Abstract

Mesoscale height and temperature fields can be extracted from the observed wind field by making use of the full divergence equation. Mass changes associated with irrotational ageostrophic motions are retained for a nearly complete description of the height field. Above the boundary layer, in the absence of friction, the divergence equation includes terms composed of the components of the wind and a Laplacian of the geopotential height field. Once the mass field is determined, the thermal structure is obtained through application of the hypsometric equation.

In this paper an error analysis of this divergence method is undertaken to estimate the potential magnitude of errors associated with random errors in the wind data. Previous applications of the divergence method have been refined in the following ways. (i) The domain over which the method is applied is expanded to encompass the entire STORM-FEST domain. (ii) Wind data from 23 profiler and 38 rawinsonde sites are combined in the analysis. (iii) Observed profiler and rawinsonde data are interpolated to grid points through a modified objective analysis, and (iv) the variation in elevation of the profiler sites is taken into account.

The results of the application of the divergence method to the combined wind data from profiler and rawinsonde sites show good agreement between the retrieved heights and temperatures and the observed values at rawinsonde sites. Standard deviations of the difference between the retrieved and observed data lie well within the precision of the rawinsonde instruments. The difference field shows features whose magnitude is significantly larger than the errors predicted by the error analysis, and these features are systematic rather than random in nature, suggesting that the retrieved fields are able to resolve mesoscale signatures not fully captured by the rawinsonde data alone.

The divergence method is also applied solely to the profiler data to demonstrate the potential of the divergence method to provide mass and thermal fields on a routine basis at synoptic times when operational rawinsonde data are not available. A comparison of the heights derived from the profiler winds with those independently measured by rawinsondes indicates that valuable information on the evolution of atmospheric height and temperature fields can be retrieved between conventional rawinsonde release times through application of the divergence method. The implications of the results for applications of the method in weather analysis and in numerical weather prediction are discussed.

Abstract

Satellite imagery and rain gauge data are combined to create mesoscale detail in the initial states of relative humidity (RH) and surface moisture availability (M) for a mesoscale model simulation. The most profound impact of inserting the mesoscale initial fields was the development of a strong vertical circulation transverse to an intensifying cold front that triggered an intense frontal rainband similar to a severe squall line that was observed to develop explosively. This paper explores the causative factors leading to the formation of this intense circulation and the sensitivity of the model to the mesoscale initial fields.

A substantial gradient in the initialized RH and M fields occurred across the cold front in the region where the observed frontal squall line formed. In contrast to the control run, the model simulations that incorporated the mesoscale initial analysis displayed considerable daytime warming just ahead of the front. This warming was due principally to a reduction in the RH (and, hence, low-level cloud cover) east of the front, although an increase in the cross-frontal M gradient did contribute about 25% of the warming. Increased sensible heat fluxes at the expense of decreased latent heat fluxes led to a much deeper and well-mixed prefrontal boundary layer, a more erect frontal surface, and an updraft jet just ahead of the front. A density current–like flow developed in the cold air immediately behind the front only in the presence of this cross-frontal gradient in sensible heating. Much improved forecasts of the location and timing of the frontal squall line and other precipitation systems resulted from the mesoscale initial analysis. The initial RH and M fields possessed sufficient resolution and consistency with the model dynamics to have a positive influence on the forecasts for a period of at least 12 h.

This study provides evidence that differential cloud cover and evapotranspiration fields can have important impacts on frontal behavior when strong synoptic dynamics are present. Future research should attempt to improve the modeling of evapotranspiration processes, develop more objective satellite-based humidity analysis techniques, and obtain in situ mesoscale data for verification of the retrieved atmospheric and soil moisture fields.

Abstract

Radar and satellite imagery suggest that strong mesoscale forcing occurred in the Palm Sunday tornado outbreak on 27 March 1994. Parallel lines of severe thunderstorms within each of three mesoscale convective systems developed just ahead of a cold front in Mississippi and Alabama on this date. Analyses of routine meteorological observations, barograph data, and forecasts from the Eta and NGM models and a mesoscale research model (MASS) are used to examine the relative roles of large-scale dynamics and mesoscale processes in triggering and organizing the mesoscale convection.

Quasigeostrophic forcing was absent in the outbreak region. Likewise, a thermally direct circulation system transverse to the upper-level jet that was present to the northwest of the outbreak region was decoupled from the strong low-level ascent occurring in northern Alabama and Mississippi at the time of the outbreak. Strong ageostrophic frontogenesis in the presence of conditional symmetric instability (CSI) was the chief cause for the intense low-level ascent along and behind the front, consistent with the line of severe storms that developed explosively along the front and an observed postfrontal precipitation band. However, the strongest supercells developed in segmented lines 100–200 km ahead of and parallel to the frontal boundary in an atmosphere that the MASS model indicates was inertially unstable due to a mesoscale midlevel jetlet. Analysis suggests that these storms developed in a manner consistent with the predictions of asymmetric inertial instability theory in the presence of convective instability.

Several mesolows were observed to have traveled along the frontal boundary and to have played a key role in focusing the frontogenesis. Similar frontal mesolows were simulated by the MASS model. Strong low-level ascent in the presence of conditional instability helped to deepen the mesolows, but they were strongly modulated by a train of gravity waves propagating on the cold side of the front. A combination of ducting and wave-CISK (conditional instability of the second kind) processes maintained the waves, which remained coupled to the jetlets as they propagated from intense convection in northeastern Texas. A time-to-space conversion objective analysis of bandpass-filtered barograph data reveals that similar waves emanated from this same region.

The lifting patterns produced by the complex interactions between the gravity waves, CSI, asymmetric inertial instability, and frontogenesis satisfactorily explains the development, configuration, spacing, and relative movement of the severe mesoconvective systems on Palm Sunday. All of these mesoscale phenomena were coupled to or strongly influenced by the jetlets, which were produced by strong convection at an earlier time within the region of quasigeostrophic forcing far removed from the tornado outbreak.

Abstract

A two-dimensional, nonhydrostatic, elastic numerical model has been used to study the generation of gravity waves for a stably stratified shear flow over an obstacle. When a low-level wind shear is included in the simulation, we find that the predictions for noticeable upstream effects based on Froude number for a uniform flow are no longer accurate. Upstream effects are encountered in the form of upstream propagating columnar disturbances and internal bores away from the obstacle. The limited parameter space studies conducted in this study suggest that the ratio of the shear depth to the obstacle height (d/H), the obstacle aspect ratio (H/L), and the Froude number (U/NH) are instrumental in determining the strength and the existence of these upstream disturbances. Thus, the present theoretical and empirical understanding of the importance of the Froude number for determining the nature of upstream effects should be modified substantially to include additional nondimensional parameters when shear is present.

Abstract

Appalachian cold-air damming (CAD) is characterized by the development of a cool, stable air mass that is advected southwestward along the eastern slopes of the Appalachian Mountains by low-level ageostrophic flow. Operational forecasters have identified the demise of CAD as a major forecasting challenge, in part because numerical weather prediction models have a tendency to erode the cold air too quickly. Previous studies have considered the role of clouds and precipitation in the initiation and maintenance of CAD; generally, precipitation is thought to reinforce CAD due to the cooling and stabilization resulting from evaporation. Here, the impact of precipitation on CAD during a situation where the lower-tropospheric air mass was near saturation prior to the arrival of precipitation is considered.

Previous studies have indicated that the passage of a cold front can bring about CAD demise, as the synoptic-scale flow becomes northwesterly behind the front and low-level stable air is scoured. Additional complexity is evident in the case of split cold fronts (or cold fronts aloft). In these situations, precipitation bands are found well to the east of the surface cold front and may be accompanied by severe weather. Here, the impact of a split-front rainband on a mature CAD event from 14 February 2000 is investigated.

The coastal front, marking the eastern boundary of the CAD region, made significant inland progress as the split-front rainband passed. Computations from Eta Model forecast fields revealed substantial latent heat release above the cold dome during the passage of the rainband. The CAD cold dome persisted longer in an MM5 model numerical simulation in which the effects of latent heat were withheld relative to both a full-physics control run and to observations. A third model simulation where the low levels of the cold dome were initially dried showed that once saturation occurred, the cold dome began to erode. Analysis of model output and observations suggests that, in this case, precipitation contributed to the retreat of the cold dome through lower-tropospheric pressure falls, an isallobaric wind response, and a resultant inland jump of the coastal front.

Abstract

A study of three years of GOES satellite imagery has been conducted to determine whether synthesis of the imagery with surface diagnostic analyses may prove useful for predicting the precise location and time of formation of squall lines generated by a particular type of frontal circulation transverse to surface cold fronts. Existence of this circulation is inferred from the development of a thin Line of shallow Convection clouds (LC) along the front simultaneously with that of a mesoscale (<100 km wide) Clear Zone (CZ) immediately behind the front and at the leading edge of a large area of stratus clouds. The observations suggest that a thermally direct circulation transverse to the surface cold front generated the line convection and clear zone (in the upward and downward branches of the circulation, respectively) in all 15 cases which met the strict criteria for an LC/CZ.

Squall lines were observed to form from the LC in 10 of the 15 cases examined, and nearly always within 90 min following the time when the CZ reached its maximum width. In addition, initial cumulonimbus development always occurred within 100 km of the diagnosed frontogenesis center at the LC. Therefore, this study suggests that both the timing and location of such squall lines should be predictable with very high accuracy. It is also shown that thermodynamic instability was insufficient for the formation of deep convection in the five non-thunderstorm cases.

Our results also strongly support the hypothesis of Koch (1984) that this mesoscale circulation was generated by differential sensible heating acting in conjunction with geostrophic deformation effects. The contrast of cloudy skies behind the front (prior to CZ formation) with nearly clear skies ahead of the front is largely responsible for creation of the differential heating pattern. This suggests that forecasters should watch for such cloud patterns near cold fronts.

Synoptic climatological conditions favoring the occurrence of this relatively rare phenomenon are also identified. The LC/CZ appears during the afternoon almost solely over the Great Plains states during spring and autumn. The line convection was found in all but one case to be parallel to, and either along or on the cyclonic side of, a prefrontal 850 mb jet. Although the LC/CZ is usually found on the anticyclonic side of upper-level jet streaks, it does not seem to prefer any particular jet quadrant. Diagnosis of the Sawyer-Eliassen equation for one case suggested that the mesoscale circulation was linked to a thermally direct circulation cell associated with the upper-level frontal zone.

The information provided in this paper should be valuable to the operational forecaster concerned with having some guidance about specific mesoscale trigger mechanisms for squall lines. This phenomenon can be isolated with conventional surface and satellite data in real time to provide accurate and timely forecasts of the formation of squall line activity.

Abstract

Planning and managing commercial airplane routes to avoid thunderstorms requires very skillful and frequently updated 0–8-h forecasts of convection. The National Oceanic and Atmospheric Administration’s High-Resolution Rapid Refresh (HRRR) model is well suited for this purpose, being initialized hourly and providing explicit forecasts of convection out to 15 h. However, because of difficulties with depicting convection at the time of model initialization and shortly thereafter (i.e., during model spinup), relatively simple extrapolation techniques, on average, perform better than the HRRR at 0–2-h lead times. Thus, recently developed nowcasting techniques blend extrapolation-based forecasts with numerical weather prediction (NWP)-based forecasts, heavily weighting the extrapolation forecasts at 0–2-h lead times and transitioning emphasis to the NWP-based forecasts at the later lead times. In this study, a new approach to applying different weights to blend extrapolation and model forecasts based on intensities and forecast times is applied and tested. An image-processing method of morphing between extrapolation and model forecasts to create nowcasts is described and the skill is compared to extrapolation forecasts and forecasts from the HRRR. The new approach is called salient cross dissolve (Sal CD), which is compared to a commonly used method called linear cross dissolve (Lin CD). Examinations of forecasts and observations of the maximum altitude of echo-top heights ≥18 dBZ and measurement of forecast skill using neighborhood-based methods shows that Sal CD significantly improves upon Lin CD, as well as the HRRR at 2–5-h lead times.

Abstract

This study documents a very rapid increase in convective instability, vertical wind shear, and mesoscale forcing for ascent leading to the formation of a highly unusual tornado as detected by a ground-based microwave radiometer and wind profiler, and in 1-km resolution mesoanalyses. Mesoscale forcing for the rapid development of severe convection began with the arrival of a strong upper-level jet streak with pronounced divergence in its left exit region and associated intensification of the low-level flow to the south of a pronounced warm front. The resultant increase in stretching deformation along the front occurred in association with warming immediately to its south as low-level clouds dissipated. This created a narrow ribbon of intense frontogenesis and a rapid increase in convective available potential energy (CAPE) within 75 min of tornadogenesis. The Windsor, Colorado, storm formed at the juncture of this warm frontogenesis zone and a developing dryline. Storm-relative helicity suddenly increased to large values during this pretornadic period as a midtropospheric layer of strong southeasterly winds descended to low levels. The following events also occurred simultaneously within this short period of time: a pronounced decrease in midtropospheric equivalent potential temperature θ _e accompanying the descending jet, an increase in low-level θ _e associated with the surface sensible heating, and elimination of the capping inversion and convective inhibition. The simultaneous nature of these rapid changes over such a short period of time, not fully captured in Storm Prediction Center mesoanalyses, was likely critical in generating this unusual tornadic event.