|
<br />FEBRUARY 1988
<br />
<br />DA V1D W. REYNOLDS AND ARUNAS P. KUCIAUSKAS
<br />
<br />155
<br />
<br />type and develop a rationale for the onset of SL W
<br />within these events. From this study it appears that of
<br />63 storms, 37 have characteristics similar to the 3
<br />storms described here. This indicates that 60% of the
<br />precipitation events might have characteristics in which
<br />satellite and radar imagery could provide information
<br />as to the onset or increase in SLW.
<br />Knowing when winter mountain clouds contain ex-
<br />cesses in SL W has a direct impact on winter cloud
<br />modification programs. These results show that the
<br />highest amounts of SL W exist in the shallow, warm-
<br />topped orographic clouds containing ernbedded con-
<br />vection and generally having low precipitation rates
<br />(<2 mm h -I). These results are in agreement with the
<br />earlier studies of Grant and Elliott (1974), Hobbs
<br />(1975), Hill (1980), Rauber and Grant (1986), Elliott
<br />(1987), and Hill (1987). Additionally, these clouds are
<br />conducive to seeding because the SL W exists in the
<br />colder more convective airmass which serves to in-
<br />crease the dispersal of seeding material and to increase
<br />the activation of AgI nucleant.
<br />
<br />Acknowledgments. Preparation of this paper was
<br />supported by the Department of Interior, Bureau of
<br />Reclamation. One author was supported under Bureau
<br />of Reclamation Contract 4-CR-81-03860. The authors
<br />wish to acknowledge the contributions to the SCPP
<br />made by many scientists, pilots, technicians, and other
<br />personnel both within and outside the Bureau of Rec-
<br />lamation. The authors would like to especially thank
<br />Professor Dusan Djuric for his detailed review and
<br />constructive criticism of the original manuscript. Ap-
<br />preciation goes to Mrs. Bernie Culver for typing the
<br />manuscript and to personnel of the Bureau's Mid-Pa-
<br />cific Region and Mrs. Carol Wilcox for drafting the
<br />figures.
<br />
<br />APPENDIX
<br />
<br />'-
<br />
<br />The 2500 Condensate Supply Rate Calculation
<br />
<br />For a simple two-dimensional, steady state, stable
<br />orographic flow as depicted in Fig. 14, the vapor flux
<br />(Fo) into the region at x = Xo is given by
<br />
<br />Fo = qoVoMoAY
<br />g
<br />
<br />while the vapor flux out (FI) at x = XI is
<br />
<br />qlVIMIAy
<br />FI = ,
<br />g
<br />
<br />where q is the specific humidity (or mixing ratio), AP
<br />the layer thickness in pressure units, V the horizontal
<br />velocity in the x-direction, Ay an arbitrary distance
<br />perpendicular to the flow, and g gravity.
<br />Since the change in vapor content between Xo and
<br />XI is due to condensation by adiabatic lift, the conden-
<br />sate supply rate (CSR) is equal to Fo - Fl, This can
<br />
<br />VO,lIPO
<br />qo
<br />
<br />VI ,t>P 1 ....l
<br />
<br />
<br />~
<br />
<br />
<br />Xl
<br />
<br />==>
<br />
<br />Xo
<br />
<br />fIa. 14. Schematic two-dimensional flow across a mountain barrier.
<br />
<br />be simplified by noting that when mass inflow equals
<br />flow out (neglecting water substance changes)
<br />
<br />APo Vo API VI
<br />
<br />g g
<br />
<br />which gives
<br />
<br />VoMol.ly
<br />CSR = -- (qo - ql)..
<br />g
<br />
<br />Changing this to a representation in units of depth
<br />per unit area gives
<br />
<br />VoAPo
<br />CSR = -(qo - ql)
<br />Pwg!:ix
<br />
<br />where Pw = density of water.
<br />At the bottom of the tirne-height sections shown in
<br />Figs. 2, 7, and 12, CSR was computed for each 5 mb
<br />layer from the surface to 5 krn assurning a 1.2 km lift
<br />for each layer, and !:ix = 70 km (i.e., a 70 km wide
<br />upslope region along the 2500 wind component, per-
<br />pendicular to the Sierra Nevada, so that
<br />
<br />CS = APoVo cos(a - 2500qo - ql)
<br />R 2500 Pwg!:ix '
<br />
<br />where a is the layer-averaged wind direction from the
<br />SHR sounding, qo - ql the adiabatic condensate frorn
<br />a lift of 1.2 km, and Vo the layer interpolated wind
<br />speed frorn the SHR sounding.
<br />Based on this fairly sirnple calculation, the values
<br />derived should be considered as rough approximations
<br />and are provided to show relative increases or decreases
<br />in available condensate.
<br />
<br />REFERENCES
<br />
<br />Bergeron, T., 1937: On the physics of fronts. Bull.. Amer. Meteor.
<br />Soc.. 18, 265-275.
<br />Browning, K. A., 1985: Conceptual models of precipitation systems.
<br />Meteorological Magazine. 114,293-319.
<br />-, and G. A. Monk, 1982: A simple model for the synoptic analysis
<br />of cold fronts. Quarterly J. Roy. Meteor. Soc.. 101, 893-900.
<br />Elliott, R. D., 1987: Review of wintertime orographic cloud seeding.
<br />Precipitation Enhancement-A Scientific Review. Meteor.
<br />Monogr., R. R. Braham, Ed., 21, 87-101.
<br />Aueck, J. H., and D. W. Reynolds, 1986: A forecast experiment on
<br />the prediction of cloud conditions suitable for treatment in the
<br />Sierra Nevada. Preprints Tenth Conf. on Weather Modification
<br />of the AMS, 27-30 May, Arlington, VA, 13-17.
<br />
|