Abstract

A Lagrangian model applicable to the overshooting region of thunderstorm tops is used to describe the temperature-height path taken by updraft core parcels as they penetrate above the tropopause, reach their maximum height and descend in the periphery of the convective tower. The model is run under a variety of ambient and in-cloud conditions in order to simulate certain temperature-height relationships observed in satellite observations.

Observations indicate that in the majority of observed storm tops, the satellite-observed cold point in the IR brightness temperature (T_B ) field is collocated with the highest point in the convective overshooting region and the T_B -height relations are near adiabatic. The parcel model quantitatively reproduces this type of relationship for model runs where the mixing parameter is relatively small.

Another type of storm has a close-in, cold-warm T_B couplet with a dimension of approximately 20–40 km and a V-shaped cold T_B pattern. In some cases of these V-shaped storms, the cold point is clearly located upwind of the high point. Model runs have been made to reproduce a number of these salient features for these types of storms. With larger values of the mixing parameters (presumably related to larger shear), the model produces temperature-height relationships that are, of course, much closer to ambient than to adiabatic, as is observed in these cases. With the larger mixing parameter, the cold-high offset is also produced, for model runs having a relatively large initial vertical velocity and under conditions of a strong inversion. The amount of the cold-high offset is shown to be a direct function of the strength of the inversion.

The cause of the close-in warm point is also explored with the simple model. As has been shown in three-dimensional cloud model results, the warm point in the cold-warm couplet can be related to internal cloud subsidence on the downwind side in association with mixing with the environment. This effect is also reproduced in the parcel model with the occurrence of a warm point being related to conditions of an intense updraft and strong mixing. The model also points to parcels subsiding from their maximum height and crossing the ambient lapse rate from negative to positive buoyancy on the downwind side and then coming into equilibrium at a relatively high level above the tropopause on the downwind side. This effect may be related to the top of the downwind anvil cloud being elevated significantly above the equilibrium point or tropopause. Another interpretation of this model result may be related to the above-anvil cirrus noted by a few investigators.

The temperature-height distributions produced by the model in a Lagrangian framework are converted to the spatial domain by the assumption of steady state conditions and are compared to temperature-height cross sections determined from GOES IR and stereoscopic height fields. The locations of cold points, high points, warm points, and the magnitude of cold-high offsets compare favorably between the model and the satellite observations.

Abstract

Observational studies of thunderstorm cloud height-rainfall rate and cloud height-volume rainfall rate relations are reviewed with significant variations being noted among climatological regimes. Analysis of the Florida (summer) and Oklahoma (spring) relations are made using a one-dimensional cloud model to ascertain the important factors in determining the individual cloud-rain relations and the differences between the two regimes. In general, the observed relations are well simulated by the model-based calculations. The generally lower predicted rain rates in Oklahoma (as compared to Florida) result from lower precipitation efficiencies which are due to a combination of larger entrainment (related to larger vertical wind shear) and drier environment. The generally steeper slope of the Oklahoma rain rate height curves is shown to be due to a stronger variation in maximum vertical velocity with cloud top height, which, in turn, is related to the greater static stability in the range of cloud tops. The impact of the regime-to-regime variations on empirical rain estimation schemes based on satellite-observed cloud height or cloud temperature information is discussed and a rain estimation approach based on model-generated cloud-rain relations is outlined.

Abstract

GOES stereoscopy is applied to the study of severe squall line cells. Short interval (3 min) GOES stereoscopic data from the 2–3 May 1979 SESAME case were used to measure cloud top heights of growing storms as a function of time. A one-dimensional cloud model was used to relate the stereoscopically derived cloud top ascent rates to thunderstorm updraft intensity. Results show ascent rates ranging from 4.4 to 7.7 m s⁻¹ for intense cells in a squall line. These results compare well in magnitude with growth rates determined from simultaneous GOES infrared observations and previous estimates of visual cloud and radar echo top growth rates of other thunderstorms.

Detailed stereoscopic cloud top height contour maps of the mature squall line on 2–3 May 1979 were constructed and are discussed here in terms of the small-scale structure and its variability. Results show that for small-scale features (e.g., 5 km diameter tropopause penetrating towers) the short-interval GOES data are not sufficient for studying the life cycle of such features. The stereoscopic height contours are compared to infrared cloud top temperature patterns observed with intense thunderstorms and used to evaluate various theories on the cause of the infrared V-shaped signatures.

Abstract

This study investigates the role of the parameterized boundary layer structure in hurricane intensity change using two retrospective HWRF forecasts of Hurricane Earl (2010) in which the vertical eddy diffusivity K _m was modified during physics upgrades. Earl undergoes rapid intensification (RI) in the low-K_m forecast as observed in nature, while it weakens briefly before resuming a slow intensification at the RI onset in the high-K_m forecast. Angular momentum budget analysis suggests that K _m modulates the convergence of angular momentum in the boundary layer, which is a key component of the hurricane spinup dynamics. Reducing K _m in the boundary layer causes enhancement of both the inflow and convergence, which in turn leads to stronger and more symmetric deep convection in the low-K_m forecast than in the high-K_m forecast. The deeper and stronger hurricane vortex with lower static stability in the low-K_m forecast is more resilient to shear than that in the high-K_m forecast. With a smaller vortex tilt in the low-K_m forecast, downdrafts associated with the vortex tilt are reduced, bringing less low-entropy air from the midlevels to the boundary layer, resulting in a less stable boundary layer. Future physics upgrades in operational hurricane models should consider this chain of multiscale interactions to assess their impact on model RI forecasts.

Abstract

Historical changes in time of once daily maximum and minimum temperature observations at cooperative climatological stations from 1905 to 1975 have introduced a systematic bias in mean temperatures. Unless corrected, this bias may be interpreted incorrectly as climatic “cooling” and may also affect the assessment of agricultural production potential and fossil fuel needs. Maximum and minimum temperature data for two years from the National Weather Service station at Indianapolis International Airport were used to evaluate the differences between mean temperatures obtained by terminating the 24 h period at the midnight observation and the mean temperatures obtained by terminating the 24 h period at 0700 and 1900 hours, typical observation times for AM and PM observing stations. The greatest mean temperature bias occurs in March when a 1900 observation day yields a monthly mean temperature 1.3°F above a midnight observation, and a 0700 observational day gives a −1.3°F bias. Since the number of AM observing stations in Indiana have increased from 10% of the total number of temperature stations in 1925 to 55% in 1975, the March mean temperature shows a decrease of 1.2°F in the last 40 years, solely because of the change in substation observational times. Unless the time of observation bias is considered, the mixture of AM and PM observations complicates interpretation of areal temperature anomaly patterns. This bias is accumulated in monthly, seasonal or annual values of the mean temperature-derived variables-heating degree days, cooling degree days and growing degree days—and may provide misleading information for applications in industry and agriculture.

Abstract

An improved shearing stress meter (drag plate) of 6 ft (183 cm) diameter has been constructed for micro-meteorological work. This size should enable representative sampling of rough surfaces and is intended particularly for farmland experimental sites where, after harvesting, the ground is either left to grain stubble or ploughed.The sensors are inductive proximity probes, whose advantages of robustness and stability enable the instrument to be handled without undue precautions against damage and to withstand gusts of over 20 dyn cm⁻² while achieving a resolution of 0.01 dyn cm⁻².Comparison experiments involving two drag plates installed in a ploughed field with a sonic anemometer mounted nearby indicate good agreement in stress measurement among all three instruments.

Abstract

An extended series of surface observations is used to compare observed surface winds with winds computed using the geostrophic relationship. These computations are done for both steady and unsteady wind regimes. Large differences are found in the comparisons of observed to computed winds. The differences exhibit pronounced seasonal and diurnal variability that appear to reflect both boundary layer stability and small-scale wind and pressure fields-for example, those attending land-sea breezes and thunderstorms.

The results of this study may be useful to those engaged in studying global datasets and to modelers, who are continually challenged to improve the treatment of parameterization of turbulent processes. However, it is not obvious that any simple parameterization can be applied to obtain an accurate estimate of the surface wind in central Florida, given only the large-scale pressure gradient or a model-predicted wind above the surface as input. The use of the pressure field to estimate surface winds is an uncertain exercise at best.

Abstract

The interannual variability (IAV) in monthly averaged outgoing infrared radiation (IR, from the NOAA polar orbiting satellites) is observed to be larger during summer than during winter over the north Pacific Ocean. A statistical analysis of the daily observations shows the daily variance to be similar during both seasons while the autocorrelation function is quite different. This leads to a seasonal difference in estimates of the climatic noise level, i.e., the variances expected in summer and winter monthly averages due to the number of effectively independent samples in each average. Because of a less vigorous tropospheric circulation, monthly means of IR during summer are affected by the passage of fewer synoptic-scale disturbances and their attendant cloudiness. Fewer independent samples imply a larger variance in the time averages. While the observed IAV is less in winter, the ratio of the observed IAV to the climatic noise level is larger, suggesting that signals of climatic variability in outgoing IR may be more readily diagnosed during winter in this region. The climatic noise level in monthly averaged IR and cloudiness is also estimated for two other climatic regimes—the quiescent subtropical north Pacific and the ITCZ in the western Pacific.

Abstract

A squall-line cloud cluster observed in the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE) is studied as an example of a mesoscale convective system in the tropics. The system is divided into convective and stratiform regions. Composite wind, vertical motion, humidity, radar and satellite data fields have been derived for the system and are used to calculate the components of the water budgets of each region. Particular attention is devoted to understanding the sources of condensate for the stratiform region. The mesoscale updraft in the stratiform cloud accounts for 25–40% of the condensate making up the stratiform cloud, while the remaining 60–75% is supplied by horizontal transfer to the stratiform region of condensate generated in the cumulonimbus towers of the convective region.

Abstract

The mesoscale structure of a squall-line system that passed over Oklahoma on 22 May 1976 is investigated by dual-Doppler radar analysis. The mature storm consisted of a leading line of deep convection, which exhibited organized multicellular structure, trailed by an extensive region of stratiform precipitation marked by a radar bright band at the melting level. These contrasting radar echo regimes were separated by a narrow band of weak reflectivity at lower levels, which has been termed the “transition zone.” While conventional and single-Doppler radar analyses documented the persistence of this precipitation structure and revealed the corresponding kinematic structure in one part of the mature storm, the dual-Doppler analysis demonstrates the pervasiveness of these features over much of the squall line's length.

The structure of a midtropospheric maximum of rearward, system-relative flow crossing the system is particularly well described by the dual-Doppler data. This mesoscale current originated ahead of the storm. gained strength while passing through the convective line, spanned the transition zone, and extended to near the back edge of the stratiform region. It strongly influenced precipitation growth and radar echo structure by promoting the transfer of ice particles from convective cells across the transition zone into the trailing stratiform region. Deep, intense updrafts occurred in association with convective cells along the leading edge of the system. Convective downdrafts were apparently active both in the lower troposphere, where thermodynamic data showed they were a source of air feeding the leading gust front, and at upper levels, where the Doppler analysis indicated they were forced by convergence of air detrained from the tops of the updrafts with slower moving ambient air. Horizontal momentum transported vertically by convective motions converged at midlevels, accelerating parcels rearward and so bolstering the front-to-rear flow.

Profiles of radar-derived mean vertical motion confirm the presence of a mesoscale updraft overlying a mesoscale downdraft in the transition and trailing stratiform regions. The mean descent in the lower troposphere was particularly deep and intense in the transition zone and may have contributed to the decreased reflectivity values observed there.