Hail Storms in the Andes Region (Mendoza, Argentina) and their Relation with Large Amplitude Mountain Waves

DE LA TORRE; R. HIERRO; P. LLAMEDO; A. ROLLA; P. M. ALEXANDER

Foz de Iguazu

Congreso; agu 2010; 2010

American Geophysical Union

In addition to 1) an environmental lapse rate conditionally unstable and 2) sufficient moisture for a rising parcel’s associated moist adiabat has a level of free convection (LFC), 3) some process by which a parcel is lifted to its LFC is required for the development of deep convection. Under certain conditions, the rising motions associated with synoptic-scale processes are to slow to lift a potentially buoyant parcel to its LFC in the required time. This is not the case in Mendoza, Argentina, a semiarid region situated at midlatitudes (roughly between 32S and 36S) at the east of the highest Andes tops. There, frequent cold fronts arriving from SW provide the required lift mechanism to trigger severe deep convection events. The frequency of these events, distributed in spring and summer, peaks in January. However, many of them are observed in the absence of cold fronts. In the absence of any synoptic-scale possible mechanism able to trigger deep convection processes like cold fronts, the lift required to raise a parcel to its level of free convection must be supplied by some other process operating on subsynoptic scales, like anabatic or katabatic winds and/or mountain waves. We analyze here recent severe storms registered at the south of Mendoza, initiated at the same local time. In these cases, large amplitude stationary mountain waves with similar wavelengths were generated, through the forcing of NW wind by the Andes Range, just before the first cell was detected in the S-band radar. Mesoscale model simulations (WRF3V, three domains, inner at 4 km) were conducted. The wave pattern was analyzed at several constant pressure levels with a Morlet wavelet. This has proven to be a useful technique for this purpose, as propagating mountain waves are well localized within a horizontal domain of some hundred kilometers. The simulated evolution in space and time of vertical wind oscillations (even better than reflectivity) reveal their influence in the genesis zone of both storms. The synoptic conditions observed (low-pressure system over NW of Argentina, slow displacement of anticyclones in Pacific and Atlantic oceans, a low level jet carrying warm and damp air from N and geopotential distribution at 1000, 500 and 300 hPa) are consistent with earlier works. We describe i) the vertical and horizontal wavelengths, ii) the direction of propagation of the main wave modes, iii) their lineal polarization and phase relation between wind and temperature, iv) the Scorer parameter and v) the validation of WRF results with two measured COSMIC GPS radio occultation temperature profiles in the inner domain along their lines-of-sight.