Socorro Medina's Webpage

Socorro Medina

Research Scientist
Department of Atmospheric Sciences
University of Washington
Seattle, WA, US
Curriculum Vitae

I am interested in understanding the different ways in which moist flow is modified by topography and on the effect that such modification has on precipitation distribution. Two particular subjects that I have studied are:

1-How orography affects precipitation when baroclinic systems pass over mid-latitude montain ranges.

2-How orography and land cover affect convection and precipitation when moist, warm flow approaches tropical and sub-tropical mountain ranges.

PUBLICATIONS

Medina, S., R. A. Houze, Jr., A. Kumar, and D. Niyogi, 2009: Summer monsoon convection in the Himalayan region: Terrain and land cover effects. Quart. J. Roy. Meteor. Soc., revised.

Romatschke, U., S. Medina, and R. A. Houze, Jr. 2009: Regional, seasonal, and diurnal variation of extreme convection in the South Asian region. J. Clim., in press.

Colle, B. A., Y. Lin, S. Medina, and B. F. Smull, 2008: Orographic modification of convection and flow kinematics by the Oregon Coastal Range and Cascades during IMPROVE-2. Mon. Wea. Rev., 136, 3894-3916.

Medina, S., E. Sukovich, and R. A. Houze, Jr., 2007: Vertical strucutres of precipitation in cyclones crossing the Oregon Cascades . Mon. Wea. Rev., 135, 3565-3586 (Paper of note highlighted in Bull. Amer. Meteor. Soc., Dec 2007 Issue).

Medina, S., B. F. Smull, R. A. Houze, Jr., and M. Steiner, 2005: Cross-barrier flow during orographic precipitation events: Results from MAP and IMPROVE. J. Atmos. Sci., IMPROVE special issue, 62, 3580-3598.

Houze, R. A., Jr., and S. Medina, 2005: Turbulence as a mechanism for orographic precipitation enhancement. J. Atmos. Sci., IMPROVE special issue, 62, 3599-3623.

Medina-Valles, M. S., 2005: Orographic enhancement of mid-latitude cyclone precipitation. Ph. D. thesis. Dept. of Atmospheric Sciences, University of Washington, Seattle, WA, 177 pp.

Medina, S., and R. A. Houze, Jr., 2003: Air motions and precipitation growth in alpine storms. Quart. J. Roy. Meteor. Soc., special MAP issue, 129, 345-371.

Medina, S., 2002: Air motions and precipitation growth in Alpine storms. M. S. thesis. Dept. of Atmospheric Sciences, University of Washington, Seattle, WA, 114 pp.

Houze, R. A., Jr., C. N. James, and S. Medina, 2001: Radar observations of precipitation and airflow on the Mediterranean side of the Alps: Autumn 1998 and 1999. Quart. J. Roy. Meteor. Soc., 127, 2537-2558.

Medina-Valles, M. S., 1999: Actividad de conveccion atmosferica en las albercas de agua caliente cercanas a Mexico (Convection over the warm pools near Mexico). M. S. thesis. Unidad Academica de los Ciclos Profesional y de Posgrado del Colegio de Ciencias y Humanidades, Universidad Nacional Autonoma de Mexico (UNAM), Mexico City, Mexico, 50 pp.

Magaña, V., J. Amador, and S. Medina, 1999: The midsummer drought over Mexico and Central America. J. Climate, 12, 1577-1588.

Medina-Valles, M. S., 1997: Variabilidad intraestacional en precipitacion en Mexico (Intraseasonal variability in precipitation in Mexico). B. S. thesis. Facultad de Ciencias, Universidad Nacional Autonoma de Mexico (UNAM), Mexico City, Mexico, 56 pp.

CONFERENCES

OTHER TALKS

RESEARCH INTERESTS

1-How orography affects precipitation when baroclinic systems pass over mid-latitude montain ranges.

When a mid-latitude cyclones passes over a mountain range, the precipitation production is rearranged such that the windward side precipitation tends to be enhanced while that on the lee-side tends to be reduced or even eliminated. To understand the windward enhancement, it is necessary to analyze both the terrain-modified airflow and the microphysical processes that convert water vapor into precipitation. Recent multi-platform, multi-institution field projects conducted over the European Alps (MAP) and the Oregon Cascade Mountains (IMPROVE-2) collected detailed meteorological data during the passage of mid-latitude cyclones over orography. In particular, Doppler radars provided continuous information on the orographic airflow and precipitation. Additionally, polarimetric radars provided information on hydrometeor type, which gives some indication on the microphysical growth mechanisms.

We have identified and documented two distinct terrain-modified cross-barrier flow patterns (termed Type A and B) based on the analysis of data collected during these two field experiments. For each flow pattern, new conceptual models of windward enhancement of mid-latitude precipitation were derived. In Type A storms the low static stability low-level air rises easily as it encounters the first peaks of the terrain (Fig. 1; Medina and Houze 2003). Lifting of the moist low-level air produces high liquid water content over these peaks, which favor growth of the pre-existing precipitation particles by coalescence below the 0 degree level and by riming above. If the upstream flow is potentially unstable, convective cells will be triggered in the upslope ascent. These cells produce pockets of especially high liquid water content where the coalescence and riming processes are accentuated.

FIG. 1. Conceptual model for Type A storms (From Medina and Houze 2003)

Type B storms exhibit a shear layer on the windward slopes (Fig. 2; Houze and Medina 2005). The combination of high shear and static stability produces conditions that support dynamical instability manifested in the form of Kevin-Helmholtz billows and turbulent overturning cells (bottom panel in Fig. 3). Aggregation of ice particles falling from the baroclinic system into the layer of cells is aided by the turbulent motions. The strong updrafts produce pockets of high liquid water content, which favor riming and coalescence.

FIG. 2. Conceptual model for Type B storms (From Houze and Medina 2005)

FIG. 3. Time-height cross-section of NOAA/ETL S-band vertically pointing radar (From Houze and Medina 2005)

Therefore, during the passage of mid-latitude cyclones over a mountain range, windward precipitation is enhanced by small-scale cellularity regardless of the static stability of the upstream flow. In Type A storms static instability is responsible for the updraft generation, whereas dynamic instability produces updraft motions in Type B storms. In both scenarios, the updrafts are strong enough to activate the accretion growth processes (coalescence, aggregation and riming), which are capable of producing large particles that fallout rapidly on the windward side of the terrain.

2-How orography and land cover affect convection and precipitation when moist, warm flow approaches tropical and sub-tropical mountain ranges.

During the Asian summer monsoon, convection occurs frequently near the Himalayan foothills (Fig. 4a). However, the nature of the convective systems varies from the western to eastern parts of the region. Systems containing extremely intense convective cells have a maximum probability in the western Himalayan region (black circle in Fig. 4b), while convective systems containing extensive stratiform echoes have a maximum likelihood in the eastern Himalayan region (black solid rectangle in Fig. 4b). We used observations and high-resolution numerical simulations elucidate the conditions that lead to these regional differences in convective structure.

FIG. 4. Southern Asia orography with (a) relevant geographical locations and (b) regions of frequent occurrence of intense convective echoes (circle) and convecitve systems containing broad stratiform echo (black solid rectangle) (From Medina et al. 2008)

Convective systems with extremely intense cells (e.g., Fig. 5) occur in the western region when low-level air collects moisture while passing over the Arabian Sea, then makes landfall and traverses the Thar desert (Fig. 4a), where extreme surface flux of sensible heat enhances buoyancy. Thus, low-level flow reaches the western end of the Himalayas with both high moisture content and large Convective Available Potential Energy (CAPE). The terrain modulates convection released in this air mass in three ways: 1) Low-level moisture is trapped and concentrated within the western concave indentation of the Himalayan barrier; 2) dry, downslope flow from the Afghan mountains caps the low-level moist flow, inhibiting premature release of instability upstream of the foothills; 3) convection is violently released when the low-level flow encounters small foothills (Fig. 6). These terrain-influenced processes resemble those leading to intense convection east of the Rocky Mountains in the U.S.

FIG. 5. Reflectivity for a convective system with intense cells as (a-b) observed by TRMM at 2200 UTC 3 Sep 2003 and (c-d) WRF-simulated at 2000 UTC 3 Sep 2003. Panels (a) and (c) show horizontal cross-sections at 4 km. Panels (b) and (d) show vertical cross-sections taken along the blacks lines in panels (a) and (c), respectively. (From Medina et al. 2008).

FIG. 6. Simulation output of terrain (shading) and vertically-integrated mixing ratio of precipitating hydrometeors 10 mm isochrones at 1900 UTC (thick white solid contour) 2000 UTC (short-dashed contour), 2200 UTC (long-dashed contour), and 2300 UTC 3 September 2003 (thin solid contour). (b) Vertical cross-section of mixing ratio of precipitating hydrometeors (black contour, 1 g/kg isoline) along the south-north oriented line in (a) at the time the convection was initialized (1900 UTC 3 Sep 2003) (From Medina et al. 2008).

Convective systems exhibiting extremely broad stratiform echoes (e.g., Fig. 7) occur in the eastern Himalayan region in association with Bay of Bengal depressions, which provide persistent and strong low-level flow that effectively extracts moisture from the Bay of Bengal. As the flow passes inland over the wetlands of the Ganges Delta (Fig. 4a), additional moisture is extracted from the diurnally heated surface. The terrain affects the convection in three ways: 1) The eastern concave indentation of the Himalayas prevents the low-level flow from continuing to move northward, thus the moisture content builds up upwind of the foothills (blue line in Fig. 8a-b); 2) convection is triggered as conditionally unstable flow is lifted upstream and over the foothills; 3) the convective cells, which evolve into mesoscale convective systems (MCSs) with convective and stratiform areas, are advected farther into the Himalayan eastern indentation, where orographic lifting enhances the stratiform precipitation (red and orange lines in Fig. 8b and cross-section in Fig. 8c). This orographic enhancement is similar to the way upslope flow enhances stratiform precipitation over the European Alps (i.e., Type A flow pattern).