Storm Area Research Articles

Development of an eye-wall like structure in a tropical cyclone model simulation

Abstract Structural changes during the intensification of a tropical storm into a hurricane in a numerical simulation are examined. A 10 layer primitive equation model that employs a horizontal grid spacing of 20 km over 4400 × 4400 km area is integrated. An elongated band in vertical motion over the storm area intensifies slowly during the first few hours. In the upper troposphere high pressures arise due to condensational heating. Between 8–12 h strong outflow winds develop in the upper troposphere due to the increased pressure gradients. Strong divergence occurs in the outflow wind region, and a large increase in the vertical motion, condensational heating and intensification rate of the storm ensues. Between 12–24 h the elongated band of the storm stage transforms into an eye-wall like structure, and the tropical storm intensifies into a hurricane. Regions with negative moist potential vorticity appear in the high troposphere. Widening of area of condensation and slanting of the convergence area occurs with height in the high level negative moist potential vorticity regions. Results suggest that the formation of anvil clouds in some cases may be due to the development of slantwise convection on the outer periphery of a hurricane's eye-wall.

Linearity of basin response as a function of scale in a semiarid watershed

Linearity of basin runoff and peak response as a function of watershed scale was examined for a set of 29 nested semiarid watersheds within the U.S. Department of Agriculture–Agricultural Research Service Walnut Gulch Experimental Watershed, located in southeastern Arizona. Watershed drainage areas range from 1.83 × 103 to 1.48 × 108 m2 (0.183–14800 ha), and all stream channels are ephemeral. Observations of mean annual runoff, database‐derived 2‐ and 100‐year peak runoff rates, ephemeral channel area, and areal rainfall characteristics derived from 304 events were examined to assess the nature of runoff response behavior over this range of watershed scales. Two types of distributed rainfall‐runoff models of differing complexity were applied to a subset of the watersheds to further investigate the scale‐dependent nature of the collected data. Contrary to the conclusions of numerous studies in more humid regions, it was found that watershed runoff response becomes more nonlinear with increasing watershed scale, with a critical transition threshold area occurring roughly around the range of 3.7 × 105 to 6.0 × 105 m2 (37–60 ha). The primary causes of increasingly nonlinear response are the increasing importance of ephemeral channel losses and partial storm area coverage. The modeling results indicate that significant error will result in model estimates of peak runoff rates when rainfall inputs from depth area‐frequency relationships are applied beyond the area of typical storm coverage. For runoff modeling in Walnut Gulch and similar semiarid environments, explicit treatment of channel routing and transmission losses from channel infiltration will be required for watersheds larger than the critical drainage area.

Impact of parameterized convection on a numerically simulated tropical Cyclone structure

ABSTRACT. Condensational heating is a primary source of energy for disturbances like a tropical storm. The resolvable scale condensation and the parameterized convection, in many fine mesh numerical models, are evaluated at intervals greater than the time step, order of a minute, used for computing dynamical processes. The latent heating may depend on the model resolution and the interval at which the precipitation physics is evaluated. Numerical results from a series of short range forecasts are compared to study the impact of varying the horizontal resolution and the interval for evaluating condensation physics, and of excluding the parameterized convective heating. A horizontal grid spacing of 40 km (coarse mesh) or 20 km (fine mesh) in National Centers for Environmental Prediction's Quasi-Lagrangian Model (QLM), and the initial data for a tropical storm case, are utilized. Resolvable scale condensation is invoked only at supersaturated grid points, and a Kuo-type convective parameterization procedure is employed.
 
 Significant structural differences are produced when the interval for computing both parameterized convection and resolvable scale heating is changed, and these differences broaden when the horizontal resolution is increased. The central warm cote structure and storm intensity are simulated better when both condensational processes are evaluated at an interval of twelve time steps than at each time step. Vertical columns in central storm area rapidly become convectively stable, and the maximum in vertical motion and strongest horizontal winds shift in the outer storm area, when both condensational processes are invoked at each time step. The central storm area remains conditionally unstable, and strongest winds develop close to the center, when both condensational processes are evaluated at intervals of twelve time steps.
 
 The central storm area remains conditionally unstable also in the fine mesh experiment in which the parameterized convective heating is excluded and the resolvable scale heating is evaluated at each time step. Intense vertical motion and vigorous heating develop in deep vertical columns, indicating that the heating on the convective scale is simulated as the resolvable scale heating. The vertical distribution of heating and the storm structure, during the first six hours in this case, are similar to those in the fine mesh run in which both condensational processes are evaluated at intervals of twelve time steps. However, the storm intensifies more rapidly after 6 h in the former than in the later case. Numerical results from additional experiments are presented to show that predicted storm structure is modified with a change in interval for invoking either or both condensational processes, and these circulation differences are not due to the initial spin up.
 
 Transfer of moisture and heat from low levels into the higher troposphere in cumulonimbus clouds takes place in several minutes. Above cited and other predictions from the QLM suggest that storm structure. intensity and motion in a mesoscale model are likely to, be improved when parameterized convective heating is included; however, a parameterization scheme that concurrently produces alterations in the entire model cloud depth should be invoked at intervals of several minutes.
 
 

Polarimetrically TunedR(Z) Relations and Comparison of Radar Rainfall Methods

Abstract The following rainfall measurements are compared: 1) the reflectivity factor–rain rate or R(Z) relation, whereby the rain is estimated point by point for mapping or area integration; 2) use of a specific differential phase KDP (between vertical and horizontal polarization) in a relation with rainfall rate for point-by-point mapping and subsequent integration over areas and time; 3) use of a R(KDP) relation together with a relation between KDP and Z to derive a polarimetrically tuned or matched R(Z) relation; and 4) use of empirical relations between the rainfall volume and the time integral of the storm area in which reflectivity is larger than a selected threshold. These methods are tested on five cases—two summer-type convections, one winter convective case, and two events of stratiform rain with embedded convection. Accumulations of rain in a dense gauge network in Oklahoma are used as a standard for comparison with radar measurements. In four of the five cases the rain totals obtained from th...

Gravity torques for surface waves

The effects of the gravity torques acting on the angular momentum of surface gravity waves are calculated theoretically. For short crested waves the gravity torque is caused by the force of gravity on the orbiting fluid particles acting down the slopes of the crests and troughs and in the direction parallel to the crests and troughs. The gravity torque tries to rotate the angular momentum vectors, and thus the waves themselves, counterclockwise in the horizontal plane, as viewed from above, in both hemispheres. The amount of rotation per unit time is computed to be significant assuming reasonable values for the along-crest and trough slopes for waves in a storm area. The gravity torque has a frequency which is double the frequency of the waves. For long crested waves the gravity torque acts in the vertical plane of the orbit and tries to decelerate the particles when they rise and accelerate them when they fall. By disrupting the horizontal cyclostrophic balance of forces on the fluid particles (centrifugal force versus pressure force) the gravity torque accounts qualitatively for the three characteristics of breaking waves: that they break at the surface, that they break at the crest, and that the crest breaks in the direction of wave propagation.

The influence of storm characteristics and catchment conditions on extreme flood response : A case study based on the brue river basin, U.K.

Methods for estimating the magnitude of extreme floods are reviewed. A method which combines a probabilistic storm transposition technique with a physically-based distributed rainfallrunoff model is described. Synthetic storms with detailed spatial and temporal distributions are generated and applied to the calibrated model of the Brue river basin, U.K. (area 135 km2). The variability of catchment response due to storm characteristics (storm area, storm duration, storm movement, storm shape and within storm variation) and initial catchment wetness conditions is investigated. A probabilistic approach to estimating the return periods of extreme catchment responses is suggested.

Estimation of probable maximum precipitation for catchments in eastern India by a generalized method

A generalized method to estimate the probable maximum precipitation (PMP) has been developed for catchments in eastern India (80° E, 18° N) by pooling together all the major rainstorms that have occurred in this area. The areal raindepths of these storms are normalized for factors such as storm dew point temperature, distance of the storm from the coast, topographic effects and any intervening mountain barriers between the storm area and the moisture source. The normalized values are then applied, with appropriate adjustment factors in estimating PMP raindepths, to the Subarnarekha river catchment (upto the Chandil dam site) with an area of 5663 km2. The PMP rainfall for 1, 2 and 3 days were found to be roughly 53 cm, 78 cm and 98 cm, respectively. It is expected that the application of the generalized method proposed here will give more reliable estimates of PMP for different duration rainfall events.

Estimation of the Fractional Coverage of Rainfall in Climate Models

The fraction of the grid cell area covered by rainfall, μ, is a very important parameter in the descriptions of land surface hydrology in climate models. A simple procedure is presented for estimating this fraction, based on extensive observations of storm areas and rainfall volumes. It is often observed that storm area and rainfall volume are linearly related. This relation is utilized in rainfall measurement to compute rainfall volume from radar observations of the storm area. The authors suggest that the same relation be used to compute the storm area from the volume of rainfall simulated by a climate model. A formula is developed for computing μ, which describes the dependence of the fractional coverage of rainfall on the season of the year, the geographical region, rainfall volume, spatial resolution of the model, and the temporal resolution of the model. The new formula is applied in computing μ over the Amazon region. Significant temporal variability in the fractional coverage of rainfall is demonstrated. The implications of this variability for the modeling of land surface hydrology in climate models are discussed.

Redistribution of sediments in three Swedish lakes

Sedimentation and redistribution of fine sediments in three Swedish lakes of different character have been investigated using settling sediment traps. The bottom shear stress from wind generated waves are calculated and the extension of erodable bottom area is related to wind conditions. Wave induced erosion and deposition during and after cessation of storms in different parts of a lake are discussed theoretically. It is shown that a single one year storm may redistribute more bottom material than the accumulated resuspension caused by frequent but smaller wind events. The settling sediment trap deposition and the concentration of suspended solids are related to the extension of erodable bottom area of particular storms. It is found that in lakes where there are relatively large areas of erosion bottoms, resuspended material from the part of the lake most susceptible to strong winds of large fetch constitutes a major part of the settled material on deep bottoms.

The estimation of convective rainfall by area integrals: 1. The theoretical and empirical basis

This work develops a unified theory for the estimation of both total rainfall from an individual convective storm over its lifetime and the areawide instantaneous rainfall from a multiplicity of such storms by use of measurements of the areal coverage of the storms within a threshold rain intensity isopleth or the equivalent threshold radar reflectivity. The method is based upon the existence of a well‐behaved probability density function (pdf) of rain rate either from the many storms at one instant or from a single storm during its life. In the first method, the lifetime storm rainfall volume is V = [〈A(τ)〉T]S(τ), where 〈A(τ)〉 is the average storm area over the life of the storm of duration, T, in which R > τ and the bracketed term is the area time integral. In the second method, the instantaneous areawide rain rate, 〈R〉 = F(τ)S(τ), where F(τ) is the fractional observed area with R > τ. In both methods, S(τ) is the climatological rain rate for the regime divided by the relative frequency with which R > τ. For thresholds exceeding some minimum value, S(τ) is essentially linear with τ for the kind of lognormal pdf which characterizes convective rain, and is a constant for specified τ. Thus both the lifetime V of the individual storm and the instantaneous 〈R〉 for a multiplicity of storms are linear functions of A(τ) and F(τ), respectively. This is in accord with the observations of Doneaud et al. (1984) and Chiu (1988a, b). Because the autocorrelation time of the areawide rain rate of convective storms in areas in excess of 104 km2 is about 5–6 hours, the snapshot 〈R〉 is representative of the rain for a few hours. This enhances the accuracy of snapshot measurements for climate purposes and also extends their utility to smaller time/space problems such as hydrology and numerical weather prediction. We also discuss ways of obtaining climatologically valid and unbiased values of S(τ) and means of establishing its values from aircraft and space.

A Numerical Study of the Effect of Island Terrain on Tropical Cyclones

A triply nested, movable mesh model was used to study the behavior of tropical cyclones encountering island mountain ranges. The integration domain consisted of a 37° wide and 45° long channel, with an innermost mesh resolution of 1/6°. The storms used for this study were embedded in easterly flows of ∼5 and ∼10 m s−1 initially. Realistic distributions of island topography at 1/6° resolution were inserted into the model domain for the region of the Caribbean, including the islands of Cuba, Hispaniola, and Puerto Rico; the island of Taiwan; and the region of Luzon in the northern Philippines. It was found that the islands affected the basic flow as well as the wind field directly associated with the storm system. The combination of these effects caused changes in the track and translational speed of the storm. In particular, in the case of the 5 m s−1 easterly flow, the storm accelerated and veered to the north well before reaching Taiwan. For the other island distributions, the northward deflection of the track and the increase of translational speed occurred near and over the islands. After landfall, the surface pressure underwent rapid filling. As the tropical cyclone passed over Hispaniola, the surface low continued to move along with the upper level vortex as it transversed the mountain range, while over Luzon it became obscure before reforming on the lee side slope of the mountain. In case of Taiwan and the 10 m s−1 easterly zonal flow, secondary surface lows developed behind the mountain range. The upper level vortex in this case became detached from the original surface low and eventually coupled with a secondary one. The intensity changes of the storm near and over the islands were strongly related to the latent energy supply and the vertical coherence of the storm system. Advection of dry air from near or above the mountain tops into the storm area caused significant weakening of all the storms moving with the weaker easterly flow. Storms leaving Hispaniola and moving over open sea quickly reintensified as their vertical structure remained coherent. On the other hand, storms leaving Luzon were disorganized and did not reintensify until several hours later when the vertical coherence of the systems was reestablished. Although these experiments were performed for an idealized experimental design and basic flow, many observed storms have exhibited similar behavior in track deviation and decay. This implies that the effect of detailed topography should be considered if an accurate forecast of the storm direction and behavior is to be made.

A Case Study of GWE Satellite Data Impact on GLA Assimilation Analyses of Two Ocean Cyclones

The impact of GWF satellite data on GLA analyses of the thermal structure of two oceanic extratropical cyclones are presented. The two storms developed nearly simultaneously in the western Pacific and Atlantic oceans during January 1979. In addition to describing the satellite data impact on temperature and static-stability distributions within each storm, implications of the impact on available potential energy and on the baroclinic growth of the cyclone are discussed. In the comparison of CLA analyses with (SAT) and without (NOSAT) the satellite temperature soundings produced by 1,be NOAA/NESS retrieval method, the results show sizable temperature differences between the two analyses for both storms. Characteristic features include the following: 1) warmer SAT temperature within NOSAT thermal through and colder SAT temperatures within NOSAT thermal ridges 2) a difference in sign or magnitude of the temperature impact between the upper troposphere and the lower troposphere, and 3) an evolution of a significant, satellite-data cold bias located within and paralleling the evolution of the low-tropospheric thermal ridge close to the cyclone center. The satellite temperature impact in the low troposphere increases tropospheric static in the central storm area and decreases troposhperic stability in the cold troughs both to the rear and ahead of the cyclone. Although the net effect of area-averaged satellite temperature impact for the storms is small, the association of warmer temperature with NOSAT thermal trough and colder temperatures with thermal ridges reduces the areal temperature stability variance within the Pacific storm area. The reduction for the Atlantic storm region is less. For the Pacific storm the satellite temperature impact led to a 10% reduction of available potential energy, a significant reduction in vertical phase tilt of the tropospheric temperature structure, and a potentially greater than 10–20% reduction of the baroclinic rate of cyclone-scale waves. The decreased baroclinic amplification is inferred from stabilization of the highest lapse rates and a reduction of the largest vertical thermal-wind shears in the low troposphere of the storm area. Overall the results stress the need to identify characteristics of satellite temperature-sounding impacts on the baroclinic structure of cyclone waves, which potentially degrade numerical weather prediction of cyclogenesis.

A discussion of non-linear temperature profiles at six closely spaced heat flow sites, southern Sohm Abyssal Plain, northwest Atlantic Ocean

Six heat flow measurement sites were occupied in June 1980 in a 10 x 10 km 2 flat area of the southern Sohm Abyssal Plain, western North Atlantic Ocean. Non-linear sediment temperature profiles, measured to depths of 5 m, indicate perturbations in the temperature field in sediments overlying 90 Ma ocean floor. Temperature gradients average 59.0 mK m −1 in the lower half of the profile and decrease by 25% to an average of 44.24 mK m −1 in the upper half. Thermal conductivities of sediment cores down to 12 m ranged from 0.74 to 2.12 W m −1 K −1 and averaged 1.06 W m −1K −1. The non-linearity of sediment temperature profiles cannot be accounted for by the variations in thermal conductivity. Vertical fluid convection in the sediments, with a predominantly downward migration on the order of 5 x 10 −8 ms −1 in the upper 3 m, could explain the perturbations. However, in this study area of high abyssal kinetic energy and abyssal storms, bottom-water temperature fluctuations are the likely source of observed sediment temperature perturbations. A bottom-water temperature change of 50 mK occurring 3 months prior to the cruise could produce sediment temperature perturbations similar to those observed. Heat flow determined from the lower gradient (3–5 m sediment depth interval), assuming the non-linearity in the upper sensors to be principally due to bottom-water temperature fluctuations, averages 59.2 mW m −2, a slightly higher value than that predicted for 90 Ma crust.

Sferics rate in relation to thunderstorm dimensions

On the basis of data from 3 days in 1978 this paper investigates the relation between thunderstorm dimensions and electrical activity to determine the relative importance of thunderstorm size and thunderstorm environment on the sferics rate. Sferics were recorded continuously by a wide‐band (100 ± 50 kHz) crossed‐loop radio direction finder located at the radar site; precipitation data during the corresponding period came from the radar volume scan. The number of sferics associated with specific storms and recorded in 5‐min intervals varied from a few tens to a few thousands, depending on the dimensions of the source storm and on its distance from the sferics receiver. For four thunderstorm situations this variation was reduced to a small scatter, with standard deviations corresponding to factors of 1.17–1.47, by fitting the data with the empirical relation: S ∝ Ax × 10yH × 10−zr, where S is the observed number of sferics per 5 min, A the cross‐sectional area, H the height, and r the range of the storm. For three of the four sets of data analyzed, x was just over unity, giving a nearly linear relation between the sferics rate and the storm's area; for a supercell storm the relation was higher than linear. For thunderstorms between 50 and 300 km distant the number of recorded sferics decreased exponentially, by a factor of about 2 every 45 km, or with z = 0.0068 ± 0.0011 per km. Thunderstorm height appeared the dominant parameter determining the rate of electrical activity; values of y for the 3 days were between 0.13 and 0.28 per km, the greater effect of thunderstorm height was found associated with a stronger wind shear in the 7.5‐ to 12‐km layer.

Moments of Catchment Storm Area

The portion of a catchment covered by a stationary rainstorm is modeled by the common area of two overlapping circles. Given that rain occurs within the catchment and conditioned by fixed storm and catchment sizes, the first two moments of the distribution of the common area are derived from purely geometrical considerations. The variance of the wetted fraction is shown to peak when the catchment size is equal to the size of the predominant storm. The conditioning on storm size is removed by assuming a probability distribution based upon the observed fractal behavior of cloud and rainstorm areas.

Focusing Mechanisms in the Texas Hill Country Flash FIOMS of 1978

During the early morning of 2 August 1978, a stationary thunderstorm complex drenched the Balcones Escarpment of Texas and unleashed flash floods in the Hill Country which killed 27 people and produced extensive damage. After the storm, an unofficial 24 h total of at 1east 79 cm of rain was observed near the town of Bluff (about 90 km northwest of San Antonio). Five distinct mechanisms or meteorological features interacted to focus and anchor the storm complex: 1) a east-west oriented mesohigh outflow boundary was situated just south of the storm area; 2) a rapidly moving ribbon of extremely moist boundary-layer air was flowing toward the storm are from the southeast 3) an elevated warm, dry layer of air extended over the area east of the Hill Country and capped the southeasterly low level inflow; 4) a deep vertical motion field associated with a midtropospheric short-wave trough was advancing toward the Hill Country from Mexico; and 5) the diurnal heating cycle with its associated production of thermally forced convective clouds was ending. The juxtaposition of the first tour mechanisms over the storm are in combination with terrain lifting and the termination of convection initiated by boundary layer heating resulted in a forced, stationary storm complex that produced catastrophic flash floods in the Hill Country.

Possible Triggering Mechanisms for Severe Storms in SESAME-AVE IV (9–10 May 1979)

The SESAME-AVE IV (9–10 May 1979) rawinsonde data were analyzed to uncover possible triggering mechanisms for severe storms that developed over western Oklahoma and Texas. The high frequency of observations (at 3 h intervals) and high vertical resolution of reported data (at 25 mb intervals) at all stations permitted investigation of the diurnal variation of the planetary boundary layer on the synoptic scale. Thunderstorms developed first just ahead of a stationary front over the Texas panhandle on the afternoon of 9 May. This area was characterized by the absence of a strong inversion (or “lid”) that represented an interface between very warm and dry air aloft, and warm moist tropical air below. Apparently, mesoscale low-level ascending motion associated with frontal lifting and/or the inland sea breeze effect led to the removal of the lid. Another noteworthy feature in this storm event was the strong vertical wind shear at low and middle levels over the storm area. When combined with the development of a deep boundary layer with weak stratification during the daytime, the Richardson number became less than one in the boundary layer in the prestorm situation. The results of our numerical linear stability analysis indicate that the observed basic states were indeed symmetrically unstable. This may suggest that the triggering processes were argumented by symmetric instability. Although a well defined dry line was present, it does not seem to have contributed directly to initiation of storms in this case. It also was observed that, as the thermal low began to weaken in the early evening, the cold air behind a stationary front started advancing eastward and helped to extend the line of thunderstorms deep into central Texas. This may be another process whereby some storms prefer to develop in the late afternoon or early evening.

A Synoptic Analysis of the First AVE-SESAME '79 Period

Fields of divergence, vertical motion, stability, and surface pressure tendency are examined at 3 h intervals for the first regional scale AVE-SESAME '79 (Atmospheric Variability Experiment—Severe Environmental Storms and Mesoscale Experiment) day. Two areas of severe storms formed during the period from 1200 GMT 10 April through 1200 GMT 11 April. The Red River Valley outbreak began during the afternoon of 10 April, while a second area formed in southwestern Texas during the early evening hours. Results show the rapid changes in environmental conditions associated with these two storm areas. The propagation of an upper level jet streak into the region was a major factor in producing the Red River Valley outbreak. This streak was associated with the formation of a strong low-level jet and a small-scale surface pressure perturbation. The sudden development of a strong upper tropospheric wind maximum over Oklahoma and Kansas corresponded with major changes in kinematic parameters at that level. Instability over the Red River Valley was released by strong upward motion producing intense convection. Similar features were responsible for the storms in southwestern Texas. Although this area was quite unstable, forcing mechanisms appear somewhat weaker than in the earlier outbreak.

Low-frequency wind-generated noise produced by the impact of spray with the ocean’s surface

Ambient noise measurements for frequencies from 50 to 1000 Hz indicate that wind-generated noise is a significant source of the total ambient noise field during high-wind/sea-state conditions. The acoustic source spectrum levels for the physical mechanism involving the impact of water droplets from ocean spray, streaks, and white caps were derived as a function of wind speed. An empirical approach using the results of Franz and measurements of Morris were used in this analysis. A full analytical derivation of the wind-generated source level using Franz’s results was not possible because knowlege of certain parameters such as size and impact velocity of ocean spray as a function of wind speed is currently unknown. Recent measurements of the ’’white capping index’’ as a function of wind speed were used in conjunction with Morris measurements and Franz’s results to derive the source spectrum levels for wind-generated noise in this frequency interval. The acoustic noise from bubble oscillation in white caps was not addressed in detail in this paper since bubble size distributions and the results of Strasberg indicate that this source is significant only above 1000 Hz. Rain noise was also not included in this paper because knowledge of raindrop size distribution as a function of wind speed and geographical extent of rain within the storm area is currently not known or predictable enough to model. The wind-generated source levels from 5 to 50 Hz from a previous analysis by the author were combined with the results of this paper to yield an estimate of wind-generated source spectrum levels as a function of wind speed for 5 to 1000 Hz. A definite change of wind-speed dependence from 5- to 50-Hz interval to the 150- to 1000-Hz interval is predicted by these results since the dominant physical mechanisms are different in these two intervals. Noise measurements by Perrone, Piggott, and Morris seem to substantiate this prediction, but more measurements are needed to prove this conclusively.

Numerical Study of Storms in the Transvaal

Abstract An automatic procedure was developed for tracking the centroids of many weather radar. echoes simultaneously by computer. This was applied to storms recorded ID the Transvaal on 19 days of radar operation during the summers of 1976–7 and 1977–8. Of the 1861 storms tracked, only 23 percent attamed 100 km2 and only 8 percent lasted more than an hour. Deviation of storm tracks to the left of the mean 500 mb winds was found to increase with increasing storm lifetimes and areas. When the 500 mb winds were strong, storm directions were usually well correlated with their areas.