A historical review of research on sea fog is presented. The period of interest is essentially the twentieth century, beginning with the celebrated work of G. I. Taylor in the aftermath of the Titanic tragedy. It has been argued that relative maxima in fog frequency over the North Atlantic (including the British Isles and the Grand Banks of Newfoundland) and the North Pacific (including the U.S. West Coast) has led to major contributions by scientists in England and the United States. The early work (pre-World War II) tended to be phenomenological—that is, conceptual with broad inference from statistical summaries. Yet, this early work laid the foundation for the numerical modeling that came with the advent of computers in the postwar period. The subtleties associated with sea fog formation and maintenance are explored by analyzing some of the results from the numerical simulations. The essay ends with a speculative view on our prospects for a more complete understanding of sea fog in light of the earlier contributions.

Abstract

The return of tropical air from the Gulf of Mexico is examined in the autumnal cool season. Results from the thermodynamic equilibrium model of Betts and Ridgway are used to calculate the equilibrium equivalent potential temperature (θ _e ) over the gulf and the northwestern Caribbean Sea. With a climatological study as a backdrop, a case of severe weather outbreak in mid-November 1988 is analyzed with emphasis on the analysis of low-level θ _e that flowed into the storm region from the Gulf of Mexico.

The primary results of the study are the following:

1. The climatological distribution of equilibrium θ _e over the gulf and the Caribbean in November serves as a useful tool for the analysis of the 1988 case study.

2. Between 5 and 15 November 1988, equilibrium in the marine layer was established over the gulf due to the absence of any deep cold-air penetrations during this period.

3. The high-valued θ _e that streamed into the severe storm region on 15 November 1988 tracked from the Yucatán straits and the northwestern Caribbean over a three-day period.

4. This air was able to maintain its high-θ _e property because of an anomalously warm gulf.

5. Significant increases in available energy for deep convection could have been anticipated by means of the upper bounds on coastal θ _e predicted by the Betts and Ridgway formulation, which was supported by observations along the Texas coast.

Abstract

A novel hybrid vertical mixing scheme, based jointly on the Kraus–Turner-type mixed layer model and Price's dynamic instability model, is introduced to aid in parameterization of vertical turbulent mixing in numerical ocean models. The scheme is computationally efficient and is capable of simulating the three major mechanisms of vertical turbulent mixing in the upper ocean, that is, wind stirring, shear instability, and convective overturning.

The hybrid scheme is first tested in a one-dimensional model against the Kraus–Turner-type bulk mixed layer model and the Mellor–Yamada level 2.5 (MY2.5) turbulence closure model. As compared with those two models, the hybrid model behaves more reasonably in both idealized experiments and realistic simulations. The improved behavior of the hybrid model can be attributed to its more complete physics. For example, the MY2.5 model underpredicts mixed layer depth at high latitudes due to its lack of wind stirring and penetrative convection, while the Kraus–Turner bulk model produces rather shallow mixed layers in the equatorial region because of its lack of shear-produced mixing. The hybrid model reproduces the good results of the MY2.5 model toward the equator and the bulk model toward high latitudes, thereby taking the advantages of those two models while avoiding their shortcomings.

The hybrid scheme is then implemented in a three-dimensional model of the tropical Pacific Ocean. This leads to an improved simulation of the large-scale equatorial circulation. Compared with the other two commonly used mixing schemes tested in this experiment, the hybrid scheme helps to produce more realistic velocity profiles in the eastern and central equatorial Pacific. This is mainly due to the improved parameterization of interior mixing related to the large shears of the Equatorial Undercurrent. Another feature in this model that is sensitive to the vertical mixing scheme is the equatorial instability waves; in the eastern Pacific Ocean these waves are most energetic when the hybrid scheme is used. The meridional heat flux associated with these waves can be locally important in the mixed layer heat budget.

Abstract

A numerical model is designed to study the effects of the strong, near-surface associated with the equatorial current system on energy transmission of time-periodic equatorial waves into the deep mean. The present paper is confined to long wavelength, low-frequency Kelvin waves forced by a longitudinally confined patch of zonal wind. Energy transmission into the deep ocean is investigated as a function of mean current shear amplitude and geometry and the forcing frequency.

Solutions form well-defined beams of energy that radiate energy eastward and vertically toward the deep ocean in the absence of mean flow. However, the presence of critical surfaces associated with mean currents inhibits low-frequency energy from reaching the deep ocean. For a given zonal wavenumber, longitudinal propagation through mean currents will be less inhibited as the frequency increases (phase speed increases). When the mean current amplitude is large enough, the beam encounters multiple critical surfaces (i.e., critical surfaces for different wavenumber components of the beam) where significant and momentum can take place with the men currents via Reynolds stress transfers. Work against the dominant vertical shear is the dominant wave energy loss for the case of a mean South Equatorial Current–Equatorial Undercurrent system, illustrating the need for high vertical resolution in equatorial ocean models.

The model also describes the possible induction of a mean zonal acceleration as well as a mean meridional circulation. Eliassen-Palm fluxes are used to diagnose these dynamics. The presence of critical surfaces result in mean field accelerations on the equator above the core of the Equatorial Undercurrent. Implications of these results with regard to observations in the equatorial waveguide are discussed.

Abstract

Nimbus II High Resolution Infrared Radiometer (HRIR) data, sensitive in the 3.4–4.2 μ window, were analyzed over several oceanic regions. Current boundaries such as the north wall of the Gulf Stream have been located consistently within 10 km of the positions indicated by airplane radiation data. With present techniques, primarily designed for meteorological purposes, the Gulf Stream boundary has been seen, at least in significant parts, in about 50 out of 175 days. Similar results have also been obtained in analyses of the Agulhas Current boundary, and the boundary between the Brazil and Falkland Currents. The satellite radiation observations suggest that the Brazil-Falkland Current boundary which is associated with a surface temperature gradient is as sharp and strong as the Gulf Stream North Wall. The Agulhas Current exhibits a similar temperature gradient along its western boundary, separating it from the Benguela Current surface waters.

Comparisons of equivalent blackbody temperatures over the Gulf Stream from Nimbus II with low flying radiometer-equipped aircraft showed that the satellite data were on the average 0.5C warmer.

Seasonal sea surface temperature variations of 9C over the Persian Gulf and Somali region and the upwelling along the Somali Coast during the southwest monsoon were clearly detected in the nighttime HRIR data.

Daytime observations within the 3.4–4.2 μ window have also shown qualitatively the location of major current boundaries.

Abstract

A reduced-gravity, primitive-equation, upper-ocean general circulation model is used to study the mean water pathways in the North Pacific subtropical and tropical ocean. The model features an explicit physical representation of the surface mixed layer, realistic basin geometry, observed wind and heat flux forcing, and a horizontal grid-stretching technique and a vertical sigma coordinate to obtain a realistic simulation of the subtropical/tropical circulation. Velocity fields, and isopycnal and trajectory analyses are used to understand the mean flow of mixed layer and thermocline waters between the subtropics and Tropics.

Subtropical/tropical water pathways are not simply direct meridional routes; the existence of vigorous zonal current systems obviously complicates the picture. In the surface mixed layer, upwelled equatorial waters flow into the subtropical gyre mainly through the midlatitude western boundary current (the model Kuroshio). There is additionally an interior ocean pathway, through the Subtropical Countercurrent (an eastward flow across the middle of the subtropical gyre), that directly feeds subtropical subduction sites. Below the mixed layer, the water pathways in the subtropical thermocline essentially reflect the anticyclonic gyre circulation where we find that the model subtropical gyre separates into two circulation centers. The surface circulation also features a double-cell pattern, with the poleward cell centered at about 30°N and the equatorward component contained between 15° and 25°N. In addition, thermocline waters that can be traced to subtropical subduction sites move toward the Tropics almost zonally across the basin, succeeding in flowing toward the equator only along relatively narrow north–south conduits. The low-latitude western boundary currents serve as the main southward circuit for the subducted subtropical thermocline water. However, the model does find a direct flow of thermocline water into the Tropics through the ocean interior, confined to the far western Pacific (away from the low-latitude western boundary currents) across 10°N. This interior pathway is found just to the west of a recirculating gyre in and just below the mixed layer in the northeastern Tropics. This equatorward interior flow and a flow that can be traced directly to the western boundary are then swept eastward by the deeper branches of the North Equatorial Countercurrent, finally penetrating to the equator in the central and eastern Pacific. Most of these results are consistent with available observations and recently published theoretical and idealized numerical experiments, although the interior pathway of subtropical thermocline water into the Tropics found in this experiment is not apparent in other published numerical simulations.

Potential vorticity dynamics are useful in explaining the pathways taken by subtropical thermocline water as it flows into the Tropics. In particular, a large-scale zonally oriented “island” of homogenous potential vorticity, whose signature is determined by thin isopycnal layers in the central tropical Pacific along about 10°N, is dynamically linked to a circulation that does not flow directly from the subtropics to the Tropics. This large-scale potential vorticity feature helps to explain the circuitous pathways of the subducted subtropical thermocline waters as they approach the equator. Consequently, waters must first flow westward to the western boundary north of these closed potential vorticity contours and then mostly move southward through the low-latitude western boundary currents, flow eastward with the North Equatorial Countercurrent, and finally equatorward to join the Equatorial Undercurrent in the thermocline.

Abstract

The spatial and temporal evolution of supercooled water fields in ten wintertime storm systems occurring over the northern Colorado Rocky Mountain region have been examined using data collected by the recently developed scanning dual-channel microwave radiometer. These data were supported by several independent datasets including vertically pointing radar data, mountaintop liquid water content measurements, low and high altitude measurements of crystal rime characteristic rawinsonde data and precipitation intensity measurements.

The ten case studies discussed in this paper represent various stages in the synoptic scale evolution of storms that affect the northern Colorado Rockies. Liquid water was found to occur in nearly all stages of most of these storms. The temporal variations in the magnitude of the liquid water content were significant.

Three common features concerning the evolution of the liquid water field were observed in the prefrontal cloud systems: 1) an inverse relationship between precipitation rate and liquid water content occurred; 2) a direct relationship between cloud top temperature and liquid water content was observed; and 3) the magnitude of the liquid water content was consistently higher over the mountain slopes.

In the postfrontal cloud systems studied, the liquid water content exhibited little variability upwind of the mountain base but varied considerably in the vicinity of the mountain. In these three storms, the magnitude of the liquid water content over the ridge was inversely related to the precipitation rate at mountain base. Liquid water production near the ridgeline was associated with both orographic and convective forcing.

Three orographic cloud systems are discussed in this paper. These clouds formed in similar synoptic environments. The three systems were shallow, had tops warmer than −22°C, and had limited horizontal extent. As in the previous cases, the changes in the liquid water field were inversely associated with changes in precipitation rate. In one case, a decrease in liquid water content was also associated with a decrease in cloud top temperature.

Abstract

This study focuses on the meso-α- and meso-β-scale manifestations of the latent-heat-induced reduction of windward-side blocking to two flood-producing precipitation events on the leeside of the Sierra Nevada. Two simulations were performed—one employing full microphysics [control (CTRL)] and a second in which the latent heating terms are turned off in the microphysics [no latent heating (NLH)]. The differences between the CTRL and NLH are consistent with upstream latent heating—the moist, divergent, and ascending flow dominates the leeside of the mountain range in the CTRL producing copious spillover precipitation while dry high-momentum/downslope-descending flow dominates the NLH simulation on the leeside. A comprehensive sequence of events for spillover precipitation is formulated as follows: 1) Ascent within the exit region of a polar jet streak develops in response to velocity divergence aloft. 2) This ascent phases with ascent from the windward-side flow to create a mesoscale region of heavy upslope precipitation. 3) The latent heat release from the upslope precipitation reduces the upstream static stability and blocking. 4) A mesoscale ridge in the thickness field builds in the upper troposphere and induces subgeostrophic flow in the jet’s exit region above the mountain range. 5) Adjustments to this ridge result in a cross-mountain midlevel jet that facilitates a river of midlevel moisture advected over to the leeside. 6) Stretching of moist isentropic surfaces in proximity to the plume of moisture fluxes causes destabilization on the leeside and formation of a leeside mesolow. 7) Boundary layer air accelerates into the leeside mesolow to form a mountain-parallel low-level flow.

Abstract

A feed forward neural network (FFNN) is developed for tropical cyclone (TC) intensity prediction, where intensity is defined as the maximum 1-min average 10-m wind speed. This deep learning model incorporates a real-time operational estimate of the current intensity and predictors derived from Hurricane Weather Research and Forecasting (HWRF; 2017 version) Model forecasts. The FFNN model is developed with the operational constraint of being restricted to 6-h-old HWRF data. Best track intensity data are used for observational verification. The forecast training data are from 2014 to 2016 HWRF reforecast data and cover a wide variety of TCs from both the Atlantic and eastern Pacific Ocean basins. Cross validation shows that the FFNN increasingly outperforms the operational observation-adjusted HWRF (HWFI) in terms of mean absolute error (MAE) at forecast lead times from 3 to 57 h. Out-of-sample testing on real-time data from 2017 shows the HWFI produces lower MAE than the FFNN at lead times of 24 h or less and similar MAEs at later lead times. On the other hand, the 2017 data indicate significant potential for the FFNN in the prediction of rapid intensification (RI), with RI defined here as an intensification of at least 30 kt (1 kt ≈ 0.51 m s⁻¹) in a 24-h period. The FFNN produces 4 times the number of hits in HWFI for RI. While the FFNN has more false alarms than the HWFI, Brier skill scores show that, in the Atlantic, the FFNN has significantly greater skill than the HWFI and probabilistic Statistical Hurricane Intensity Prediction System RI index.

Abstract

The synoptic structure of two case studies of heavy “spillover” or leeside precipitation—1–2 January 1997 and 30–31 December 2005—that resulted in Truckee River flooding are analyzed over the North Pacific beginning approximately 7 days prior to the events. Several sequential cyclone-scale systems are tracked across the North Pacific, culminating in the strengthening and elongation of a polar jet stream’s deep exit region over northern California and Nevada. These extratropical cyclones separate extremely cold air from Siberia from an active intertropical convergence zone with broad mesoscale convective systems and tropical cyclones. The development of moisture surges resulting in leeside flooding precipitation over the Sierra Nevada is coupled to adjustments within the last wave in the sequence of cyclone waves. Stage I of the process occurs as the final wave moves across the Pacific and its polar jet streak becomes very long, thus traversing much of the eastern Pacific. Stage II involves the development of a low-level return branch circulation [low-level jet (LLJ)] within the exit region of the final cyclone scale wave. Stage III is associated with the low-level jet’s convergence under the upper-level divergence within the left exit region, which results in upward vertical motions, dynamic destabilization, and the development of mesoscale convective systems (MCSs). Stage IV is forced by the latent heating and subsynoptic-scale ridging caused by each MCS, which results in a region of diabatic isallobaric accelerations downstream from the MCS-induced mesoridge. During stage IV the convectively induced accelerating flow, well to the southeast of the upper-level jet core, organizes a midlevel jet and plume of moisture or midlevel atmospheric river, which is above and frequently out of phase with (e.g., southeast of) the low-level atmospheric river described in ahead of the surface cold front. Stage V occurs as the final sequential midlevel river arrives over the Sierra Nevada. It phases with the low-level river, allowing upslope and midlevel moisture advection, thus creating a highly concentrated moist plume extending from near 700 to nearly 500 hPa, which subsequently advects moisture over the terrain.

When simulations are performed without upstream convective heating, the horizontal moisture fluxes over the Sierra Nevada are reduced by ∼30%, indicating the importance of convection in organizing the midlevel atmospheric rivers. The convective heating acts to accelerate the midlevel jet flow and create the secondary atmospheric river between ∼500 and 700 hPa near the 305-K isentropic surface. This midlevel moisture surge slopes forward with height and transports warm moist air over the Sierra Nevada to typically rain shadowed regions on the lee side of the range. Both observationally generated and model-generated back trajectories confirm the importance of this convectively forced rapid lifting process over the North Pacific west of the California coast ∼12 h and ∼1200 km upstream prior to heavy leeside spillover precipitation over the Sierra Nevada.