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Michael Dickinson and John Molinari

Abstract

A large-amplitude mixed Rossby–gravity wave packet is identified in the western Pacific using 6–10-day bandpass-filtered winds. Individual disturbances of 2300–3000-km wavelength propagated westward as the packet moved slowly eastward. The packet first appeared, and subsequently amplified, within a region of active convection associated with the Madden–Julian oscillation (MJO), which was isolated by low-pass-filtered outgoing longwave radiation. The packet lasted about 5 weeks, then rapidly dispersed as the active MJO moved away from it to the east.

West of 150°E, individual disturbances within the packet turned northwestward away from the equator, indicating an apparent transition from mixed Rossby–gravity waves to off-equatorial tropical depression (TD)-type disturbances. Cyclones filled with cloud and anticyclones cleared during the transition. Nevertheless, convective structure consistent with mixed Rossby–gravity waves remained outside the circulation centers, and three tropical cyclones formed on the edges of three consecutive cyclonic gyres as they moved off the equator. Although the expected Rossby–Kelvin wave structure was present in the background winds within the active MJO, tropical cyclone genesis did not occur within the trailing Rossby gyres, but 2500 km to the west and north.

This case study provides evidence that equatorial modes, under the right conditions, can supply precursor disturbances for repeated formation of tropical cyclones. It is argued based on previous work in the literature that this sequence of events is not uncommon.

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Michael Dickinson and John Molinari

Abstract

A 10-yr climatology (1986–95) was performed using ECMWF gridded analyses on isentropic surfaces to identify regions where the lower-tropospheric meridional potential vorticity (PV) gradient changes sign across Africa and Australia during their respective summer seasons. While an African sign reversal has been documented, no similar study has been performed for the Australian region, which also has desert on the poleward side of open ocean. In each hemisphere, a northward decrease of PV is sufficient to produce a sign reversal.

It was found that PV decreases northward in the lower troposphere across northern Australia, with the maximum reversal on the 315-K surface. It had comparable magnitude but smaller zonal extent (∼3000 km) than that on the 320-K surface in Africa (∼5000 km). In each region the sign reversal was associated with cyclonic PV anomalies on the equatorward side and anticyclonic anomalies on the poleward side.

OLR was used as a proxy for deep convective heating in order to evaluate the total convective forcing of PV. The vertical distribution of heating was specified. In both regions the maximum total convective forcing of PV was largest on the equatorward edge of the sign reversal region. The effects of dry convection were not included in the PV budget. Dry convection, located poleward of the maximum deep convection, acts as a lower-tropospheric PV sink and produces anticyclonic PV anomalies. In both regions these anticyclonic anomalies were larger in magnitude and areal coverage than the cyclonic anomalies associated with deep convection.

The potential instability implied by the sign reversal regions has traditionally been associated with the growth of easterly waves. In support of this argument, bandpass-filtered (2–6 day) meridional wind variance on the 320-K surface nearly triples from east to west along the African sign reversal. In Australia, little evidence was found of such waves in the 2–10-day meridional wind variance. Possible explanations for the lack of growing disturbances over Australia are discussed.

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Michael J. Dickinson and David J. Knight

Abstract

A two-dimensional, hydrostatic, nearly adiabatic primitive equation model is used to study the evolution of a front passing across topography. Frontogenesis is forced by shearing deformation associated with the nonlinear evolution of an Eady wave. This study extends previous work by including an upper-level potential vorticity (PV) anomaly and a growing baroclinic wave in a baroclinically unstable basic state.

Results for the Eady wave simulations show that the mountain retards and blocks the approaching front at the surface while the upper-level PV anomaly associated with the front moves across the domain unaffected. Warm advection ahead of the lee trough forces convergence and cyclonic vorticity growth near the base of the lee slope. This vorticity growth is further encouraged by the approach of the upper-level PV anomaly. The upper-level PV anomaly then couples with this new surface vorticity center and propagates downstream. The original surface front remains trapped on the windward slope. Thus when the upstream blocking is strong, frontal propagation is discontinuous across the ridge. This evolution occurs for tall mountains and narrow mountains, as well as weak fronts. For low mountains, wide mountains, and strong fronts, only weak retardation is observed on the windward slope. The surface front remains coupled with the upper-level PV anomaly. The front moves continuously across the mountain.

The net result, regardless of mountain size and shape, is that the front reaches the base of the lee slope stronger, sooner, and with a decreased cross-front scale compared to the “no-mountain” case. Well downstream of the mountain, no position change of the surface front is observed.

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Eric D. Maloney and Michael J. Dickinson

Abstract

The tropical intraseasonal oscillation (ISO) causes variations in the large-scale flow over the western North Pacific during June–August that strongly impact the energetics of tropical depression (TD)-type disturbances. An energetics analysis is conducted with NCEP–NCAR reanalysis data during June–August of 1979–2001. Composite TD-type disturbance perturbation kinetic energy (PKE) is significantly higher during ISO 850-hPa westerly periods than during ISO 850-hPa easterly periods. ISO westerly periods are associated with enhanced barotropic conversion and enhanced perturbation available potential energy (PAPE) to PKE conversion. ISO easterly periods are characterized by diminished TD-type PKE, negligible barotropic conversion, and weakened PAPE to PKE conversions, as compared to composite TD-type disturbances during ISO westerly periods and the entire June–August record. Barotropic conversion accounts for a larger fraction of the PKE generation during ISO westerly periods than during the entire June–August record, and vertically averaged barotropic conversion during ISO westerly periods is 3–4 times that during ISO easterly periods. Barotropic conversion during ISO westerly periods maximizes in the lower troposphere, coincident with the maximum in TD-type disturbance kinetic energy. PAPE to PKE conversion maximizes in the upper troposphere, where it is redistributed to the lower-troposphere and tropopause levels, and horizontally, by the perturbation geopotential flux. PAPE is primarily generated through convective heating associated with the TD-type disturbances and is converted to PKE through the negative correlation of pressure velocity and temperature.

The effect of western Pacific ISO flow variations on the energy budgets of TD-type disturbances may help explain the ISO-related modulation of tropical cyclones observed by Liebmann et al. Energetic TD-type disturbances during ISO westerly periods may provide suitable seed disturbances from which tropical cyclones may form.

June–August TD-type disturbance structure and energetics (unstratified by ISO phase) were compared to the results of Lau and Lau, who used a different analysis product, lower-resolution dataset, and shorter data record. TD-type disturbance structure and energetics are consistent with those shown in Lau and Lau. The largest deviation in the present analysis from that of Lau and Lau is the strong destruction of PKE found at 150 hPa, a level not resolved in their study. Although the sign of the 150-hPa signal is consistent with southwest–northeast-tilted TD-type disturbances interacting with strongly sheared easterly flow aloft, the nonlinear nature of the energy budget calculations may also amplify the effects of unrelated variability.

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Kristen L. Corbosiero, Michael J. Dickinson, and Lance F. Bosart

Abstract

Forty-six years of summer rainfall and tropical cyclone data are used to explore the role that eastern North Pacific tropical cyclones (TCs) play in the rainfall climatology of the summer monsoon over the southwestern United States. Thirty-five TCs and their remnants were found to bring significant rainfall to the region, representing less than 10% of the total number of TCs that formed within the basin. The month of September was the most common time for TC rainfall to occur in the monsoon region as midlatitude troughs become more likely to penetrate far enough south to interact with the TCs and steer them toward the north and east. On average, the contribution of TCs to the warm-season precipitation increased from east to west, accounting for less than 5% of the rainfall in New Mexico and increasing to more than 20% in southern California and northern Baja California, with individual storms accounting for as much as 95% of the summer rainfall. The distribution of rainfall for TC events over the southwest United States reveals three main categories: 1) a direct northward track from the eastern Pacific into southern California and Nevada, 2) a distinct swath northeastward from southwestern Arizona through northwestern New Mexico and into southwestern Colorado, and 3) a broad area of precipitation over the southwest United States with embedded maxima tied to terrain features. Differences in these track types relate to the phasing between, and scales of, the trough and TC, with the California track being more likely with large cutoff cyclones situated off the west coast, the southwest–northeast track being most likely with mobile midlatitude troughs moving across the intermountain west, and the broad precipitation category generally exhibiting no direct interaction with midlatitude features.

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John Molinari, David Knight, Michael Dickinson, David Vollaro, and Steven Skubis

Abstract

A significant sign reversal in the meridional potential vorticity gradient was found during the summer of 1991 on the 310-K isentropic surface (near 700 mb) over the Caribbean Sea. The Charney–Stern necessary condition for instability of the mean flow is met in this region. It is speculated that the sign reversal permits either invigoration of African waves or actual generation of easterly waves in the Caribbean.

During the same season, a correlation existed between the strength of the negative potential vorticity gradient in the Caribbean and subsequent cyclogenesis in the eastern Pacific. The meridional PV gradient, convective heating measured by outgoing longwave radiation data, and eastern Pacific cyclogenesis all varied on the timescale of the Madden–Julian oscillation (MJO). It is hypothesized that upstream wave growth in the dynamically unstable region provides the connection between the MJO (or any other convective forcing) and the associated enhanced downstream tropical cyclogenesis.

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John Molinari, David Vollaro, Steven Skubis, and Michael Dickinson

Abstract

The genesis of Hurricane Hernan (1996) in the eastern Pacific was investigated using gridded analyses from the European Centre for Medium-Range Weather Forecasts and gridded outgoing longwave radiation. Hernan developed in association with a wave in the easterlies that could be tracked back to Africa in longitude–time plots of the filtered υ component of the wind (2–6-day period) at 700 mb. The wave crossed Central America near Lake Nicaragua with little change in its southwest–northeast tilt, but the most intense convection shifted from near the wave axis in the Caribbean to west of the wave axis in the Pacific. The wave intensified as it moved through a barotropically unstable background state (defined by a low-pass filter with a 20-day cutoff) in the western Caribbean and eastern Pacific. A surge in the southwesterly monsoons and enhanced convection along 10°N occurred to the west of the 700-mb wave in the Pacific and traveled with the wave. This had the effect of enhancing low-level vorticity over a wide region ahead of the 700-mb wave. Available evidence suggests that additional low-level vorticity was produced by enhanced flow from the north through the Isthmus of Tehuantepec as the 700-mb wave approached. Depression formation did not occur until 6–12 h after the 700-mb wave reached this region of large low-level vorticity in the Gulf of Tehuantepec.

Eastern Pacific SST and vertical wind shear magnitude are typically favorable for tropical cyclone development in Northern Hemisphere summer and early fall. Because the favorable mountain interaction and the surge in the low-level monsoons appear to relate directly to the wave in the easterlies, it is argued that the strength of such waves reaching Central America from the east is the single most important factor in whether subsequent eastern Pacific cyclogenesis occurs. Possible parallels with western Pacific cyclogenesis are discussed.

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James B. Elsner, Thomas H. Jagger, Michael Dickinson, and Dail Rowe

Abstract

Hurricanes cause drastic social problems as well as generate huge economic losses. A reliable forecast of the level of hurricane activity covering the next several seasons has the potential to mitigate against such losses through improvements in preparedness and insurance mechanisms. Here a statistical algorithm is developed to predict North Atlantic hurricane activity out to 5 yr. The algorithm has two components: a time series model to forecast average hurricane-season Atlantic sea surface temperature (SST), and a regression model to forecast the hurricane rate given the predicted SST value. The algorithm uses Monte Carlo sampling to generate distributions for the predicted SST and model coefficients. For a given forecast year, a predicted hurricane count is conditional on a sampled predicted value of Atlantic SST. Thus forecasts are samples of hurricane counts for each future year. Model skill is evaluated over the period 1997–2005 and compared against climatology, persistence, and other multiseasonal forecasts issued during this time period. Results indicate that the algorithm will likely improve on earlier efforts and perhaps carry enough skill to be useful in the long-term management of hurricane risk.

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Ty A. Dickinson, Michael B. Richman, and Jason C. Furtado

Abstract

Extreme precipitation across multiple time scales is a natural hazard that creates a significant risk to life, with a commensurately large cost through property loss. We devise a method to create 14-day extreme-event windows that characterize precipitation events in the contiguous United States (CONUS) for the years 1915–2018. Our algorithm imposes thresholds for both total precipitation and the duration of the precipitation to identify events with sufficient length to accentuate the synoptic and longer time scale contribution to the precipitation event. Kernel density estimation is employed to create extreme-event polygons that are formed into a database spanning from 1915 through 2018. Using the developed database, we clustered events into regions using a k-means algorithm. We define the “hybrid index,” a weighted composite of silhouette score and number of clustered events, to show that the optimal number of clusters is 15. We also show that 14-day extreme precipitation events are increasing in the CONUS, specifically in the Dakotas and much of New England. The algorithm presented in this work is designed to be sufficiently flexible to be extended to any desired number of days on the subseasonal-to-seasonal (S2S) time scale (e.g., 30 days). Additional databases generated using this framework are available for download from our GitHub. Consequently, these S2S databases can be analyzed in future works to determine the climatology of S2S extreme precipitation events and be used for predictability studies for identified events.

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Suzanne Dickinson, Kathryn A. Kelly, Michael J. Caruso, and Michael J. McPhaden

Abstract

There was an opportunity to compare 10 months of collocated National Aeronautics and Space Administration scatterometer (NSCAT) wind vectors with those from the Tropical Atmosphere Ocean (TAO) buoy array, located in the tropical Pacific Ocean. Over 5500 data pairs, from nearly 70 buoys, were collocated in the calibration/validation effort for NSCAT. These data showed that the wind speeds produced from the NSCAT-1 model function were low by about 7%–8% compared with TAO buoy winds. The revised model function, NSCAT-2, produces wind speeds with a bias of about 1%. The scatterometer directions were within 20° (rms), meeting accuracy requirements, when compared to TAO data. The mean direction bias between the TAO and the NSCAT vectors (regardless of model function) is about 9° with the scatterometer winds to the right of the TAO winds, which may be due to swell. The statistics of the two datasets are discussed, using component biases in lieu of the speed bias, which is naturally skewed. Using ocean currents and buoy winds measured along the equator, it is shown that the scatterometer measures the wind relative to the moving ocean surface. In addition, a systematic effect of rain on the NSCAT wind retrievals is noted. In all analyses presented here, winds less than 3 m s−1 are removed, due to the difficulty in making accurate low wind measurements.

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