Twenty-one warm-season heavy-rainfall events in the central United States produced by mesoscale convective systems (MCSs) that developed above and north of a surface boundary are examined to define the environmental conditions and physical processes associated with these phenomena. Storm-relative composites of numerous kinematic and thermodynamic fields are computed by centering on the heavy-rain-producing region of the parent elevated MCS. Results reveal that the heavy-rain region of elevated MCSs is located on average about 160 km north of a quasi-stationary frontal zone, in a region of low-level moisture convergence that is elongated westward on the cool side of the boundary. The MCS is located within the left-exit region of a south-southwesterly low-level jet (LLJ) and the right-entrance region of an upper-level jet positioned well north of the MCS site. The LLJ is directed toward a divergence maximum at 250 hPa that is coincident with the MCS site. Near-surface winds are light and from the southeast within a boundary layer that is statically stable and cool. Winds veer considerably with height (about 140) from 850 to 250 hPa, a layer associated with warm-air advection. The MCS is located in a maximum of positive equivalent potential temperature e advection, moisture convergence, and positive thermal advection at 850 hPa. Composite fields at 500 hPa show that the MCS forms in a region of weak anticyclonic curvature in the height field with marginal positive vorticity advection. Even though surface-based stability fields indicate stable low-level air, there is a layer of convectively unstable air with maximum-e CAPE values of more than 1000 J kg 1 in the vicinity of the MCS site and higher values upstream. Maximum-e convective inhibition (CIN) values over the MCS centroid site are small (less than 40 J kg 1) while to the south convection is limited by large values of CIN (greater than 60 J kg 1). Surface-to-500-hPa composite average relative humidity values are about 70%, and composite precipitable water values average about 3.18 cm (1.25 in.). The representativeness of the composite analysis is also examined. Last, a schematic conceptual model based upon the composite fields is presented that depicts the typical environment favorable for the development of elevated thunderstorms that lead to heavy rainfall.

A 30-yr climatology of the snow-to-liquid-equivalent ratio (SLR) using the National Weather Service (NWS) Cooperative Summary of the Day (COOP) data is presented. Descriptive statistics are presented for 96 NWS county warning areas (CWAs), along with a discussion of selected histograms of interest. The results of the climatology indicate that a mean SLR value of 13 appears more appropriate for much of the country rather than the often-assumed value of 10, although considerable spatial variation in the mean exists. The distribution for the entire dataset exhibits positive skewness. Histograms for individual CWAs are both positively and negatively skewed, depending upon the variability of the in-cloud, subcloud, and ground conditions.

This paper presents a reexamination of the Earth radiation budget' parameterization of energy balance climate models in light of data collected over the last 12 years. The study consists of three parts: (1) an examination of the infrared terrestrial radiation to space and its relationship to the surface temperature field on time scales from 1 month to 10 years; 2) an examination of the albedo of the Earth with special attention to the seasonal cycle of snow and clouds; (3) solutions for the seasonal cycle using the new parameterizations with special attention to changes in sensitivity. While the infrared parameterization is not dramatically different from that used in the past, the albedo in the new data suggest that a stronger latitude dependence be employed. After retuning the diffusion coefficient the simulation results for the present climate generally show only a slight dependence on the new parameters. Also, the sensitivity parameter for the model is still about the same (1.25øC for a 1% increase of solar constant) for the linear models and for the nonlinear models that include a seasonal snow line albedo feedback (1.34øC). One interesting feature is that a clear-sky planet with a snow line albedo feedback has a significantly higher sensitivity (2.57øC) due to the absence of smoothing normally occurring in the presence of average cloud cover. 1.relating the outgoing infrared flux to the surface temperature to be determined from data (these are part of the subject of this study); Q is the solar constant divided by 4; S(x, t) is the normalized seasonal distribution of heat flux entering the top of the atmosphere; and a[x, T(•, t)] is the coalbedo, which may be dependent on the local temperature as well as position (also a subject of the present study). In solving the equation we require the boundary condition that the horizontal heat flux into the poles vanish.When the temperature dependence of the coalbedo is held fixed, the model is linear and it is convenient to use the discrete Fourier representation since the harmonics are uncoupled. The nonlinearity stemming from the temperature dependence of the albedo is sufficiently mild that even when it is included the harmonic representation is often a useful approximation. Most of the studies have concentrated on applications that exploit the ability of the linear version of the model to reproduce the ensemble average seasonal cycle in the present and altered climates [Hyde et al., 1990; Crowley and North, 1988; Short et al., 1991; Baum and Crowley, 1991]. In addition, some studies have introduced noise forcing to simulate fluctuations at frequencies away from the forced seasonal cycle and its harmonics [North and Cahalan, 1982; Leung and North, 1990; Leung and North, 1991; Kim and North, 1991]. All of these studies have relied on parameterizations derived from satellite data taken from the 1970s [cf. North and Coakley, 1979]. Nonlinearity enters the model as the snowcover (and possibly cloud) movements alter the albedo leading to a feedback which can increase cl...

A case study of a long, narrow band of heavy snowfall is presented that illustrates those processes that force and focus the precipitation in a unique linear fashion. System-relative flow on isentropic surfaces shows how the trough of warm air aloft (trowal) formed to the north-northwest of a weak synoptic-scale surface cyclone. To the north of the trowal, midtropospheric frontogenesis formed as the warm, moist, high-θe air in the trowal canyon became confluent with cold, dry air to the northwest of a closed midlevel circulation. Within the trowal airstream, isentropic uplsope is shown to contribute to vertical motion, while transverse to this flow, mesoscale lift is enhanced on the warm side of a frontogenetical zone in the presence of weak symmetric stability and conditional symmetric instability. Further, it is shown that a sloping zone of small positive to negative equivalent potential vorticity forms to the southeast of the midtropospheric system-relative closed circulation as low-θe air associated with the dry conveyor belt, seen in water vapor imagery, overruns warm, moist high-θe air associated with the warm conveyor belt. In this way cold season instability forms due to differential moisture advection on the warm side of the frontogenesis axis. Finally, a conceptual model is shown that encapsulates the key processes that contributed to the extensive, narrow band of heavy snow in the presence of a weak synoptic-scale surface cyclone.

No abstract