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Diane H. Portis and Peter J. Lamb

Abstract

We investigate the method-dependence of large-scale vertical motion (LSVM) estimates given by four variants of the kinematic approach (Endlich-Clark triangle; Chien-Smith pentagon; Objective analysis; Kung optimization) and the Limited-area Fine-Mesh II (LFM-II) model of the U.S. National Weather Service. The treatment spans 54 rawinsonde sounding times from three contrasting periods during the 1979 summer in the central United States that each included both widespread and abundant rainfall and intervening dry spells. Quantitative LSVM intercomparisons and evaluations for 500 and 700 mb are are with respect to cloud cover and rainfall data for 138 locations.

The Kung (especially) and LFM-II estimates have the narrowest frequency distributions. A particularly broad distribution is evident for the Endlich-Clark method. The Kung and LFM-II approaches yield significantly more frequent estimates of upward LSVM for overcast and rainy conditions (64–74 percent, depending on level and weather category) than the other methods (57–66 percent). However, the LFM-II also gives significantly more frequent estimates of upward LSVM for the fair weather condition of “0–5/10 cloud cover” (44–47 percent, depending on level) than all of the other techniques (36–41 percent). The Kung method gives very low such frequencies (38–39 percent). The above fair weather result, together with more detailed frequency distribution information, suggest that the LFM-II may be biased toward giving upward LSVM at 500 and 700 mb.

The foregoing findings are supported by additional analyses that intercompare the LSVM methods with respect to (i) their full frequency distributions when the more extreme cloud/rainfall conditions prevail and (ii) the frequency of occurrence of various cloud/rainfall categories when strong LSVM (defined separately for each method) is estimated. The results collectively suggest the Kung method to be superior to the other kinemtic approaches, in some cases substantially so, and also to the LFM-II. They offer guidance for the treatment of LSVM in meso- and synoptic-scale studies and climate dynamics.

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Chester F. Ropelewski, Peter J. Lamb, and Diane H. Portis

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Paul C. Etter, Peter J. Lamb, and Diane H. Portis

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Monthly multi-annual mean heat budgets are calculated for the Caribbean Sea; previous results from the Gulf of Mexico are included to portray fields for the combined Central American seas. Oceanic heat storage rates (QT) for the upper 200 meters in the Caribbean are calculated directly from vertical subsurface temperature data for the decade 1967–76; spatial distribution of QT are contoured on maps for February, May, August and November. In the Gulf of Mexico, QT was found to be determined principally by the surface heat exchange. In the Caribbean Sea, QT is related primarily to convergence and divergence of heat transport; QT patterns in the southern Caribbean can be associated with Ekman pumping and heat advection due to currents. The monthly mean surface heat exchanges are defined by the averages of Bunker's unpublished data and the atlas data of Hastenrath and Lamb. Comparisons are also made with the results of both Budyko and Coló in the Caribbean Sea for historical perspective. Monthly mean oceanic heat transport divergences are then derived as residuals in the heat budget equation. Partial verification is obtained by directly computing the horizontal component of heat advection using estimates of water transport in the Central American seas.

Estimates of the seasonal freshwater budgets in the Central American seas are calculated using the oceanic precipitation rates (P) of Dorman and Bourke and the averaged evaporation rates (E) obtained from Bunker and from Hastenrath and Lamb. Annual mean E – P values of 104 and 112 cm are obtained for the Caribbean Sea and Central American seas, respectively. The freshwater continuity is examined by including estimates of river discharge rates; it is shown that river discharge does not compensate for the net water loss caused by an excess of evaporation over precipitation. An analysis of the freshwater flux in the Central American seas, using typical salinity data, indicates a convergence of freshwater over the region consistent with the earlier observation of excessive evaporation.

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Peter J. Lamb, Diane H. Portis, and Abraham Zangvil

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The atmospheric moisture budget and surface interactions for the southern Great Plains are evaluated for contrasting May–June periods (1998, 2002, 2006, and 2007) as background for the Cloud and Land Surface Interaction Campaign (CLASIC) of (wet) 7–30 June 2007. Budget components [flux divergence (MFD), storage change (dPW), and inflow (IF/A)] are estimated from North American Regional Reanalysis data. Precipitation (P) is calculated from NCEP daily gridded data, evapotranspiration (E) is obtained as moisture budget equation residual, and the recycling ratio (PE/P) is estimated using a new equation. Regional averages are presented for months and five daily P categories. Monthly budget results show that E and E − P are strongly positively related to P; EP generally is positive and balanced by positive MFD that results from its horizontal velocity divergence component (HD, positive) exceeding its horizontal advection component (HA, negative). An exception is 2007 (CLASIC), when EP and MFD are negative and supported primarily by negative HA. These overall monthly results characterize low P days (≤0.6 mm), including for nonanomalous 2007, but weaken as daily P approaches 4 mm. In contrast, for 4 < P ≤ 8 mm day−1 EP and MFD are moderately negative and balanced largely by negative HD except in 2007 (negative HA). This overall pattern was accentuated (including for nonanomalous 2007) when daily P > 8 mm. Daily P E/P ratios are small and of limited range, with P category averages 0.15–0.19. Ratios for 2007 are above average only for daily P ≤ 4 mm. CLASIC wetness principally resulted from distinctive MFD characteristics. Solar radiation, soil moisture, and crop status/yield information document surface interactions.

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John E. Walsh, William L. Chapman, and Diane H. Portis

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Arctic radiative fluxes, cloud fraction, and cloud radiative forcing are evaluated from four currently available reanalysis models using data from the North Slope of Alaska (NSA) Barrow site of the Atmospheric Radiation Measurement Program (ARM). A primary objective of the ARM–NSA program is to provide a high-resolution dataset of direct measurements of Arctic clouds and radiation so that global climate models can better parameterize high-latitude cloud radiative processes. The four reanalysis models used in this study are the 1) NCEP–NCAR global reanalysis, 2) 40-yr ECMWF Re-Analysis (ERA-40), 3) NCEP–NCAR North American Regional Reanalysis (NARR), and 4) Japan Meteorological Agency and Central Research Institute of Electric Power Industry 25-yr Reanalysis (JRA25). The reanalysis models simulate the radiative fluxes well if/when the cloud fraction is simulated correctly. However, the systematic errors of climatological reanalysis cloud fractions are substantial. Cloud fraction and radiation biases show considerable scatter, both in the annual mean and over a seasonal cycle, when compared to those observed at the ARM–NSA. Large seasonal cloud fraction biases have significant impacts on the surface energy budget. Detailed comparisons of ARM and reanalysis products reveal that the persistent low-level cloud fraction in summer is particularly difficult for the reanalysis models to capture creating biases in the shortwave radiation flux that can exceed 160 W m−2. ERA-40 is the best performer in both shortwave and longwave flux seasonal representations at Barrow, largely because its simulation of the cloud coverage is the most realistic of the four reanalyses. Only two reanalyses (ERA-40 and NARR) capture the observed transition from positive to negative surface net cloud radiative forcing during a 2–3-month period in summer, while the remaining reanalyses indicate a net warming impact of Arctic clouds on the surface energy budget throughout the entire year. The authors present a variable cloud radiative forcing metric to diagnose the erroneous impact of reanalysis cloud fraction on the surface energy balance. The misrepresentations of cloud radiative forcing in some of the reanalyses are attributable to errors in both simulated cloud amounts and the models’ radiative response to partly cloudy conditions.

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Abraham Zangvil, Diane H. Portis, and Peter J. Lamb

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The relative contributions of locally evapotranspired (i.e., recycled) moisture versus externally advected water vapor for the growing-season precipitation of the U.S. Corn Belt and surrounding areas (1.23 × 106 km2) are estimated in this paper. Four May–August seasons with highly contrasting precipitation and crop yields (1975, 1976, 1979, and 1988) are investigated. A simple recycling equation—developed from the traditional atmospheric moisture budget and involving regional evapotranspiration and atmospheric water vapor inflow—is applied on daily, monthly, and seasonal time scales. Several atmospheric moisture budget components {moisture flux divergence [MFD], storage change [or change in precipitable water (dPW)], and inflow [IF]} are evaluated for 24-h periods using standard finite difference and line integral methods applied to objectively analyzed U.S. and Canadian rawinsonde data (50-hPa vertical resolution, surface to 300 hPa) for 0000 and 1200 UTC. Daily area-averaged precipitation (P) totals are derived from approximately 600 evenly distributed (but ungridded) recording rain gauges. Evapotranspiration (E) is estimated as the residual of the moisture budget equation for 24-h periods; values compare favorably with the few existing observations.

Traditional budget results show the following: E is weakly related to P on monthly and seasonal time scales; there is surprising interannual constancy of seasonal E cycles and averages given the large variation in resulting crop yields; and monthly and seasonal variability of the export of the EP surplus is determined largely by the horizontal velocity divergence component of MFD. New recycling analyses suggest that the contribution of local E to P (i.e., P E/P) is relatively small and remarkably consistent (largely 0.19–0.24) for monthly and seasonal periods, despite large P and crop yield variations. However, the monthly/seasonal averaging process is found to completely mask a striking decrease of daily P E/P (from approximately 0.30 to 0.15) with increasing P from 0 to 8 mm day−1. Unique and detailed analyses of P-stratified daily moisture budget results provide key insights into apparent contradictions between daily and monthly/seasonal recycling and related results and concomitant interannual variability, especially for the very dry 1988 season. Interpretation is facilitated by the use of modeled daily global radiation values, measured (instantaneous) and modeled (monthly) soil moisture, United States Department of Agriculture (USDA) crop yield estimates, and satellite normalized difference vegetation index (NDVI) imagery. This paper shows that land–atmosphere interactions are intimately involved in pronounced seasonal climate anomalies for the world's richest agricultural region, but apparently with considerable complexity that includes plant behavior, solar radiation forcing, and challenging time-scale interrelations.

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Abraham Zangvil, Diane H. Portis, and Peter J. Lamb

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Atmospheric moisture budget components are evaluated for a large area (1.23 × 106 km2) in the midwestern United States for all 12-h (1200–0000, 0000–1200 UTC) and 24-h (1200–1200 UTC) periods during the contrasting summers (May–August) of 1975, 1976, 1979, and 1988. The atmospheric moisture flux divergence (MFD, separated into horizontal and vertical advection components, HA and VA) and storage change (dPW) are estimated using a standard finite-difference method applied to objectively analyzed U.S. and Canadian rawinsonde data (50-hPa vertical resolution, surface–300 hPa) for 0000 and 1200 UTC. Area-averaged precipitation (P) totals are derived from approximately 600 relatively evenly distributed (but ungridded) recording rain gauges. Evapotranspiration (E) is estimated as a residual of the moisture budget equation and compares favorably with the few existing observations, especially when totaled for periods of 1 month or longer. Relationships between the budget components are established for the daily, monthly, and seasonal timescales using stratification, correlation, and cross-spectral analyses.

On monthly and seasonal timescales, the surface is a net source of water vapor (positive EP) and the bulk of this surplus is exported from the region, largely through HA. For the daily budget, a threshold P rate (∼4 mm day−1) separates surplus E−P budgets from deficit budgets. On all timescales, most of the P variance is reflected in the VA component of MFD, while HA explains ∼80% of the variation in dPW. For the monthly and (especially) daily budgets, E has bimodal distributions with P where the minimum E occurs at P ∼ 2.6 mm day−1 (monthly) and P ∼ 4–5 mm day−1 (daily). For drier daily P regimes, relatively high E is associated with increased (decreased) dry VA (HA). The correlation of E with P becomes substantially more positive from the daily-to-monthly timescale, confirming the importance of land–atmosphere interactions over longer periods. The above stratification and correlation results are complemented by cross-spectral analyses that identify strong associations between P–HA and P–dPW previously masked by phase differences. The cross-spectral results also prompt the development of a conceptual model that describes the temporal relationships among the budget components for eastward-moving large-scale, “wavelike” disturbances with 3–10-day timescales. The suggested sequence of interactions—moist HA is accompanied by a pronounced PW increase and then followed by a moist VA maximum; this horizontal and then vertical moisture redistribution is first associated with an E minimum and then culminates in a P maximum; after the P event, atmospheric drying occurs through increased (diminished) dry HA (moist VA), which leads to an E maximum and then P minimum.

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Diane H. Portis, Michael P. Cellitti, William L. Chapman, and John E. Walsh

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Hourly data from 17 relatively evenly distributed stations east of the Rocky Mountains during 54 winter seasons (1948/49 through 2001/02) are used to evaluate the low-frequency variability of extreme cold air outbreaks (CAOs). The results show no overall trend in CAO frequency, despite an increase in mean temperature over the Midwest and especially upstream into the CAO formation regions of high-latitude North America. However, there are regionally based trends in the intensity of long-duration (5 day) CAOs.

Daily heat budgets from reanalysis data are also used to investigate the thermodynamic and dynamic processes involved in the evolution of a subset of the major CAOs. The cooling of the air masses can be generally traced in the heat budget analysis as the air masses track southward along the Rocky Mountains into the Midwest. The earliest cooling begins in northwestern Canada more than a week before the cold air mass reaches the Midwest. Downstream in southwestern Canada, both diabatic and advective processes contribute to the cumulative cooling of the air mass. At peak intensity over the Midwest, diabatic processes and horizontal advection cool the air mass, but warming by subsidence offsets this cooling. By contrast, to the west of the CAO track into the Midwestern United States, vertical advection by orographic lifting cumulatively cools the air in the upslope flow regime associated with the low-level airflow around a cold air mass, and this cooling is offset by diabatic warming. Diabatic processes have strong positive correlations with temperature change over all regions (especially in central Canada) except for the mountainous regions in the United States that are to the west of the track of the cold air mass. Correlations of vertical advection with horizontal advection and diabatic processes are physically consistent and give credibility to the vertical advection field.

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John E. Walsh, Adam S. Phillips, Diane H. Portis, and William L. Chapman

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Reanalysis output for 1948–99 is used to evaluate the temporal distributions, the geographical origins, and the atmospheric teleconnections associated with major cold outbreaks affecting heavily populated areas of middle latitudes. The study focuses on three subregions of the United States and two subregions of Europe. The cold outbreaks affecting the United States are more extreme than those affecting Europe, in terms of both the regionally averaged and the local minimum air temperatures. There is no apparent trend toward fewer extreme cold events on either continent over the 1948–99 period, although a long station history suggests that such events may have been more frequent in the United States during the late 1800s and early 1900s. The trajectories of the coldest air masses are southward or southeastward over North America, but westward over Europe. Subsidence of several hundred millibars is typical of the trajectories of the coldest air to reach the surface in the affected regions. Sea level pressure anomalies evolve consistently with the trajectories over the 1–2 weeks prior to the extreme outbreaks, and precursors of the cold events are apparent in coherent antecedent anomaly patterns. Negative values of the North Atlantic oscillation index and positive anomalies of Arctic sea level pressure are features common to North American as well as European outbreaks. However, the strongest associated antecedent anomalies of sea level pressure are generally shifted geographically relative to the nodal locations of the North Atlantic and Arctic oscillations.

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Diane H. Portis, John E. Walsh, Mostafa El Hamly, and Peter J. Lamb

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Monthly sea level pressure (SLP) data from the National Centers for Environmental Prediction reanalysis for 1948–99 are used to develop a seasonally and geographically varying “mobile” index of the North Atlantic oscillation (NAOm). NAOm is defined as the difference between normalized SLP anomalies at the locations of maximum negative correlation between the subtropical and subpolar North Atlantic SLP. The subtropical nodal point migrates westward and slightly northward into the central North Atlantic from winter to summer. The NAOm index is robust across datasets, and correlates more highly than EOF coefficients with historical measures of westerly wind intensity across North Atlantic midlatitudes. As measured by this “mobile index,” the NAO’s nodes maintain their correlation from winter to summer to a greater degree than traditional NAO indices based on fixed stations in the eastern North Atlantic (Azores, Lisbon, Iceland). When the NAOm index is extended back to 1873, its annual values during the late 1800s are strongly negative due to negative contributions from all seasons, amplifying fluctuations present in traditional winter-only indices. In contrast, after the mid-1950s, the values for different seasons sufficiently offset each other to make the annually averaged excursions of NAOm smaller than those of winter-only indices. Global teleconnection fields show that the wider influence of the NAO—particularly in the western North Atlantic, eastern North America, and Arctic—is more apparent during spring–summer–autumn when the NAOm is used to characterize the NAO. Thus, the mobile index should be useful in NAO investigations that involve seasonality.

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