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Eric P. James
and
Stanley G. Benjamin

Abstract

A set of observation system experiments (OSEs) over three seasons using the hourly updated Rapid Refresh (RAP) numerical weather prediction (NWP) assimilation–forecast system identifies the importance of the various components of the North American observing system for 3–12-h RAP forecasts. Aircraft observations emerge as the strongest-impact observation type for wind, relative humidity (RH), and temperature forecasts, permitting a 15%–30% reduction in 6-h forecast error in the troposphere and lower stratosphere. Major positive impacts are also seen from rawinsondes, GOES satellite cloud observations, and surface observations, with lesser but still significant impacts from GPS precipitable water (PW) observations, satellite atmospheric motion vectors (AMVs), and radar reflectivity observations. A separate experiment revealed that the aircraft-related RH forecast improvement was augmented by 50% due specifically to the addition of aircraft moisture observations. Additionally, observations from en route aircraft and those from ascending or descending aircraft contribute approximately equally to the overall forecast skill, with the strongest impacts in the respective layers of the observations. Initial results from these OSEs supported implementation of an improved assimilation configuration of boundary layer pseudoinnovations from surface observations, as well as allowing the assimilation of satellite AMVs over land. The breadth of these experiments over the three seasons suggests that observation impact results are applicable to general forecasting skill, not just classes of phenomena during limited time periods.

Open access
Eric P. James
and
Richard H. Johnson

Abstract

Climatological characteristics of mesoscale convective vortices (MCVs) occurring in the state of Oklahoma during the late spring and summer of four years are investigated. The MCV cases are selected based on vortex detection by an objective algorithm operating on analyses from the Rapid Update Cycle (RUC) model. Consistent with a previous study, true MCVs represent only about 20% of the mesoscale relative vorticity maxima detected by the algorithm. The MCVs have a broad range of radii and intensities, and their longevities range between 1 and 54 h. Their median radius is about 200 km, and their median midlevel relative vorticity is 1.2 × 10−4 s−1. There appears to be no significant relationship between MCV longevity and intensity. Similar to past estimates, approximately 40% of the MCVs generate secondary convection within their circulations.

The mean synoptic-scale MCV environment is determined by the use of a RUC-based composite analysis at four different stages in the MCV life cycle, defined based on vortex detection by the objective algorithm. MCV initiation is closely tied to the diurnal cycle of convection over the Great Plains, with MCVs typically forming in the early morning, near the time of maximum extent of nocturnal mesoscale convective systems (MCSs). Features related to the parent MCSs, including upper-level divergent outflow, midlevel convergence, and a low-level jet, are prominent in the initiating MCV composite. The most significant feature later in the MCV life cycle is a persistent mesoscale trough in the midlevel height field. The potential vorticity (PV) structure of the composite MCV consists of a midlevel maximum and an upper-level minimum, with some extension of elevated PV into the lower troposphere as the vortex matures. The environment immediately downshear of the MCV is more conducive to secondary convection than the environment upshear of the MCV.

This midlatitude MCV climatology represents an extension of past individual case studies by providing mean characteristics of a large MCV population; these statistics are suitable for the verification of MCV simulations. Also presented is the first high-resolution composite analysis of the MCV environment at different stages of the MCV life cycle, which will aid in identifying and forecasting these systems.

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Eric P. James
and
Richard H. Johnson

Abstract

Surface pressure manifestations of mesoscale convective vortices (MCVs) that traversed Oklahoma during the periods May–August 2002–05 are studied using the Weather Surveillance Radar-1988 Doppler (WSR-88D), the Oklahoma Mesonet, and the NOAA Profiler Network data. Forty-five MCVs that developed from mesoscale convective systems (MCSs) have been investigated, 28 (62%) of which exhibit mesolows detectable at the surface. Within this group, three distinct patterns of precipitation organization and associated mesolow evolution have been identified. The remaining 17 (38%) of the cases do not contain a surface mesolow. Two repeating patterns of precipitation organization are identified for the latter group.

The three categories of MCVs possessing a surface mesolow are as follows. Nineteen are classified as “rear-inflow-jet MCVs,” and tend to form within large and intense asymmetric MCSs. Rear inflow into the MCS, enhanced by the development of an MCV on the left-hand side relative to system motion, produces a rear-inflow notch and a distinct surface wake low at the back edge of the stratiform region. Hence, the surface mesolow and MCV are displaced from one another. Eight are classified as “collapsing-stratiform-region MCVs.” These MCVs arise from small asymmetric MCSs. As the stratiform region of the MCS weakens, a large mesolow appears beneath its dissipating remnants due to broad subsidence warming, and at the same time the midlevel vortex spins up due to column stretching. One case, called a “vertically coherent MCV,” contains a well-defined surface mesolow and associated cyclonic circulation, apparently due to the strength of the midlevel warm core and the weakness of the low-level cold pool. In these latter two cases, the surface mesolow and MCV are approximately collocated.

Within the group of MCVs without a surface mesolow, 14 are classified as “remnant-circulation MCVs” containing no significant precipitation or surface pressure effects. Finally, three are classified as “cold-pool-dominated MCVs;” these cases contain significant precipitation but no discernible surface mesolow.

This study represents the first systematic analysis of the surface mesolows associated with MCVs. The pattern of surface pressure and winds accompanying MCVs can affect subsequent convective development in such systems. Extension of the findings herein to tropical oceans may have implications regarding tropical cyclogenesis.

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Dennis P. Lettenmaier
,
Eric F. Wood
, and
James R. Wallis

Abstract

Spatial patterns in trends of four monthly variables: average temperature, precipitation, streamflow, and average of the daily temperature range were examined for the continental United States for the period 1948–88. The data used are a subset of the Historical Climatology Network (1036 stations) and a stream gage network of 1009 stations. Trend significance was determined using the nonparametric seasonal Kendall's test on a monthly and annual basis, and a robust slope estimator was used for determination of trend magnitudes. A bivariate test was used for evaluation of relative changes in the variables, specifically, streamflow relative to precipitation, streamflow relative to temperature, and precipitation relative to temperature.

Strong trends were found in all of the variables at many more stations than would be expected due to chance. There is a strong spatial and seasonal structure in the trend results. For instance, although annual temperature increases were found at many stations, mostly in the North and West, there were almost as many downtrends, especially in the South and East. Among the most important trend patterns are (a) increases in March temperature at almost half of the stations; (b) increases in precipitation from September through December at as many as 25 percent of the stations, mostly in the central part of the country; (c) strong increases in streamflow in the period November–April at a maximum of almost half of the stations, with the largest trend magnitudes in the north-central states; (d) changes in the temperature range (mostly downward) at a large number of stations beginning in late spring and continuing through winter, affecting as many as over half of the stations. The observed trends in streamflow are not entirely consistent with the changes in the climatic variables and may be due to a combination of climatic and water management effects.

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Eric P. James
,
Stanley G. Benjamin
, and
Brian D. Jamison

Abstract

Weather observations from commercial aircraft constitute an essential component of the global observing system and have been shown to be the most valuable observation source for short-range numerical weather prediction (NWP) systems over North America. However, the distribution of aircraft observations is highly irregular in space and time. In this study, we summarize the recent state of aircraft observation coverage over the globe and provide an updated quantification of its impact upon short-range NWP forecast skill. Aircraft observation coverage is most dense over the contiguous United States and Europe, with secondary maxima in East Asia and Australia/New Zealand. As of late November 2019, 665 airports around the world had at least one daily ascent or descent profile observation; 400 of these come from North American or European airports. Flight reductions related to the COVID-19 pandemic have led to a 75% reduction in aircraft observations globally as of late April 2020. A set of data denial experiments with the latest version of the Rapid Refresh NWP system for recent winter and summer periods quantifies the statistically significant positive forecast impacts of assimilating aircraft observations. A special additional experiment excluding approximately 80% of aircraft observations reveals a reduction in forecast skill for both summer and winter amounting to 30%–60% of the degradation seen when all aircraft observations are excluded. These results represent an approximate quantification of the NWP impact of COVID-19-related commercial flight reductions, demonstrating that regional NWP guidance is degraded as a result of the decreased number of aircraft observations.

Open access
Eric A. Hendricks
,
James D. Doyle
,
Stephen D. Eckermann
,
Qingfang Jiang
, and
P. Alex Reinecke

Abstract

During austral winter, and away from orographic maxima or “hot spots,” stratospheric gravity waves in both satellite observations and Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data reveal enhanced amplitudes in a broad midlatitude belt extending across the Southern Ocean from east of the Andes to south of New Zealand. The peak latitude of this feature slowly migrates poleward from 50° to 60°S. Wave amplitudes are much weaker across the midlatitude Pacific Ocean. These features of the wave field are in striking agreement with diagnostics of baroclinic growth rates in the troposphere associated with midlatitude winter storm tracks and the climatology of the midlatitude jet. This correlation suggests that these features of the stratospheric gravity wave field are controlled by geographical variations of tropospheric nonorographic gravity wave sources in winter storm tracks: spontaneous adjustment emission from the midlatitude winter jet, frontogenesis, and convection.

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Eric A. Hendricks
,
Wayne H. Schubert
,
Richard K. Taft
,
Huiqun Wang
, and
James P. Kossin

Abstract

The asymmetric dynamics of potential vorticity mixing in the hurricane inner core are further advanced by examining the end states that result from the unforced evolution of hurricane-like vorticity rings in a nondivergent barotropic model. The results from a sequence of 170 numerical simulations are summarized. The sequence covers a two-dimensional parameter space, with the first parameter defining the hollowness of the vortex (i.e., the ratio of eye to inner-core relative vorticity) and the second parameter defining the thickness of the ring (i.e., the ratio of the inner and outer radii of the ring). In approximately one-half of the cases, the ring becomes barotropically unstable, and there ensues a vigorous vorticity mixing episode between the eye and eyewall. The output of the barotropic model is used to (i) verify that the nonlinear model approximately replicates the linear theory of the fastest-growing azimuthal mode in the early phase of the evolution, and (ii) characterize the end states (defined at t = 48 h) that result from the nonlinear chaotic vorticity advection and mixing. It is found that the linear stability theory is a good guide to the fastest-growing exponential mode in the numerical model. Two additional features are observed in the numerical model results. The first is an azimuthal wavenumber-2 deformation of the vorticity ring that occurs for moderately thick, nearly filled rings. The second is an algebraically growing wavenumber-1 instability (not present in the linear theory because of the assumed solution) that is observed as a wobbling eye (or the trochoidal oscillation for a moving vortex) for thick rings that are stable to all exponentially growing instabilities. Most end states are found to be monopoles. For very hollow and thin rings, persistent mesovortices may exist for more than 15 h before merging to a monopole. For thicker rings, the relaxation to a monopole takes longer (between 48 and 72 h). For moderately thick rings with nearly filled cores, the most likely end state is an elliptical eyewall. In this nondivergent barotropic context, both the minimum central pressure and maximum tangential velocity simultaneously decrease over 48 h during all vorticity mixing events.

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Daniel P. Brown
,
John L. Beven
,
James L. Franklin
, and
Eric S. Blake

Abstract

The 2008 Atlantic hurricane season is summarized and the year’s tropical cyclones are described. Sixteen named storms formed in 2008. Of these, eight became hurricanes with five of them strengthening into major hurricanes (category 3 or higher on the Saffir–Simpson hurricane scale). There was also one tropical depression that did not attain tropical storm strength. These totals are above the long-term means of 11 named storms, 6 hurricanes, and 2 major hurricanes. The 2008 Atlantic basin tropical cyclones produced significant impacts from the Greater Antilles to the Turks and Caicos Islands as well as along portions of the U.S. Gulf Coast. Hurricanes Gustav, Ike, and Paloma hit Cuba, as did Tropical Storm Fay. Haiti was hit by Gustav and adversely affected by heavy rains from Fay, Ike, and Hanna. Paloma struck the Cayman Islands as a major hurricane, while Omar was a major hurricane when it passed near the northern Leeward Islands. Six consecutive cyclones hit the United States, including Hurricanes Dolly, Gustav, and Ike. The death toll from the Atlantic tropical cyclones is approximately 750.

A verification of National Hurricane Center official forecasts during 2008 is also presented. Official track forecasts set records for accuracy at all lead times from 12 to 120 h, and forecast skill was also at record levels for all lead times. Official intensity forecast errors in 2008 were below the previous 5-yr mean errors and set records at 72–120 h.

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Stanley G. Benjamin
,
Eric P. James
,
Edward J. Szoke
,
Paul T. Schlatter
, and
John M. Brown

Abstract

The Marshall Fire on 30 December 2021 became the most destructive wildfire costwise in Colorado history as it evolved into a suburban firestorm in southeastern Boulder County, driven by strong winds and a snow-free and drought-influenced fuel state. The fire was driven by a strong downslope windstorm that maintained its intensity for nearly 11 hours. The southward movement of a large-scale jet axis across Boulder County brought a quick transition that day into a zone of upper-level descent, enhancing the midlevel inversion providing a favorable environment for an amplifying downstream mountain wave. In several aspects, this windstorm did not follow typical downslope windstorm behavior. NOAA rapidly updating numerical weather prediction guidance (including the High-Resolution Rapid Refresh) provided operationally useful forecasts of the windstorm, leading to the issuance of a High-Wind Warning (HWW) for eastern Boulder County. No Red Flag Warning was issued due to a too restrictive relative humidity criterion (already published alternatives are recommended); however, owing to the HWW, a countywide burn ban was issued for that day. Consideration of spatial (vertical and horizontal) and temporal (both valid time and initialization time) neighborhoods allows some quantification of forecast uncertainty from deterministic forecasts—important in real-time use for forecasting and public warnings of extreme events. Essentially, dimensions of the deterministic model were used to roughly estimate an ensemble forecast. These dimensions including run-to-run consistency are also important for subsequent evaluation of forecasts for small-scale features such as downslope windstorms and the tropospheric features responsible for them, similar to forecasts of deep, moist convection and related severe weather.

Significance Statement

The Front Range windstorm of 30 December 2021 combined extreme surface winds (>45 m s−1) with fire ignition resulting in an extraordinary and quickly evolving, extremely destructive wildfire–urban interface fire event. This windstorm differed from typical downslope windstorms in several aspects. We describe the observations, model guidance, and decision-making of operational forecasters for this event. In effect, an ensemble forecast was approximated by use of a frequently updated deterministic model by operational forecasters, and this combined use of temporal, spatial (horizontal and vertical), and other forecast dimensions is suggested to better estimate the possibility of such extreme events.

Open access
Eric M. Kemp
,
Jerry W. Wegiel
,
Sujay V. Kumar
,
James V. Geiger
,
David M. Mocko
,
Jossy P. Jacob
, and
Christa D. Peters-Lidard

Abstract

This article describes a new precipitation analysis algorithm developed by NASA for time-sensitive operations at the United States Air Force. Implemented as part of the Land Information System—a land modeling and data assimilation software framework—this NASA–Air Force Precipitation Analysis (NAFPA) combines numerical weather prediction model outputs with rain gauge measurements and satellite estimates to produce global, gridded 3-h accumulated precipitation fields at approximately 10-km resolution. Input observations are subjected to quality control checks before being used by the Bratseth analysis algorithm that converges to optimal interpolation. NAFPA assimilates up to 3.5 million observations without artificial data thinning or selection. To evaluate this new approach, a multiyear reanalysis is generated and intercompared with eight alternative precipitation products across the contiguous United States, Africa, and the monsoon region of eastern Asia. NAFPA yields superior accuracy and correlation over low-latency (up to 14 h) alternatives (numerical weather prediction and satellite retrievals), and often outperforms high-latency (up to 3.5 months) products, although the details for the latter vary by region and product. The development of NAFPA offers a high-quality, near-real-time product for use in meteorological, land surface, and hydrological research and applications.

Significance Statement

Precipitation is a key input to land modeling systems due to effects on soil moisture and other parts of the hydrologic cycle. It is also of interest to government decision-makers due to impacts on human activities. Here we present a new precipitation analysis based on available near-real-time data. By running the program for prior years and comparing with alternative products, we demonstrate that our analysis provides better accuracy and usually less bias than near-real-time satellite data alone, and better accuracy and correlation than data provided by numerical weather models. Our analysis is also competitive with other products created months after the fact, justifying confidence in using our analysis in near-real-time operations.

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