Abstract

A semiphysical solar radiation (SR) model is implemented to generate a new historical daily SR database for 53 locations in nine Midwestern and six adjacent states (available from the Midwestern Climate Center). This model estimates daily SR using standard hourly meteorological observations (surface atmospheric pressure and dewpoint temperature; cloud height and fractional sky cover by layer) as well as time of day, day of year, latitude/longitude, and the daily presence/absence of snow cover as input. Because of an extensive effort to interpolate for missing input (especially cloud) data, the daily SR dataset generated is 92% complete for all 53 stations for 1948–91, and 99% complete for the 43 stations with continuous hourly meteorological observations that commenced during 1945–50 and extended through 1991. Consistent with previous work, the model validates favorably against sets of daily SR measurements from (three) contrasting parts of the study region, and so its output is used here without adjustment.

Analyses of the dataset document the basic Midwestern spatial and temporal SR variability since the mid-to late 1940s. The spatial variation of calendar monthly mean SR is dominated by a near-meridional (north-eastward) decrease in fall and winter. This fundamental pattern is substantially perturbed from midspring through summer by subregional-to-mesoscale variability around and across the Great Lakes. Time series of individual monthly station mean SR values exhibit a pronounced, regionwide 1945–91 downtrend for August–November. This decline is strongest (∼12%) and most statistically significant (>99% level) for October in a belt extending east-southeastward from west-central Wisconsin across southern lake Michigan and western Lake Erie to western Pennsylvania. The SR trends for December–July are largely positive but of lesser spatial coherence, temporal consistency, and statistical significance.

Abstract

The procedure to calculate the active layer depth of the upper ocean, as proposed by Van den Dool and Horel (DH), was applied to the Atlantic Ocean from 20°S to 70°N. In this method, the observed climatological annual cycle in SST is employed to invert a simple linear energy balance. The results for the Atlantic are similar to those for the Pacific Ocean in several ways. The active layer is considerably shallower than the annual mean mixed layer (which is calculated from in situ sea temperature profiles). Just as for the Pacific, however, the patterns of active and mixed layer depth show a remarkable spatial match.

Using Bunker's datasets for SST and heat transfer over the Atlantic Ocean, the forcing used in the energy balance equation was made increasingly more realistic, from (i) astronomical solar radiation, through (ii) empirical estimates of absorbed solar radiation including the modifying effect of clouds to (iii) the complete empirically determined net ocean surface heat gain. No matter what forcing was used, the calculated active layer is always much shallower than the mixed layer depth. The best pattern match was found using the simplest forcing of all—the astronomical solar forcing.

Increasingly, atmospheric models are being coupled to an oceanic slab in which the SST evolves in response to local heat gains and losses. The key question is how deep that slab should be. Our study implies that, in order to match the observed annual cycle in SST, the oceanic stab should be quite shallow, and certainly shallower than the mixed layer depth. The shallowness of the active layer implies that ocean heat transport contributes to the forcing of the annual cycle in SST in the midlatitudes of the Atlantic Ocean.

Abstract

Budyko's model for estimating the contributions of locally evaporated and advected moisture to regional precipitation is extended to two dimensions. It is shown that a simple extension by analogy of the one-dimensional Budyko's formula to a two-dimensional region is inconsistent unless the flow in the region is parallel and uniform. The correct extension based on the two-dimensional equations of conservation of water vapor in the region leads to a generalization of Budyko's formula that includes a correction factor depending on the atmospheric flow structure. A general procedure for calculating the correction factor for a given atmospheric flow field is presented. Calculations of the correction factor for specific flow structures show that the deviations of the flow from the rectilinear structure can significantly affect the degree to which the local evaporation contributes to precipitation.

Abstract

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Abstract

The major objective of this study is to re-evaluate the ocean–land transport of moisture for rainfall in West Africa using 1979–2008 NCEP–NCAR reanalysis data. The vertically integrated atmospheric water vapor flux for the surface–850 hPa is calculated to account for total low-level moisture flux contribution to rainfall over West Africa. Analysis of mean monthly total vapor fluxes shows a progressive penetration of the flux into West Africa from the south and west. During spring (April–June), the northward flux forms a “moisture river” transporting moisture current into the Gulf of Guinea coast. In the peak monsoon season (July–September), the southerly transport weakens, but westerly transport is enhanced and extends to 20°N owing to the strengthening West African jet off the west coast. Mean seasonal values of total water vapor flux components across boundaries indicate that the zonal component is the largest contributor to mean moisture transport into the Sahel, while the meridional transport contributes the most over the Guinea coast. For the wet years of the Sahel rainy season (July–September), active anomalies are displaced farther north compared to the long-term average. This includes the latitude of the intertropical front (ITF), the extent of moisture flux, and the zone of strong moisture flux convergence, with an enhanced westerly flow. For the dry Sahel years, the opposite patterns are observed. Statistically significant positive correlations between the zonal moisture fluxes and Sudan–Sahel rainfall totals are most pronounced when the zonal fluxes lead by 1–4 pentads. However, although weak, they still are statistically significant at lags 3 and 4 for meridional moisture fluxes.

Abstract

This study investigates the El Niño–Southern Oscillation (ENSO) contribution to Philippine tropical cyclone (TC) variability, for a range of quarterly TC metrics. Philippine TC activity is found to depend on both ENSO quarter and phase. TC counts during El Niño phases differ significantly from neutral phases in all quarters, whereas neutral and La Niña phases differ only in January–March and July–September. Differences in landfalls between neutral and El Niño phases are significant in January–March and October–December and in January–March for neutral and La Niña phases. El Niño and La Niña landfalls are significantly different in April–June and October–December. Philippine neutral and El Niño TC genesis cover broader longitude–latitude ranges with similar long tracks, originating farther east in the western North Pacific. In El Niño phases, the mean eastward displacement of genesis locations and more recurving TCs reduce Philippine TC frequencies. Proximity of La Niña TC genesis to the Philippines and straight-moving tracks in April–June and October–December increase TC frequencies and landfalls. Neutral and El Niño accumulated cyclone energy (ACE) values are above average, except in April–June of El Niño phases. Above-average quarterly ACE in neutral years is due to increased TC frequencies, days, and intensities, whereas above-average El Niño ACE in July–September is due to increased TC days and intensities. Below-average La Niña ACE results from fewer TCs and shorter life cycles. Longer TC durations produce slightly above-average TC days in July–September El Niño phases. Fewer TCs than neutral years, as well as shorter TC durations, imply less TC days in La Niña phases. However, above-average TC days occur in October–December as a result of higher TC frequencies.

Abstract

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 (P_E /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; E − P 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 E − P 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 E − P 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.

Abstract

Ice storms are an infrequent but significant hazard in the U.S southern Great Plains. Common synoptic profiles for freezing precipitation reveal advection of low-level warm moist air from the Gulf of Mexico (GOM), above a shallow Arctic air mass ahead of a midlevel trough. Because the GOM is the proximal basin and major moisture source, this study investigates impacts of varying GOM sea surface temperature (SST) on the thermodynamic evolution of a winter storm that occurred during 28–30 January 2010, with particular emphasis on the modulation of freezing precipitation. A high-resolution, nested ARW sensitivity study with a 3.3-km inner domain is performed, using six representations of GOM SST, including control, climatological mean, uniform ±2°C from control, and physically constrained upper- and lower-bound basin-average anomalies from a 30-yr dataset. The simulations reveal discernable impacts of SST on the warm-layer inversion, precipitation intensity, and low-level dynamics. Whereas total precipitation for the storm increased monotonically with SST, the freezing-precipitation response was more varied and nonlinear, with the greatest accumulation decreases occurring for the coolest SST perturbation, particularly at moderate precipitation rates. Enhanced precipitation and warm-layer intensity promoted by warmer SST were offset for the highest perturbations by deepening of the weak 850-hPa low circulation and faster eastward progression associated with enhanced baroclinicity and diabatic generation of potential vorticity. Air-parcel trajectories terminating within the freezing-precipitation region were examined to identify airmass sources and modification. These results suggest that GOM SST can affect the severity of concurrent ice-storm events in the southern Great Plains, with warmer basin SST potentially exacerbating the risk of damaging ice accumulations.

In response to the need to improve climate services at the local, state, and regional levels, a national network of regional climate centers has developed. This paper provides the background to this development, and outlines the functions of the centers and identifies their benefits and beneficiaries. The centers are a source of climate expertise and maintain multifaceted interfaces with the public and private sectors. Each center a) performs services, including the management of the basic data for its region and the delivery of specialized products; b) conducts applied climate studies, including the monitoring of anomalous conditions and the promoting of regional research; and c) acquires and maintains specialized regional datasets. Examples are given for each function. The paper concludes by elaborating on the potential for focused, applied research to enhance the services provided by the regional centers. This includes reference to the current irrigation scheduling information service operated by the High Plains Regional Climate Center.

Abstract

Most investigations of relationships between tropical Pacific sea surface temperature anomaly (SSTA) events and regional climate patterns have assumed the teleconnections to be linear, whereby the climate patterns associated with cold SSTA events are considered to be similar in structure and morphology but opposite in sign to those linked to warm SSTA events. In contrast, and motivated by early evidence of nonlinearity in the above regard, this study identifies characteristic (i.e., composite) calendar monthly central and eastern North American precipitation patterns separately for warm and cold SSTA events in different regions of the tropical Pacific (central, eastern, west-central“horseshoe,” far western) identified through principal component analysis. The precipitation anomaly patterns are computed from an approximately 1° lat–long set of monthly station data for 1950–92. Their robustness and nonlinearity are established using local, regional, and field statistical significance tests and a variance analysis.

This combination of unique SSTA analyses, resulting composite selection, and characteristic precipitation anomaly determination from a fine-resolution dataset increases our understanding of tropical Pacific–North American precipitation teleconnections in several respects. First, significant linkages to the two SSTA modes related to traditional warm and cold events (central and eastern tropical Pacific) are identified for all months except September and October, with all exhibiting some nonlinear characteristics. The most important of those nonlinearities involve associations with eastern tropical Pacific SSTAs, which affect precipitation near the southern Atlantic and Gulf of Mexico coasts (dry for cold Novembers), around the Great Lakes and in the Ohio River valley (dry, warm, January–February), in the southeastern United States (dry, warm, July–August), and across the northern Great Plains (dry, warm, November–January). Conversely, the regions confirmed to have essentially linear associations with traditional warm and cold events include the Gulf of Mexico coast (positive relation with eastern tropical Pacific, January–March), Ohio River valley (negative, central tropical Pacific, February), and mid-Atlantic coast (negative, eastern tropical Pacific, July–August). However, only nonlinear precipitation teleconnections are associated with SSTAs in tropical Pacific regions largely unrelated to ENSO. These principally involve anomalously dry conditions in much of the eastern half of the United States during January–March and in the central United States in July–October (warm SSTAs in west-central tropical Pacific horseshoe), and in a strip from Texas to New England in January and along the central gulf coast and lower Mississippi valley in April (warm SSTAs in far western tropical Pacific). The results thus demonstrate the sensitivity of central and eastern North American precipitation teleconnections to the location and extent of tropical Pacific SSTAs. In the appendix, the present results are also compared to the observed climate anomalies during the 1997–98 El Niño event.