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Forecasting Mesoscale Convective Complex Movement in Central South America

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Forecasting Mesoscale Convective Complex Movement in Central South America FORECASTING MESOSCALE CONVECTIVE COMPLEX MOVEMENT IN CENTRAL SOUTH AMERICA Marc R. Gasbarro 551 st Electronic Systems Wing Electronic Systems Center Hanscom AFB, Massachusetts Ronald P. Lowther Air and Space Science Directorate Air Force Weather Agency Offutt AFB, Nebraska Stephen F. Corfidi NOANNational Weather Service Storm Prediction Center Norman, Oklahoma Abstract 1. Introduction A method for operationally predicting the movement Mesoscale convective complexes (MCCs) are respon­ of the centroid, or coldest cloud tops, of mesoscale con­ sible for producing severe weather and flooding rains vective complexes (MCCs) in Central South America in Central South America (CSA). This region includes (CSA) is presented (Gasbarro 2003). The procedure of Paraguay, Uruguay, Northern Argentina, and predicting the movement of an MCC centroid, which Southeastern Brazil (Velasco and Fritsch 1987). They primarily relates to the area of heaviest precipitation are responsible for producing damaging winds, hail, within the MCC, is modified from the work ofCorfidi et injuries, and occasionally even deaths. Furthermore, al. (1996). * This process is based on the concept that the MCCs significantly change and/or influence upper­ motion of quasi-stationary or backward-propagating atmospheric wind fields, presenting problems with convective systems can be found as the sum of the aviation safety and efficiency problems with flight advective component, defined by the mean motion of the scheduling (CFM96). In addition, the sparse data net­ cells comprising the system, and the propagation com­ work of South America (SA) further adds to the diffi­ ponent, defined by the rate and location of new cell for­ culty of accurately forecasting MCC movement due to mation relative to existing cells. These concepts and the the lack of synoptic observations. forecast procedure are examined using 22 mesoscale There are many similarities between North convective systems (MCSs), 20 of which were classified America (NA) and SA MCCs (Velasco and Fritsch as MCCs. 1987). As in NA, SA MCCs are nocturnal storms that It is found that the advective component of MCS owe their existence partly due to a moist, poleward motion is well correlated to the mean flow in the cloud advecting low-level jet (LLJ) that is comparable in layer. Similarly, the propagation component is shown to strength to the United States (US) LLJ. The steep be directly proportional, but opposite in sign, and well Andes mountain chain helps to initiate convection and correlated to the direction of the low-level jet. channel the South American low-level jet (SALLJ) Correlation coefficients between forecast and observed (Saulo et aI. 2000) poleward. This process is similar to values for the speed and direction of the MCSs and the process associated with the North American LLJ MCCs for CSA are 0.72 and 0.81, respectively. This in the lee of the Rocky Mountains. Like NA MCCs, compares well to correlation coefficients of 0.80 and those in SA form mainly during the warm season 0.78 for the MCC or MCS speeds and directions, respec­ [November through April in the southern hemisphere tively, of the CFM96 method for North American MCC (SH)]. SA MCCs generally exhibit a similar dynamic and MCS movement. Mean absolute errors of the cen­ structure to NA MCCs. MCCs in both hemispheres troid speed and direction are 2.1 m S·l and 16.4 0 respec­ usually require the presence of a quasi-stationary tively. These errors, comparing well to the CFM96 method, are sufficiently small so that the forecast path of the centroid would be well within the heavy rain *Cordifi et al. (1996) will be referred to as "CFM96" for the swath of a typical MCC. remainder of this paper. 68 Volume 30 December 2006 boundary associated with moderately intense, tran­ '" .? ... -~....... _&--.- r..J sient upper-level shortwaves. The shortwaves promote ~ ./ ........--..,...... /-,,-;~;-o--a~ ...,, - ~~-~- storm development by destabilizing the atmosphere "'"r ,.,..;.. ..,.".// ......... __......... ..-.• ~ ...t.-~'""""" __ and enhancing upper-level divergence. MCCs require minimal upper-level shear; therefore, very strong lOS shortwaves are not conducive to MCC growth. Surface temperatures on both continents are similar around the location of MCC genesis. Finally, the favored region ofMCC genesis in both NA and SA shifts west­ ward throughout the warm season months as the respective North or South Atlantic subtropical highs build westward (Velasco _and Fritsch 1987). SA MCCs also exhibit various differences from their NA counterparts (Velasco and Fritsch 1987). One major difference is size. SA MCCs are, on average, 60% larger than NA MCCs. Velasco and Fritsch (1987) found that the average size of the -32°C cloud shield in sow 40W 30W 2 SA MCCs is around 400,000 km , compared to only 300,000 km2 for NA. SA MCC lifespan averages 11.5 hours vs. 9 to 9.5 hours in NA (Maddox et al. 1986; Fig. 1. Mean positions of the North Argentine Depression, South Velasco and Fritsch 1987). SA MCCs form more equa­ Atlantic Convergence Zone (dashed line), South Atlantic ridge (zig­ torward and with less latitudinal variability than NA zag line), and 850 mb fectors (m s·') during December to February MCCs. NA MCCs generally form from 30° to 50° N, (modified from Lenters and Cook 1999). while MCCs in SA typically are generated only 30W between 25° and 35° S. Unlike in NA, SA MCCs are present into late austral summer and early autumn (Velasco and Fritsch 1987). Surface dewpoints in which MCCs spawn are 3-5°C higher in SA than in NA (Davison 1999; Velasco and Fritsch 1987). Also, the tropopause in SA MCCs averages about 100 mb vs. 150 to 200 mb in NA (Velasco and Fritsch 1987). A higher tropopause along with higher surface dewpoints implies greater thermodynamic instability associated with MCCs in SA. The moisture source region for SA MCCs also aids in fueling very unstable conditions. In NA, the moisture source for the LLJ is the Gulf of Mexico; however, rather than a body of water, the SALLJ feeds off the Amazon Basin, a very warm, shal­ low, land moisture source. Finally, the SALLJ owes its existence mainly due to a tight pressure gradient between a thermal low, the North Argentine Depression, and the South Atlantic High (Saulo et al. Fig. 2. Mean 200 mb position of the Bolivian High for January 2000). This differs from NA where the US LLJ princi­ (modified from Davison 1999). pally forms from boundary layer frictional differences and a nocturnal inversion (Bonner 1968). Consequently, the SALLJ lasts longer in the day and outflow in the upper levels (Fig. 2), create conditions occurs more frequently than in NA (Velasco and for MCC development and intensification. In addition, Fritsch 1987). increases in the strength of the Bolivian High, an Various synoptic features in SA working in cohesion upper-level high climatologically centered over generate conditions necessary for MCC formation. Bolivia, relate to a stronger STJ (Fig. 2). A stronger jet Climatologically, the North Argentine Depression and crossing the Andes leads to increased shortwave per­ the South Atlantic High are present through the aus­ turbations, which ultimately helps initiate MCSs and tral warm season (Fig. 1) (Lenters and Cook 1999). MCCs (Davison 1999). When one or both of the two pressure features intensi- CFM96 proposed a method to determine movement -fy, the pressure gradient between the two systems of MCCs in NA using the principle that movement of tightens, which increases the intensity of the SALLJ MCCs is affected by both cloud layer advection and and the low-level and moisture flux convergence propagation components. CFM96 hypothesized that (Saulo et al. 2000). Quasi-stationary boundaries such the advective component of MCC movement is propor­ as slow moving fronts, squall lines, and the South tional to the mean cloud layer flow. They also hypoth­ Atlantic Convergence Zone (SACZ) (Fig. 1) greatly esized that the propagation component is equal and enhance low-level convergence (Lenters and Cook opposite of the LLJ. After successfully verifying that 1999). These ingredients, combined with tremendous both components do significantly influence MCC 70 National Weather Digest relates to increased radar echo returns, this research produces results comparable to those of CFM96. Sections 2 and 3 describe the methodology and results for validating the advective and propagation components, and then verify the forecasted MCC movement against the actual MCC movement. A brief summary along with concluding remarks are given in Section 4. 2. Data and Methodology Twenty-two MCC and MCS cases were analyzed to verify the CFM96 method of MCC movement for SA. Two of the cases are in January 2001 with the remain­ der from September to December 2002. Twenty of the 22 cases are MCCs. The International Desks section of the Hydrometeorological Prediction Center (HPC) at the NOAAlNational Center for Environmental Prediction (NCEP) provided GOES-8 satellite imagery Fig.3. MCC over South America at 0245 UTC 25 Nov 2002. Black for the 2001 case studies (Davison 2002), while the dot represents the centroid of the system (coldest cloud tops) Cooperative Institute for Research in the Atmosphere (modified from CIRA 2002). (CIRA) provided GOES-8 satellite imagery for the 2002 cases (CIRA 2002). This study only utilized three­ hourly, channel four IR imagery for the detection of cold cloud tops. Due to very sporadic and inconsistent upper air sounding data in the region of study, this research uti­ lized upper air reanalysis data from the Fleet Numerical Meteorological and Oceanography Detachment (FNMOD), Asheville, NC, for verification of the CFM96 method. The U.S. Navy runs the Navy Operational Global Atmospheric Prediction System (NOGAPS) model to produce reanalysis data twice per day at 0000 and 1200 UTC. The FNMOD in Asheville, NC, stores the archived NOGAPS reanalysis data for future use. Satellite imagery was used to track 20 MCCs and 2 MCSs by tracking the centroid of the system.
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