Computation Of Vertical Motion Research Articles

Diagnosis of Anafronts and Katafronts

Abstract Diagnostic techniques are presented that aid in determining whether a cold front is of the anafront (upslope flow) or katafront (downslope flow) type, as well as in measuring the intensity of the vertical motions in the vicinity of the frontal zone. Anafronts are characterized by postfrontal cloudiness and precipitation, while katafronts typically have precipitation in a band along or ahead of the cold front. The techniques described include: 1) examination of the vertical profile of front-normal and front-parallel winds, 2) construction of an isentropic cross-section normal to the front including front-normal wind components, 3) computation of vertical motion using either the Bellamy triangle approach or adiabatic vertical motion on isentropic surfaces, and 4) analysis of cold air soundings for inversions, relative humidity profiles, and the degree of backing of winds in the vertical. Case studies are then shown in which we apply the above techniques and prove their efficacy in classifying, anaf...

Winter Forecast Problems Associated with Light to Moderate Snow Events in the Mid-Atlantic States on 14 and 22 February 1986

Abstract Two cases of light and moderate snow events over the Washington, DC area are presented for 14 and 22 February 1986. These cases were selected since the upper-tropospheric and surface features were quite similar, yet significantly different snowfall patterns occurred in the mid-Atlantic status. The numerical guidance and local forecasters displayed minimal skill in forecasting the amount of snow for each case, especially in the Washington, DC area. Analyses in isobaric and isentropic coordinates are presented which focus on the structure and evolution of the thermal fields and associated temperature advection patterns in the lower troposphere. These analyses reveal that the low-level upslope flow (as derived from vertical motion computations in isentropic coordinates) and the associated warm-air advection patterns (as viewed in isobaric coordinates) were important factors in determining the intensity and spatial distribution of precipitation for each case. Three-hourly soundings from the Genesis o...

The Transformation of Tropical Storm Agnes into an Extratropical Cyclone. Part I: The Observed Fields and Vertical Motion Computations

Abstract This is the first of two papers dealing with the transformation of tropical storm Agnes (June 1972) into an extratropical cyclone. Synoptic analyses and vertical motion patterns are used to describe the behavior of Agnes over a five-day period subsequent to initial landfall during which time Agnes regained tropical storm strength prior to transforming into an extratropical cyclone and dissipating. Input data for all calculations are obtained from optimum interpolation objective analyses with a resolution of 1° latitude-longitude in the horizontal and 100 mb in the vertical. A modified, 9-level version of Krishnamurti's (1968a) diagnostic balance model is discussed and applied to the data set to determine the fields of vertical motion. Redevelopment of Agnes subsequent to initial landfall is the result of the spread of an area of appreciable cyclonic vorticity advection aloft over the periphery of the low-level circulation devoid of significant baroclinicity. Unlike a corresponding midlatitude sto...

The lapse rate of temperature following an air parcel

AbstractThe lapse rate of temperature following an individual air parcel is derived assuming continuity of temperature, density, motion, and non‐vertical ascent in a changing pressure field. The result differs substantially in form and sometimes in magnitude from that obtained by the usual theory in which the ascent is strictly vertical and the parcel is discontinuous. When non‐vertical ascent and continuous fields are taken into account, important terms arise when the pressure field varies rapidly with time or along the horizontal coordinates. The importance of the departure of the unsaturated parcel lapse rate from gc in the computation of vertical motion by the adiabatic method is noted.

HEAVY WARM-SECTOR RAINS FROM ILLINOIS TO MIDDLE ATLANTIC COAST, MAY 26–28, 1956

During the weekend of May 26-28, 1956, a large, cold, high pressure system moved slowly off the east coast of the United States followed by a broad current of moist tropical air from the Gulf of Mexico. As this moist current overran the retreating cold air mass, widespread warm-frontal-type rains spread over the northeastern United States. The warm front was poorly defined at the surface, with a broad zone of gradual transition. Aloft the front was somewhat more distinct but still quite broad. To the rear of this frontal zone the warm sector air mass extended uninterruptedly to the Gulf of Mexico. The synoptic situation on May 27 is presented in figure 1A. It was in the extensive belt of mT air that the rains described in this paper occurred. The rains began in Illinois early Saturday morning of May 26 and spread eastward with the warm front to the coast by Sunday morning. Surprisingly enough, however, they continued throughout a broad strip from central Illinois eastward all day Saturday, Sunday, and part of Monday. The rain over most of this area ceased when a southwardmoving cold front brought dry air and subsiding currents to the region. East of the Appalachian Mountains the warm-sector rains were accompanied by a deepening trough at the surface and aloft. Here the rains began when the flow at the 700-mb. level became cyclonic and ended when the warm-sector flow lost its cyclonic curvature after passage of the 700-mb. trough. The principles relating surface precipitation to cyclonic curvature aloft [l] have long been recognized so the rains occurring in this part of the country were not unusual. Development of heavy rains which fell farther to the west in the warm sector were not so easily explained. The upward motion in the warm sector must have been quite widespread to produce such a large area of precipitation and therefore should be subject to detection or computation by several of the methods which have been developed for the study of vertical motions. The cause and distribution of these vertical motions will be investigated in this paper by examination of each of the following types of charts: 1. precipitable moisture, 2. Showalter stability index, 3. differential advection, 4. surface isobaric convergence, 5. sea level pressure change (Laplacian of), 6. curvature of flow pattern at 700 mb. and higher, 7. jet stream, 8. tropopause, and 9. JNWP vertical motion computations (900400 mb.).

METHODS OF COMPUTING VERTICAL MOTION IN THE ATMOSPHERE

Abstract The theory of a number of different techniques for the computation of vertical motion in the atmosphere is discussed. A comparison of two independent techniques shows that both usually yield vertical velocities of the correct sign and order of magnitude.