and instead peaks in summer and fall—when the moisture-flux convergence associated with the low-level jet decreases and precipitation is balanced predominantly by local evaporation. Finally, over the southwestern United States and northwestern Mexico, the fractional evaporative contribution to precipitation is found to contain a wintertime minimum as well as a secondary minimum during summer. This latter feature is due to a substantial increase in low-level atmospheric moisture-flux convergence associated with the large-scale monsoon circulation that influences this region during this time.

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