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Michael D. Hudlow

Abstract

As a part of the GARP Atlantic Tropical Experiment (GATE), quantitative precipitation measurements were made during the summer of 1974 with four C-band digital radars complemented by shipboard raingages. Isohyetal maps covering a 125 000 km2 array centered at 8°30′N, 23°30′W are presented for each of three, approximately 20-day observational phases of GATE. Large mean rain rates exist for all three phases, with the largest ones corresponding to accumulations exceeding 500 mm for some of the maximum isohyets during Phase I. The mean rainfall rate averaged over the B-scale array for all three phases, 11.3 mm day−1, is apparently not significantly different from pre-GATE rainfall climatology. Another striking characteristic of the phase-mean precipitation patterns is the large spatial gradients; e.g., gradients as large as 200 mm in 16 km are observed.

Latitude shifts in the zone of maximum confluence (intertropical convergence zone) and in the tracks of the synoptic disturbances are reflected by interphase changes in the precipitation patterns. Also presented is a time-latitude rain cross section constructed from hourly precipitation amounts, which shows that the significant precipitating convection occurred most frequently in the vicinity of the troughs of African wave disturbances during Phase III.

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Robert W. Reeves, Chester F. Ropelewski, and Michael D. Hudlow

Abstract

Upper air and surface data from the GARP Atlantic Tropical Experiment (GATE) are used to examine the interrelationships between convective-scale precipitation and the larger scale wind field. The upper air winds from the inner (B) and outer (A/B) hexagonal observational arrays are fit with second-order polynomials to provide smooth estimates of the vorticity, divergence and vertical motion in the observational array. In these analyses we examined archived validated data from all three phases of the experiment and we formed averages based on the radar-estimated precipitation rates.

Mean profiles for 19-day periods during each of the three observational phases establish the basic similarity of the kinematics during each phase. Strong boundary-layer convergence balanced, for the most part, by upper tropospheric divergence, is common to all three phases.

Radar-estimated precipitation rates are used to define suppressed (precipitation rates <0.1 mm h−1) and highly disturbed (precipitation rates >0.5 mm h−1) states over the observational array. Mean profiles for the disturbed states in each phase show weaker easterly winds and much larger upward vertical velocities than do the mean profiles for the suppressed states. The mean vorticity profiles for each state do not show such clear-cut differences.

Time series of 12 h averages indicate that the precipitation events in Phase III corresponded very closely to the cyclonic maxima of the 700 mb relative vorticity, reflecting the influence of the easterly waves described by Reed et al. (1977). During Phases I and II, when easterly waves were poorly organized, the precipitation events did not correspond closely to the cyclonic vorticity maxima. On the other hand, precipitation events showed good correspondence with the large-scale (A/B) 700 mb upward vertical velocity maxima and surface meridional convergence ∂v/∂y during all three phases. This shows that the precipitation is clearly related to events on a larger scale.

The effects of convective activity on the large-scale flow are examined through the vorticity budget. The vorticity budget residual profiles were similar from phase to phase with cyclonic production maxima in the mid and upper troposphere. The upper tropospheric residual maximum is as strong during the suppressed state as it is during the highly disturbed side. At the surface, individual values of the residual are almost always opposite in sign to the vorticity. The mean vorticity budget for the A/B array shows the tipping term to have magnitudes comparable to other terms in the vorticity budget.

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Konstantine P. Georgakakos and Michael D. Hudlow

Quantitative hydrologic forecasting usually requires knowledge of the spatial and temporal distribution of precipitation. First, it is important to accurately measure the precipitation falling over a particular watershed of interest. Second, especially for small watersheds and/or for longer forecast lead times, forecasts of precipitation are critical to the achievement of the greatest possible hydrologic forecast accuracy and longest possible lead time. This paper describes the current hydrologic forecasting program of the U.S. National Weather Service (NWS) and highlights the relevance of Quantitative Precipitation Forecasting (QPF) products to real-time hydrologic forecasting. Specific requirements for QPF products in support of hydrologic forecasting applications are defined and current operational QPF procedures are reviewed to determine to what extent they meet these requirements. It is concluded that no known QPF procedures capable of fulfilling all desired requirements are currently available operationally, although much valuable QPF information is available to meet parts of these requirements. Some recent advances in mesoscale QPF research are examined and these techniques are treated in two categories: those uncoupled dynamically from and those dynamically coupled to hydrologic forecasting procedures. Finally, a summary of possible future directions toward achieving improved use of QPF information in hydrologic forecasting applications is presented.

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